Post on 20-Feb-2015
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
1. INTRODUCTION
This report deals with regional geology, geological and engineering geological
condition as well as the geotechnical condition, tectonics and seismicity, construction
material and muck disposal survey of the Deumai Hydropower Project. This study has
been carried out as per feasibility study requirement of the guideline of DoED.
The proposed Deumai Hydropower Project is located between downstream from
Gajurmukhi village and upstream from the junction between the Mai Khola and
Deumai Khola, Ilam District, Mechi Zone, Eastern Development Region of Nepal. The
intake area is located about 15 km east of Ilam Bazaar.
The Deumai Khola is a rain fed river and a main tributary of the Mai River and finally
drains out to the Mechi River. The proposed Deumai HEP is a storage or reservoir
scheme project. The powerhouse is located on the left bank of the Deumai Khola
about 0.5 km southeast from the confluence between the Mai Khola and Deumai
Khola along the Deumai Khola. The water will be diverted into the headrace tunnel to
the powerhouse to generate power. Related structures are located at the left bank of
the Deumai Khola on bedrocks as well as alluvial and colluvial deposits.
1.1 Objectives
The main objectives of the present geological study are as follows:
To obtain information on regional geology and geomorphology of the project area
To study detail geological and engineering geological condition of the locations of
proposed project structures
To prepare detailed engineering geological map (1:2,000), geological cross-sections
and profiles of the locations of major project structures like reservoir, tunnel
alignment, intake and weir axis, surge tank and penstock alignment and
powerhouse and tailrace areas
To carry out construction material survey and locate the disposal area for the
project area
To assess the slope stability of the project area including especially the tunnel
alignment
To evaluate the geotechnical condition of the project area
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
1.2 Scope of Work
The present study comprises of the following works:
Collect and review available literatures, topographical and geological maps,
photographs and landsat images
Collection and study of geological and geomorphologic information of previous
studies
Conduct field survey to collect and verify geological information prior to general and
detail geological mapping, engineering geological mapping of project components
and particular structures
Identify geological and seismic hazards such as faults, thrusts and landslides
Measurement of discontinuities to analyze slope stability and collect geotechnical
information of the rock mass to identify the rock mass classification
Prepare maps (engineering geological map) at the scale mentioned in DoED’s
guideline.
1.3 Methodology
To accomplish the objectives and scope of work, desk study, field visit and field data
analysis were carried out. During the desk study, available geological information and
geological maps of the Deumai Khola section relevant to the project area was
thoroughly studied. After the desk study, the field visit to the project was conducted.
During the field visit, discontinuity survey and geological as well as engineering
geological mapping of the project area including intake and weir area, and reservoir
area, tunnel alignment, surge tank, penstock alignment, powerhouse and tailrace
area was done. The instability and mass wasting area and necessary geological data
were also collected. After field observation, the detail analysis of geological data was
carried out which includes graphical analysis, slope stability analysis and rock mass.
The 2D-Electrical Resistivity Survey has been conducted and proposed the location
of the core drilling point in the headworks as well as the powerhouse area.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar2
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 1: Location Map of Project Area
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
Project Area
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
1.4 Background Information
Eastern Region of Nepal within varied geomorphic scenario and with complex
geological set up offers immense scope for utilization of water resources. The water
resources of the Deumai Khola which drains into the Kankai Khola at Mahaguna
village and finally drains out into the Mechi River still remains unutilized. These rivers
are perennial and carry considerable quantity of water. Since the rainfall of the
catchment area is high, these exists a steady discharge of water in these rivers
throughout the year making them ideal for hydropower development in tandem. In
view of above, number of hydropower projects were identified and awarded to Private
Developers by the Government of Nepal to harness these vast natural resources for
the hydropower generation. The proposed Deumai Hydro Electric Project is a self
identified project which has been awarded to Dolakha Nirman Company (P) Ltd.,
Nepal by the Department of Electricity Development (DoED) under the Ministry of
Energy, Government of Nepal, development of hydropower in Mai Khola basin,
through construction of a 20 MW hydropower station utilizing the water resources of
the Deumai Khola in Ilam District, Mechi Zone, Eastern Development Region of
Nepal. Installed capacity has been worked out as a storage scheme.
1.5 Project Layout and Salient Features
The proposed Deumai Hydro Electric Power envisage construction of a 50 m high
and 50 m long concrete dam with central spillway across Deumai Khola, two 9.5 m
diameter and 511 m and 610 m long diversion tunnels, two 8 m diameter intake
tunnels, two 8 m diameter and 298 m long underground pressure shaft, a dam toe
powerhouse and two 10.4 m diameter and 4.1 km long HRT for generation of 30 MW
of hydropower utilizing a gross head of about 70.77 m. It is a reservoir project and all
the appurtenant structures are located along the left bank of the Deumai Khola.
Salient features of the Deumai HEP are given in Table 1.
Table 1: Salient Features of Deumai HEP
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
To be inserted by Consultant
1.6 Present Investigation
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar2
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
In order to fulfil the objectives and scope of work, the present studies were focused
mainly on general and detailed geological mapping and subsurface explorations. The
main activities performed during the present investigation include the following:
Geological Mapping
-General geological mapping of the project area in 1:4,000 scale
-Detailed geological mapping of the headworks/powerhouse in 1:1,000 scale
-Geological section of the headrace tunnel in 1:4,000 scale
-Mapping of the mucking area and source of the construction materials in 1:50, 000
scale.
Geotechnical Investigation
-Rock mass classification (Q and RMR Values) of the rock mass of headwork site,
tunnel alignment and powerhouse area for suggesting support system,
-Construction material survey and laboratory testing,
-Stereographic projection of major discontinuities of the rock of the project area.
1.7 Sub-surface Exploration
In order to study the sub-surface geology of the area eight lines of the ERT covers
about 1,500 m in the reservoir and intake area. Bedrocks are found at shallow depth
in each line.
2. HIMALAYA IN GENERAL
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar3
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The Himalaya is the largest mountain range of the world, which extends for a total
length of about 2,400 km. This lengthy mountain chain is geologically divided into five
sections from west to east (Figure 2; Gansser, 1964). The brief descriptions are as
follow:
2.1 Punjab Himalaya
The Punjab Himalaya (about 550 km) lies between the Indus River in the west and
Sutlej River in the east.
Figure 2: Physiographic Subdivision of the Himalayan Arc (after Gansser, 1964)
2.2 Kumaon Himalaya
It borders the Sutlej River in the west and the Mahakali River in the east and extends
about 320 km.
2.3 Nepal Himalaya
The Nepal Himalaya (800 km) lies between the Mahakali River in the west and the
Mechi River in the east.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar4
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
2.4 Sikkim-Bhutan Himalaya
It starts from the Mechi River and extends along Sikkim and Bhutan for a length of
400 km.
2.5 NEFA (North East Frontier Agency) Himalaya
It stretches for 440 km from eastern boundary of Bhutan to the Tsangpo River in the
east.
3. GEOLOGY OF THE NEPAL HIMALAYA
The Nepal Himalaya is situated in the central part of the Himalayan arc and has
covered about one third part. The Nepal Himalaya is located between Kumaon
Himalaya in the west and the Sikkim-Bhutan Himalaya in the east. The Nepal
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar5
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Himalaya is subdivided into the following five major tectonic zones from south to north
(Upreti and Le Fort, 1999; Figure 3).
Indo-Gangetic Plain (Terai)
---- Himalayan Frontal Thrust (HFT) ----
Sub-Himalaya (Siwalik or Churia Group)
---- Main Boundary Thrust (MBT) ----
Lesser Himalaya
---- Main Central Thrust (MCT) ----
Higher Himalaya
---- South Tibetan Detachment System (STDS) ---
Tibetan-Tethys Himalaya
3.1 Indo-Gangetic Plain (Terai)
This zone represents the northern edge of the Indo-Gangetic Plain and forms the
southernmost tectonic division, represents Pleistocene to Recent in age and has an
average thickness of about 1,500 m. This zone lies in the southern part of the
Himalaya, basically composed of the boulder to clay. The uppermost part of the Indo-
Gangetic Plain is the Bhabar zone and it comprises of boulder to pebble. The Middle
part (Marshy zone) is composed of sands whereas the clays are dominant in the
southern Terai.
3.2 Sub-Himalaya (Siwaliks or Churia Group)
The Sub-Himalaya (Siwaliks or Churia Group) is developed in the southern part of the
country and is represented by low hills of the Churia Range. The Siwalik Group of
Nepal is composed of 5-6 km thick fluvial sediments of the middle Miocene to early
Pleistocene age. The sediments are generally layers of mudstone, sandstone and
conglomerate. The Siwalik Group is divided into the Lower, Middle and Upper
Siwaliks in ascending order based on the lithology and increasing grain size. The
Lower Siwalik is comprised of mudstone and sandstone, whereas the Middle Siwalik
represented by thick-bedded, coarse-grained, "pepper and salt" appearance
sandstone. The Upper Siwalik is identified with the presence of conglomerate.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar6
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
3.3 Lesser Himalaya
The Lesser Himalaya lies in between the Sub-Himalaya (Siwalik Group) in the south
and Higher Himalaya in the north. Both the southern and northern limits of this zone
are represented by thrusts, the Main Boundary Thrust (MBT) and the Main Central
Thrust (MCT), respectively. Tectonically, the entire Lesser Himalaya consists of
allochthonous and para-autochthonous rocks. Rock sequences have developed with
nappes, klippes and tectonic windows, which have complicated the geology. The
Lesser Himalaya is made up of mostly the unfossiliferous sedimentary and
metasedimentary rocks, consisting of quartzite, phyllite, slate and limestone ranging
in age from Pre-Cambrian to Miocene.
3.4 Higher Himalaya
This zone is geologically as well as morphologically well defined, and consists of a
huge pile of highly metamorphosed rocks. It is situated between the fossiliferous
sedimentary zone (the Tibetan-Tethys Himalaya} in the north, separated by STDS
and the Lesser Himalaya, separated by the MCT in the south. Paradoxically it is
made up of the oldest rocks of Pre-Cambrian metamorphic and granitic gneiss. The
north-south width of the unit varies from place to place. This zone consists of almost
10 km thick succession of the crystalline rocks also known as the Tibetan Slab (Le
Fort, 1975). This sequence can be divided into four main units. From bottom to top
these units are: Kyanite-Sillimanite gneiss (Formation I), Pyroxenes marble and
gneiss banded gneiss (Formation II) and Augen gneiss (Formation III).
3.5 Tibetan-Tethys Himalaya
Rocks of the Tibetan-Tethys Himalayan zone are made up of thick pile of richly
fossiliferous sediments and their age ranges from early Paleozoic to Cretaceous. This
zone is about 40 km wide and composed of sedimentary rocks such as shale,
limestone and sandstone. In Nepal, these fossiliferous rocks of the Tibetan-Tethys
Himalaya are well developed in the Thak Khola (Mustang), Manang and Dolpa.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
HHSS Paleozoic
HHSS Mesozoic
LH Paleozoic
HH leucogranite
LH granite
Thrust
Terai
Duns & recent filling
Siwaliks
Lesser Himalayan zone
LH crystalline nappe
Higher Himalaya cryst.and crystalline nappe
TIBET
INDIA
Langtang Ri
Janakpur
Biratnagar
Everest Kanchenjunga
AnnapurnaDhaulagiri
Manaslu
Kanjiroba
Api
Pokhra
Bhairawa
Piuthan
Nepalganj
JajarkotDhangadhi
Baitadi
Jumla
Simikot
Okhaldunga
SindhuliDhankuta
Taplejung
30° 30°
28°
26°
28°
26°
80° 84° 88°
88°
25 25 50 75 100 km0
MCT
MCT
MBT
MBT
MT
Gosainkund
Kathmandu
STDS
MT
MCT
Tila
MBT
Geological map of Nepal (after Upreti and Le Fort, 1999).
Deumai HEP
Figure 3: Geological Map of the Nepal Himalaya (after Upreti and Le Fort, 1999)
7
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The proposed Deumai Hydropower Project belongs to the rocks of the Lesser
Himalaya, Eastern Nepal Himalaya.
4. REGIONAL GEOLOGY OF THE PROJECCT AREA
The area around Deumai Khola area lies in the Lesser Himalaya and equivalent to
the rocks of the Kathmandu Group (DMG, 1987) of Eastern Nepal and comprised of
gneiss, schist and schistose gneiss (Figure 4). Structurally, the area lies north of the
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar8
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Main Central Thrust (MCT) and Main Boundary Thrust (MBT). The lithostratigraphy of
the Lesser Himalaya of the eastern Nepal has been given in Figure 3 and Table 2.
Table 2: Lithostratigraphy of Eastern Nepal Himalaya
Group Formation Lithology Age
HIMALAYAN FRONTAL THRUST (HFT)
Siwalik
Upper Conglomerate/sands/muds
NeogeneMiddle Sandstone/mudstone
Lower Mudstone/sandstone
MAIN BOUNDARY THRUST (MBT)
Midland
Lakharpata Limestone, limestone, shale
Upper Pre-Cambrian
-Late Paleozoic
Syangja Quartzite, limestone, shale
Sangram Shale, limestone, quartzite
Galyang Slate, sandstones, calcareous slate
Seti* Gritty phyllite, phyllite, quartzite
Ulleri Augen gneiss
Takure* Shale, schist, conglomerates
Kathmandu
Sarung Khola* Schist, gneiss, schistose gneiss
Upper Pre-Cambrian
-Late Paleozoic
Tawa Khola Schist, quartzite
Maksana Schist
Shiprin Khola* Schist, quartzite, metabasic rocks
Udaipur Schist, marble, quartzite
MAIN CENTRAL THRUST (MCT)
HIGHER HIMALAYA
* Rock exposed in the project area
4.1 Lesser Himalaya
This Himalaya consists of high-grade metamorphic rocks like gneiss, schist as well as
schistose gneiss. The rocks of the area are subdivided into the Kathmandu and
Midland groups. The rocks of the Kathmandu Group are also comparable with the
rocks of the Higher Himalaya.
4.1.1 Midland Group
This group comprises five formations, which consist of phyllite, dolomite and
metasedimentary rocks
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar9
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
4.1.1.1 Seti Formation is comprised of alteration of greenish-grey, crenulated phyllite
and gritty phyllite and grey to greenish grey, fine-grained quartzite. This formation
attains more than 3 km thickness. This unit is exposed in the tunnel alignment.
Around the area only the 500 m rocks are exposed.
4.1.1.2 Naudanda Quartzite is represented by presence of thick-bedded, white,
coarse-grained quartzite with frequently developed rippled marks. Total thickness of
the lithounit is about 400 m.
4.1.1.3 Galyang Formation is characterized by presence of dark grey to black
phyllite and spotted white, fine-grained quartzite. Total thickness of the lithounit is
about 1,000 m.
4.1.1.4 Syangja Formation has grey metasandstone intercalates with dark grey
phyllite and dolomite. Total thickness of the lithounit is about 800 m.
4.1.1.5 Lakharpata Formation is represented by presence of bluish-grey dolomite.
This formation attains 500 to 1,000 m thickness.
4.1.1.6 Takure Formation is represented by presence of black shale and schist with
layers of the conglomerates. Rocks unit is exposed in the tunnel alignment. The area
has attained about 650 m thick bedrock.
4.1.2 Kathmandu Group
This group has high grade metamorphic rocks like schist, gneiss. The group has been
subdivided into more than five lithounits.
4.1.1 Sarung Khola Formation is the youngest rock unit of the Kathmandu Group of
the Lesser Himalaya and consists of grey, coarse-grained, garnetiferous schist
intercalated with grey gneiss and schistose gneiss. The unit attains about 4,000 m
thick but proposed are has more than 2,300 m in thickness.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar10
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
4.1.2 Tawa Khola Formation is represented by presence of grey schist intercalate
with quartzite. It has about 1,200 m thickness.
4.1.3 Maksana Formation is characterized by presence of grey schist. The unit is
about 800 m thick.
4.1.4 Shiprin Khola Formation is comprised of dark grey, garnetiferous schist and
quartzite and meta basic rocks. The area has about 1250 m thick bedrock exposed.
4.1.5 Udaipur Formation is represented by presence of schist, marble and quartzite.
4.2 Siwalik Group
The Siwalik Group is composed of sedimentary rock contains of mudstone,
sandstone and conglomerates. Based on the rock types as well as grain size Siwalik
is divisible into Lower, Middle and Upper Siwaliks.
The Lower Siwalik is comprised of thick mudstone and sandstone. Mudstone is
slightly greater than the sandstone.
The Middle Siwalik is represented by tick beeded sandstone with mudstone and
pebbly sandstone.
The Upper Siwalik is characterised by presence of the conglomerates and lenses of
muds and sands.
The project area is geologically located partly in the rocks of the Sarung Khola
Formation and partly in the rocks of the Shiprin Khola Formation of the Kathmandu
Group and partly in the Seti and Takure formations of the Midland Group. The surge
tanka and powerhouse lies in the rocks of the Lower Siwalik. Most part of the project
area falls in the rocks of the Sarung Khola and Shiprin Khola formations which units
are composed of mainly garnetiferous schist, gneiss and schistose gneiss.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar11
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
4.2 Geolgical Structures
In proposed area, the Main Boundary Thrust (MBT) and the Main Central Thrust
(MCT) represents the large-scale geological structure (Figure 3). The foliation and
fold area the example of small-scale structure.
4.2.1 Main Central Thrust (MCT)
The MCT strikes in E-W direction and separates the high-grade metamorphic rocks of
the Higher Himalaya to the north and low-grade metamorphic rocks of the Lesser
Himalaya to the south. This thrust is located less than 0.1 km (aerial distance) north
of the proposed project area. This thrust separates the rocks of the Seti Formation in
south and Shiprin Khola Formation in north.
4.2.1 Main Boundary Thrust (MBT)
The MBT strikes in E-W direction and separates the low-grade metamorphic rocks of
the Lesser Himalaya to the north and sedimentary rocks of the Siwalik Group to the
south. This thrust is located 0.1 km (aerial distance) south of the proposed project
area. The MBT separates the rocks of the Lower Siwalik in south and the Takure
Formation of the Lesser Himalaya in the north.
Thrust
The thrust developed between the Takure Formation in the south and the Seti
Formation in the north.
4.2.2 Foliation
The trends of the foliation plane of the project area are northeast to southwest dipping
towards north. The dip directions of rocks range from 0300 to 0400 and dipping
towards south (500 to 750).
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar12
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
LS=Lower Siwalik, MS-Middle Siwalik, MCT-Main Central Thrust, MBT-Main Boundary Thrust
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
MCT
MBT
10 km
13
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 3: Geological Map of Ilam Area, Eastern Nepal (after DMG, 1987)
4.2.3 Fold
A regional syncline fold is developed within the rocks of the Sarung Khola Formation
and the fold can be seen south from the project area (Figure 3). The project area is
located northern limb of the syncline fold.
Proposed intake and reservoir area lies about 5 km north from the MCT and MBT
area but the powerhouse and tailrace area is located in between the MCT and MBT
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar14
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
zones. The tunnel alignment crosses the fold. The activities of the MBT and MCT and
fold seem to be nominal.
Figure 4: Geological map of the Project area
5. DETAIL GEOLOGICAL STUDIES OF THE PROJECT AREA
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
Sarung Khola Formation
Shiprin Khola Formation
Seti Formation
Takure Formation Formation
Lower Siwalik
Middle Siwalik
15
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The project area lies partly in the rocks of the Sarung Khola Formation and Shiprin
Khola Formation, Kathmandu Group and partly lies in the rocks of the Midland Group
(e.g., the Seti and Takure formations) and powerhouse component is located
geologically the rocks of the Siwalik Group. The units of the Lesser Hilamaya are
comprised of gneiss, schistose gneiss and quartzite as well as metabasic rocks, slate
and conglomerates whereas the units of the Siwalik is copmposed of sandstone and
mudstone. The dip directions of rocks range from 0300 to 0500 and dips (600 to 750).
The powerhouse and tailrace are located on the alluvial deposits and bedrocks of the
Lower Siwalik Formation. Thickness of the alluvial deposit is expected to be about 5
m, and high possibility to meet the bedrocks at shallow depth at the powerhouse as
well as in the weir axis area. The tunnel alignment passes through rocks of the
Shiprin and Sarung Khola formations of the Kathmandu Group and rocks of the
Takure and Seti formations whereas the weir axis and intake are located in the rocks
of the Sarung Khola Formation. The surge tank, portal outlet and powerhouse as well
as tailrace and penstock alignment lies in the rocks of the Lower Siwalik Formation.
Colluvial deposits are sparsely found in the project area. Geologically, the area of
powerhouse lies south of the MBT. The tunnel alignment crosses the MBT and MCT.
The detailed geological map of the project area is shown in Figure 4 (1:50,000 scale).
5.1 Reservoir Area
The proposed reservoir area is extended from weir axis to Gajurmukhi village.
Geologically, the proposed basin area also lies in the Sarung Khola Formation (Figure
5). Around the proposed area, thick bedded gneiss and schistose gneiss can be seen
on uphill side. Superficially, the proposed project is covered with shallow depth
alluvial deposits on the bedrocks. The bedrocks are exposed along the both banks of
the Deumai Khola around the reservoir area. Individual thickness of gneiss is more
than 2 m. The proposed area at uphill side covered with bushes as well as dry and
wet cultivated area and has gentle topography. The cross-sections of the reservoir
are shown in Figure 6. The sections are taken 500 m interval to show the distribution
of the soil and rocks.
5.2 Intake and Weir Axis Area
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar16
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The proposed weir axis area is located about 3.0 km downstream from the
Gajurmukhidham. The intake area is lies about 100 m upstream from the proposed
area of the weir axis area. Geologically, the area is located in the Sarung Khola
Formation (Figure 7). Around the proposed intake area, thick bedded gneiss and and
schistose are exposed on the both banks of the Deumai Khola. Superficially, thick
alluvial deposits on the bedrocks of the Sarung Khola Formation can be seen along
the riverbed. Individual thickness of gneiss is more than 2 m whereas schistose
gneiss can be seen thickness range of 0.5 to 1 m. Presently, the sparsely forest and
cultivated land at uphill can be found around the proposed structure.
5.3 Diversion Tunnel Alignment
The proposed Diversion tunnel passes though the rocks of the Salung Formation.
This unit has thick gneiss and schistose gneiss (Figure 7). Thickness of the beds
range from 2 to 5 m. The proposed diversion tunnel follows the left bank of the
Deumai Khola.
5.4 Tunnel Alignment Area
The proposed tunnel alignment follows on the left bank of the Deumai Khola, also
belongs to the rocks of the Sarung Khola Formation and rocks of the Shiprin Khola
Formation (Figure 8). Intercalation of gneiss and schist as well as schistose gneiss
and quartzite can be found along proposed tunnel alignment. Last portion of the
tunnel alignment passes through the rocks of the Takure and Seti Formations. Thin
layers (< 1 m thick) of colluvial deposits are found on the hill slope. Individual
thickness of schistose gneiss is more than 3 m. The proposed alignment area is
covered by bush and dry cultivated and the forest can be found around the proposed
structure. The geological cross-section shows the detailed geological condition of the
tunnel alignment (Figure 9).
5.5 Surge Tank and Penstock Alignment Area
The proposed surge tank and penstock alignment area is located geologically in the
rocks of the Takure Formation (Figures 10 and 11). Around the proposed surge tank
area, thick gneiss can be observed. The beds of the schistose gneiss range from 1 to
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar17
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
3 m. The proposed area has covered barren to forest. Geological cross-section is
shown in Figure 11.
5.6 Powerhouse and Tailrace Area
The powerhouse and tailrace lies on the left bank of the Deumai Khola about 1 km
upstream from the confluence between the Deumai Khola and Mai Khola.
Geologically, proposed area belongs to the Shirin Khola Formation (Figure 9). The
rocks of the Shiprin Khola Formation at the powerhouse and tailrace area are
exposed on hillside. The beds of schist and quartzite range from 1 to 3 m and 0.5 to 1
m, respectively. At the proposed location, the alluvial terrace is wide and deposited
by the Deumai Khola. The ground surface of recent alluvial deposits of the Deumai
Khola is almost flat. Uphill side has gentle topography and bedrocks are well
exposed. Geologica cross-section is given in Figure 11.
5.7 Adit Area
The proposed adit area is located in the rocks of the Shiprin Khola Formation. Around
the proposed area thick schist can be observed. The tentative length of the proposed
adit shall be less than 500 m.
5.8 Geomorphology
The Deumai Khola is one of the major tributaries of the Mai Khola. The Deumai Khola
originates from the northern of the Ilam within the Lesser Himalayan zone. The
catchment area of the river is characterised by very rugged and steep to mild
topography, which was resulted by the upliftment of the Himalayan Range. It is mainly
composed of sharp crested ridges, steep to very steep slopes and very little spaces
are left for gently sloping lowlands in the valley. The river valley has steep slope and
rocky area. The slope of the both banks of the river valley range from 50 to 60
degrees and somewhere vertical topography can be seen. The proposed weir axis
area has gentle topography. The tunnel alignment has gentle slope whereas the
penstock alignment has also gentle slope. The Sawa Khola and Fawa Khola are two
major tributaries of the Deumai Khola between the reservoir and intake area.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar18
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
6. ENGINEERING GEOLOGICAL STUDIES OF THE PROJECT AREA
The project layout map also represents engineering geological map of the project
area, has been prepared in 1:1,000 scale for the main project structure sites such as
weir axis site, tunnel alignment (1:4,000 scale), surge tank and penstock alignment,
powerhouse and tailrace area.
The dip directions of rocks range from 0300 to 0400 and dips (300 to 750). The
powerhouse and tailrace located superficially in the alluvial deposits which are
expected to be less than 5 m in thickness. Colluvial deposits are sparsely found in hill
slope in the project area. The exposed rocks on the riverbed as well as on the hill
slope has two sets of the joints, are predominant in the rock mass. They are rough
and stepped. The filling materials in the joints are silty sand. Details of discontinuities
present in the rock mass were measured for the analysis of slope stability of the
project area including stability of the tunnel alignment.
6.1 Quaternary Deposits
The quaternary deposits are found around the project area. The deposits include the
recent river and colluvial deposits.
6.1.1 Recent River Deposits
The alluvial deposits are unconsolidated low-level recent flood plain deposits. These
deposits consist of younger river deposits along the actual riverbed. Low-level
terrace deposits consist of mainly pebble (40-50%) to boulder sized (50-60%), sub
rounded gravels of quartzite and gneiss (90%), and other (10%) filled with loose to
semi-consolidated coarse sand and very fine clay deposits. Thickness of deposits at
powerhouse site is more than 5 m. Remarkable recent river deposits are found along
the river valley of the Deumai Khola.
6.1.2 Colluvial Deposits
Colluvial deposits consist of washed out debris from slope areas and landslide
materials as well as disintegrated and weathered rock fragments. The accumulations
of colluvial deposits are on the hill slope just above the alluvial deposits of the Deumai
Khola.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar19
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
6.2 Description of the Rock Types
6.2.1 Gneiss
The rock of the Sarung Khola Formation consists of gneiss as well as associated with
schistose gneiss. They are massive in thickness, grey colour. Gneiss is exposed at
powerhouse and tailrace area of the project area along the Deumai Khola project.
6.2.2 Schistose gneiss
The schistose gneiss is intercalated with gneiss and exposed concordantly with
gneiss. The proportion of schistose gneiss is less than and that of gneiss. Schist and
quartzite bands are well exposed in the tunnel alignment area. These rocks are
generally found in the Sarung Khola and Shiprin Khola Formation.
6.2.4 Slate
Grey slate of the Seti Formation is well exposed on the tunnel alignment area.
Thickness of the beds are range from 0.2 to 0.5 m.
6.2.5 Sandstone/mudstone
Thick beaded, fine- to medium-grained sandstone and mudstone with calcareous
sandstone can be seen in the Lower Siwalik Group around the surge tank and
powerhouse area.
6.3 Description Structure wise
6.3.1 Reservoir Area
The reservoir area is extended from weir axis to Gajurmukhi village. Geologically, the
area is located in the Sarung Khola Formation. Around the proposed area, thick
bedded gneiss and schistose gneiss is exposed on the both banks of the Deumai
Khola very rare. Most of the area is covered by the alluvial deposits. The exposed
rocks are fresh to moderately weathered in nature. Generally, two sets of the joints
are observed in rock mass. Individual thickness of the bed varies from 1 to 4 m. The
spacing along the foliation and joints are large (more than 2 m). Superficially, thick
alluvial deposits can be seen along the riverbed. The Fawa Khola and the Sawa
Khola are the main tributaries of the Deumai Khola between the location of the weir
axis and the Gajurmukhi village. The area is covered with thick alluvial deposits
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar20
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
because the Fawa Khola has brought thick debris and deposited on the riverbed
along the Deumai Khola riverbed. The alluvial deposits are composed of boulders and
gravels of the recent alluvial deposits are generally subangular to subrounded in
shape exposed up and downstream from the proposed weir axis area. Majority of
clasts along the riverbed is composed of gneiss and schistose gneiss. Thickness of
deposits is expected more than 4 m along the reservoir area. Maximum diameter of
the boulder is more than 2 m. Both banks, the topography are steep slope gentle
slope but comparatively the left bank has steeper slope than the right bank. Detailed
geological condition of the reservoir area is shown in Figures 5 and 6.
6.3.1.1 Findings
1. The proposed reservoir area is covered thick alluvial deposits and
intermittently bedrock at riverbed and the bedrocks can be seen on both
banks. So, the survey of the ERT has been recommended only in the Fawa
Khola to know the structures.
2. The proposed area has no unstable rock mass as well as the landslides and
shows good stability.
3. The irregular pattern of the ERT result shows some big boulders are along the
riverbed.
6.3.1.2 Recommendations
1. To know the condition geotechnical properties of the rock mass exposed along
the reservoir area, two boreholes have been recommended even the bedrocks
area visually can be seen along the river section at proposed area.
2. Seismic refraction survey is required for determining of the overburden as well
geological structures like the lineaments, faults. It helps to make safe from the
reservoir leakage.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar21
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Plate 1: Right bank of the reservoir area Plate 2: Left bank of the reservoir
area
Plate 3: Thick alluvial deposits along the Deumai Plate 4: Bedrock at reservoir area
Khola (reservoir area)
6.3.2 Intake and Weir Axis Area
The weir axis area is located about 3 km downstream from the Gajurmukhi village.
Geologically, the area is located in the Sarung Khola Formation. Around the proposed
weir axis area, thick bedded gneiss schistose gneiss is exposed on the both banks of
the Deumai Khola. The exposed rocks are fresh to moderately weathered in nature.
Generally, two sets of the joints are observed in rock mass. Individual thickness of the
bed varies from 1 to 4 m. The spacing along the foliation and joints are large (more
than 2 m). Superficially, thick alluvial deposits can be seen along the riverbed. The
alluvial deposits are composed of boulders and gravels of the recent alluvial deposits
are generally sub angular to subrounded in shape exposed up and downstream from
the proposed weir axis area. Majority of clasts along the riverbed is composed of
gneiss and schistose gneiss. Thickness of deposits is expected less than 4 m at
headworks site. Both banks, the topography is steep slope but comparatively the
slope is more flat than the right bank. Maximum diameter of the boulder is more than
2 m.
6.3.2.1 Findings
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar22
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
1. The proposed intake and weir axis area is covered bedrock at riverbed and
the bedrocks can be seen on both banks. So, the survey of the ERT has
been recommended.
2. The proposed area shows good stability in rock.
6.3.2.2 Recommendations
1. To know the condition geotechnical properties of the rock mass exposed at
the weir axis area of the subsurface bedrock, 9 boreholes have been
recommended even the bedrocks area visually can be seen along the river
section at proposed area.
Plate 5: Right bank of the weir axis area
Plate 6: Bedrock at right bank of the Deumai
at weir axis area
Plate 7: Bedrocks of left bank of Deumai
Plate 8: Downstream from weir axis area
Khola at weir axis area
2. Two holes are
recommended in the intake
area to know the subsurface
geological condition
6.3.3 Tunnel Alignment Area
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar23
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The proposed tunnel alignment follows on the left bank of the Deumai Khola, also lies
initially in the Sarung Khola Formation. Schist of the Sarung Khola Formation and
schistose gneiss, quartzite intercalation can be found. Then, passes through the
gneiss of the Shiprin Khola Formation. Last portion of the alignment also passes
through the rocks of the Seti and Takure formations. Thin to thick layers of colluvial
deposits area found on the bedrocks. Thickness of the beds range from 2 to 4 m
generally found in the gneiss and schist whereas the thickness of schistose gneiss
varies from 0.5 to 1 m. Slates and schist of the of the Seti and Takure formations can
be seen at last section of the tunnel. The orientation of the foliation plane of the
exposed rock is directed toward northeast, which is nearly perpendicular to the tunnel
alignment. Other characters of the exposed rock along the alignment are thick-
bedded, fresh in nature, containing one to two joint sets. The spacing of the
discontinuities is large. Land use pattern at proposed area is barren to forest. The
topography at proposed area has moderate steep slope.
6.3.3.1 Findings
1. The whole alignment of the tunnel alignment lies in the bedrocks except some
locations but at depth there shall be found the bedrocks.
2. The orientation of the foliation plane of the rock is favorable for the tunnel
alignment direction, the angle between foliation plane and direction of tunnel
alignment is more than 60 degrees.
3. The alignment passes through the rocks of gneiss and schistose gneiss of the
Sarung Khola and Shiprin Khola formations. The exposed rock mass is slightly
in weathering condition.
4. The slope stability is good due to characters of the rock mass with long
spacing joints.
5. Last portion tunnel alignment lies in the rocks of the Takure Foramtion
6. The tunnel alignment crosses the MBT and MCT at the last portion.
6.3.3.2 Recommendations
1. Almost all area covered with bedrocks, It should be better to do the 2D-
Electrical Resistivity Survey in some crossing area to know the groundwater
potentiality.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar24
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
2. First option-It is better to determine the seismic activities of the thrust in the
DPR stage if the tunnel alignment passes through the MCT and MBT.
3. Second option- It is better to align the pipe on the surface to avoid the area of
the thrust.
4. Between These options which one is better can be cleared during the DPR
phase of the project.
6.3.4 Surge Tank and Penstock Alignment Area
The proposed surge tank and penstock alignment area belongs to geologically in the
rocks of the Lower Siwalik Formation. Around the proposed surge tank area, thick
bedded sandstone and mudstone can be observed. Thickness of the beds varies
from 1 to 2 m generally found in the sandstone and mudstone. The rocks around the
proposed structure area are slightly weathered. Two sets of the joints are visible in
the rocks exposed. The spacing of the rock mass is moderate. The land use pattern is
forest.
6.3.4.1 Findings
1. The bedrocks are clearly observed at the proposed area.
2. The proposed penstock alignment shall follow gentle slope.
3. Moderate space joints are seen in slightly weathered rock mass.
4. No any colluvial deposits on the hill slope along the proposed penstock
alignment.
6.3.4.2 Recommendations
1. At least two line of the 2D-Electrical Resistivity survey around the surge tank
area has recommended.
2. Based on the ERT report at least a core hole can be proposed.
3. At least two pitholes are dug up along the penstock alignment to determine the
bearing capacity.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar25
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
6.3.5 Powerhouse and Tailrace Area
The proposed powerhouse and tailrace lies on the left bank of the Deumai Khola.
Geologically, proposed area belongs to the Lower Siwalik Formation. The rocks of the
Lower Siwalik Formation at the powerhouse and tailrace area are exposed on hillside.
But, superficially, thick river deposits have covered the proposed area. At the
proposed location, the alluvial terrace is wide. The ground surface of recent alluvial
deposit of the Mai Khola is almost flat. It is assumed that thickness of alluvial deposits
more than 5 m thick at the proposed powerhouse. The tailrace also lies on thick
alluvial deposits and is more than 10 m from the riverbed. The riverbed of the Deumai
Khola is composed of boulder (> 70%) and fine materials (< 30%). More than 70% of
the boulders of gneiss as well as sandstone are found. The material found at the
proposed sites is fine- to medium-grained, grey sand with rounded to sub rounded
gravels. Thick-bedded, fresh to slightly weathered, greenish-grey to grey sandstones
are exposed uphill side of the proposed powerhouse area. Two sets of the joints are
prominent in the sandstone. The persistency of the rock mass of the discontinuities is
moderate.
6.3.5.1 Findings
1. Thickness of alluvial deposits at proposed powerhouse and tailrace area is
more than 5 m can be observed along the left bank of the Deumai Khola.
These thick deposits need to excavate for the foundation of the
powerhouse.
2. Uphill side topography has gentle slope.
6.3.5.2 Recommendations
1. At least two lines of the ERT are required to know the basement rocks
depth.
2. At least two core drilling for determination of the geochemical properties of
the rocks at powerhouse is proposed.
3. The river deposits can be used as construction materials.
6.3.6 Adit Area
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar26
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The adit area is located on the left bank of the Deumai Khola and shall be about 200
m in length. The area is geologically located in the rocks of the Sarung Khola
Formation and almost on the central portion of the tunnel alignment. The topography
of the area is gentle and the exposed rocks are composed of thick schist and
quartzite.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar27
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
7. GEOTECHNICAL ASPECTS OF PROJECT AREA
Rock mass classification was carried out based on the NGI “Q” and CSIR “RMR”
system. Based on the computed “Q” and “RMR” values the rock mass could be
classified into very good to excellent, good, good to fair, poor, very poor rock,
extremely poor and exceptionally poor rock. Classified rock masses are given in
Table 3. The calculated values can be used for rock support in the headrace tunnel
alignment as well as the underground structures.
Table 3: Rock Mass Classification
(Bieniawaski, 1989); (Barton, 1995)
Descriptions Range of Q-values Range of RMR-values
Rock Class Quality descriptions Minimum Maximum Minimum Maximum
Class 1 Very good to excellent 100 1000 85 100
Class 2 Good 10 100 65 85
Class 3 Fair to good 4 10 56 65
Class 4 Poor 1 4 44 56
Class 5 Very poor 0.1 1 35 44
Class 6 Extremely poor 0.01 0.1 20 35
Class 7 Exceptionally poor 0.001 0.01 5 20
7.1 Reservoir Area
The hill slope along the proposed section starts abruptly with an average slope of 30
degrees and increasing of up to 70 degrees in the high uphill slope at right bank and
also in the left bank the natural slope is upto 40 degrees. The bedrock exposed at the
site has foliation attitude (Dip Direction/Dip Amount) of 025/36. One major (330/66)
and other three minor joint systems (162/29 and 256/75) are observed at the area
in Table 4.
7.1.1 Rock Classification
Geotechnical classification for jointed rock mass of the reservoir using CSIR
classification was carried out based on the detailed surface discontinuity (Table 5).
Most of the Rock Mass Rating (RMR) of the headwork area falls in the range 60 to 70
and it indicates that the rock mass of headwork site is categorized as a Class II-III
type, which is defined as the fair to good rock (Table 5).
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar28
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Table 4: Attitudes of Rock Mass (Dip Direction/Dip Amount) of Project Area
Location Natural Hill Slope Foliation Joint (J1) Joint (J2) Joint (J3)
Reservoir Area
Left bank 092/81 025/36 330/66 256/75 162/29
Right bank 247/78 048/67 332/65 247/78 148/67
Weir Axis Area
Left bank 135/30 056/78 321/78 235/67
Right bank 280/69 077/77 342/65 224/52 150/38
Diversion Tunnel Area
Left Bank 280/69 070/70 322/59 245/80 165/32
Tunnel Alignment
CH. 0+000 - CH. 0+725 066/60 318/72 144/35 248/78
CH. 0+725 - CH. 2+250 084/60 330/68 136/36 158/60
CH. 2+250 - CH. 4+125 080/50 310/64 120/22 141/78
CH. 4+125 - CH. 4+850 045/62 314/72 140/40 224/74
CH. 4+850 - CH. 5+178 054/62 330/60 235/60 110/50
Surge Tank/Penstock Alignment 075/34 305/80 220/80 115/75
Powerhouse Area 075/30 300/80 202/60 072/48
Source: Field data
Table 5: Geotechnical Parameters of the Rock Mass
7.1.2 Weathering and Strength
Rock mass in the reservoir area is fresh to slightly weathered (Table 5). Generally,
the rocks along riverbank are fresh rock and slightly weathered at higher hill slope.
Gneiss is strong rock. The compressive strength of the augen gneiss range from 200
to 250 MPa. Because of presence of the high percentage of the feldspar, the
weathering shall be quite quick when it reacts with water.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar29
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
7.1.3 Slope Stability
Slope stability assessment analysis of the both banks hill slope was carried out on the
basis of aerial photos interpretation and geological observations. An analysis of
foliations to determine the stability of the rock mass due to the presence and
orientations of the foliations in the rock mass at the reservoir was done using Lower
Hemisphere Projection of the foliation planes in Schmidt’s equal area net. The
wedges formed by the foliation planes and joints were then analyzed with respect to
the hill slope surface. The dipping of the foliation plane is favorable to the natural hill
slope and the relation between them is opposite to the hill slope so less possibility to
occur failure. The wedge formed by the intersection of the joints (J1, J2 and J3) may
occur stable because these wedges are developed opposite to the natural hill slope.
The slope stability condition of the rock mass is presented in Figures 12 and 13.
Similarly, the stability condition is more or less similar to the left bank (Figures 14 and
15).
Figure 12: Contour Density Map of the Rock Mass on Left Bank of the Reservoir Area
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar30
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 13: Stereographic Projection of the Rock Mass on Left Bank of the Reservoir Area
Figure 14: Stereographic Projection of the Rock Mass on Right Bank of Reservoir Area
Figure 15: Contour Density Map of the Rock Mass on Right Bank of the Reservoir Area
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar31
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
7.2 Intake and Weir Axis Area
The intake and weir axis area along the Deumai is HEP is located on the left bank
and in the bedrocks. The hill slope starts abruptly with an average slope of 52
degrees and increasing of up to 80 degrees in the high uphill slope at left bank. The
natural slope is comparatively gentler than right bank. The bedrock exposed at the
site has foliation attitude (Dip Direction/Dip Amount) of 056/78. One major (321/78)
and other three minor joint systems (235/69) are observed at the area in Table 4.
Table 6: Rock Mass Rating of the Project Area
7.2.1 Rock Classification
Geotechnical classification for jointed rock mass of the headwork using CSIR
classification was carried out based on the detailed surface discontinuity. Most of the
Rock Mass Rating (RMR) of the headwork area falls in the range 59 to 64 and it
indicates that the rock mass of headwork site is categorized as a Class III type, which
is defined as the fair to good rock (Table 6).
7.2.2 Weathering and Strength
Rock mass in the headwork area is fresh to slightly weathered (Table 5). Generally,
the rocks along riverbank are fresh rock and slightly weathered at higher hill slope.
Gneiss is fairly strong rock.
7.2.3 Slope Stability
Slope stability assessment analysis of the left bank hill slope was carried out on the
basis of aerial photos interpretation and geological observations. An analysis of
foliations to determine the stability of the rock mass due to the presence and
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar32
1
1
2
2
3
3
4
4
N
S
EW
Orientations
ID Dip / Direction
1 30 / 135
2 69 / 235
3 78 / 321
4 78 / 056
Equal AngleLower Hemisphere
54 Poles54 Entries
1
1
2
2
3
3
4
4
5
5
N
S
EW
Orientations
ID Dip / Direction
1 38 / 150
2 77 / 077
3 69 / 280
4 65 / 342
5 52 / 224
Equal AngleLower Hemisphere
11 Poles11 Entries
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
orientations of the foliations in the rock mass at the intake site was done using Lower
Hemisphere Projection of the foliation planes in Schmidt’s equal area net. The
wedges formed by the planes were then analyzed with respect to the hill slope
surface. The dipping of the foliation plane is favorable to the natural hill slope and the
relation between them is opposite to oblique so very less possibility to occur failure.
The wedge formed by the intersection of the joints (J1 and J2) may occur failure. The
slope stability condition of rock mass is presented in Figures 16 and 17.
Figure 16: Stereographic Projection of the Rock Mass on Left Bank of the Proposed Weir Axis
Area
Figure 17: Stereographic Projection of the Rock Mass on Right Bank of the Proposed Weir Axis
Area
7.3 Diversion Tunnel Alignment Area
The exposed rock beds are competent and are favourably dipping against slope face
direction. The attitude of the rock bands are more or less same as the geotechnical
characters of the rock exposed at proposed intake and weir axis area i.e., 070/70,
322/59, 245/80, 165/32 foliation, joint 1, joint 2 and joint 3, repetitively. The exposed
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar33
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
rock is fresh to slightly weathered with average joint spacing 2 to 3 m. The joint
surfaces are rough and have some silty sand fillings in the exposed areas.
7.3.1 Rock Classification
Geotechnical classification for rock mass tunnel alignment using CSIR classification
was carried out based on the detailed surface discontinuity measurements on
exposed rock outcrops in the tunnel alignment. The results of the geotechnical
classification are presented in Table 5.
NGI Tunneling Index ‘Q’ was also carried out based on the detailed surface
discontinuity measurements on exposed rock outcrops in the tunnel alignment in
some location. The results of the geotechnical classification at the tunnel alignments
are presented Table 6 which is more or less same as the initial section of the tunnel
alignment from the intake area.
7.3.2 Weathering and Strength
Rock mass in the headwork area is fresh to slightly weathered (Table 5). Generally,
the rocks along riverbank are fresh rock and slightly weathered at higher hill slope.
Gneiss is fairly strong rock.
7.3.3 Slope Stability
Slope stability assessment analysis of the left bank hill slope was carried out on the
basis of aerial photos interpretation and geological observations. An analysis of
foliations to determine the stability of the rock mass due to the presence and
orientations of the foliations in the rock mass at the intake site was done using Lower
Hemisphere Projection of the foliation planes in Schmidt’s equal area net (Figures 18,
19). The internal friction angle has been adopted as 30 degrees for the stability
calculation. The dipping of the foliation plane is favorable to the natural hill slope and
the relation between them is opposite to oblique so very less possibility to occur
failure. The wedge formed by the intersection of the joints (J1 and J2) may occur
failure.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar34
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 18: Contour Density Map of the Rock Mass along the Diversion Tunnel Alignment Area
Figure 19: Stereographic Projection of the Rock Mass along Diversion Tunnel Alignment Area
7. 4 Tunnel Alignment Area
The conveyance of water is proposed to be done by tunnel. This has been proposed
considering the following aspects:
a) Morphological conditions of the alignment route,
b) Surfacial slope stability conditions,
c) Rock mass property observations and
d) Economical aspects
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar35
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The slope along the proposed tunnel is generally favorable and stable. The
foundation of the tunnel is on the bedrock. This most of the alignment is on gneiss
and schistose gneiss bedrock of the Sarung, Shiprin Khola formations and is over
moderate steep-to-steep hill slope. The hill slope starts abruptly with an average
slope of 45 degrees. The attitude of foliation of the rock mass in the area is shown in
chaingae wise in the sections (Table 10). The exposed has average joint spacing of 1
to more than 4 m (Table 5). The joint surfaces are rough and steeped; and have
some silty clay fillings in the exposed areas. Measured discontinuities are given in
Table 4.
7.4.1 Rock Classification
Geotechnical Classification for rock mass tunnel alignment using CSIR classification
was carried out based on the detailed surface discontinuity measurements on
exposed rock outcrops in the tunnel alignment. The results of the rock mass
classification are presented in Table 6. Most of the alignment is covered with the good
to good to fair rock are seen at the initial and last part of the tunnel but poor and very
poor rock mass can be expected at the last section of the tunnel alignment because
of the presence of some geological structures as well as the lithological variation.
NGI Tunneling Index ‘Q’ was also carried out based on the detailed surface
discontinuity measurements on exposed rock outcrops in the tunnel alignment in
some location. The results of the geotechnical classification at the tunnel alignments
are presented Table 7. Most of the alignment is covered by the good rock and some
of the locations are good rock are seen at the initial and middle part of the tunnel but
poor and very poor rock mass can be expected at the last section of the tunnel
alignment because of the presence of some geological structures as well as the
lithology.
7.4.2 Weathering and Strength
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar36
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Rock mass in most of the tunnel alignment is fresh to slightly weathered, with some
moderately weathered rock exposed. The rock at initial portion is competent and hard
rock but last portion of the rocks are weak in nature due to presence of low grade
metamorphic rock and sedimentary rocks.
Table 12: Rock Mass Classification Using NGI Tunneling Index Q Classification
Note: Q = RQD/Jn x Jr/Ja x Jw/SRF
7.4.3 Slope Stability
An analysis of foliations to determine the stability of the rock mass due to the
presence and orientations of the foliations in the rock mass along the tunnel was
done using Lower Hemisphere Projection of the foliation planes in Schmidt’s equal
area net. The internal friction angle has been adopted as 30 for the stability
calculation. Analysis of slope stability has been given in Figures 20-27.
Between the CH. 0+000 and 2+250
More or less stable in condition but wedge formed by intersection of F and J2 is some
critical. But wedge formed by intersection of the F and J1 is more or less stable
because of low angle wedge. But the size of the wedges is blocky. Three sets of the
joints are seen and very tightly joint can be found. Mainly the rock of the gneiss and
schist can be found with large persistency.
Between the CH. 2+250 and 4+125
Stability is good but the intersection of the F and J1, J2 as well as J3 are danger and
may occur failure. The section is partly covered with gneiss and schist. Large
persistency is generally found in the rock mass.
Between 4+125 and 4+850
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar37
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
More or less the stability condition is same as the chainage between 4+125 and
4+850. The section is covered with slate and schist. moderate persistency is found in
the rock mass in general.
Between 4+850 and 5+178
Stable in condition very less possibility to occur failure but the presence of the highly
jointed rocks may create the problem. The area is influenced by the thrust and
sedimentary as well as soft rocks.
Figure 20: Contour Density along the Tunnel Alignment Area CH. 0+000-0+725
Figure 21: Stereographic Projection of the Rock Mass along the Tunnel Alignment Area CH.
0+000-0+725
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar38
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 22: Contour density Map Mass along the Tunnel Alignment Area CH. 0+725-2+250
Figure 23: Stereographic Projection of the Rock Mass along the Tunnel Alignment Area CH.
0+725-2+250
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar39
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 24: Contour Density Map along the Tunnel Alignment Area CH. 2+250-4+125
Figure 25: Stereographic Projection of the Rock Mass along the Tunnel Alignment Area CH.
2+250-4+125
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar40
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 26: Contour Density along the Tunnel Alignment Area CH. 4+125-5+178
Figure 27: Stereographic Projection of the Rock Mass along the Tunnel Alignment Area CH. -
4+125-5+178
Assessment of Initial Support Requirement For Tunnel
Empirical Method
Empirical assessment of rock enforcement requirement for the tunnel has been
empirically assessed based on the rock mass classification and stability analysis
carried out based on assumed/extrapolated data. Once the excavation begins, the
parameters used to determine the rock mass quality must be re-evaluated
continuously.
These parameters include:
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar41
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
1. Number of joints per unit volume and their orientations
2. Joint conditions such as tightness, loose openings and in-fill materials
3. Continuity of joints
4. Joint surface conditions such as roughness, degree of weathering and
coatings.
5. Joint water conditions
6. Presence and orientation of shear zones, clay seams or loose open joints
crossing the tunnel excavation or the presence of squeezing of swelling rock
7. The rock strength with ratio to the major principal rock stress expected at the
tunnel periphery.
Support requirement based on the Q value: A Chart of equivalent dimension De,
plotted against the Q value, is used to define a number of support categories (Barton
et al., 1974).
De = Excavation span, diameter of height (m) / ESR (excavation support ratio)
For headrace tunnels of Hydropower, ESR is taken to be 1.6, hence for an excavation
of span of maximum 4m, De = 1.63.
About 93 % of the whole tunnel length is expected to be in fair to good rock with Q= 5
to 11 and only about 07 % in poor to very poor rocks with some highly fracture zones.
From the Chart, the 93% of the tunnel section falls in Category 1 showing
unsupported span. However, such conditions will not be anticipated in real practice.
For tunnel section with poor rock Q < 4, De = nearly 2.0, support category 4 is
required by the chart.
The Length of Rock bolt can be estimated from excavation width B and Excavation
Support Ratio ESR, as follows
L = (2 + 0.15B)/ESR
Substituting, B = 4.0 m and ESR = 1.6, the minimum length of rock bolt comes to be
1.625 m (~ 2.0 m).
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar42
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
7.5 Estimated Rock Support
Rock support in the tunnel and underground cavern is provided to improve the stability
and to safeguard the opening with respect to safety of the working crew. The guiding
principle of rock support design is that it is capable to response the actual ground
conditions that is encountered in the tunnel and the safety requirement at the tunnel face
is met. This requires provision of flexible rock support methods that can be quickly
adjusted to meet continuously changing heterogeneous rock mass (Table 8). The best
way to achieve such flexibility is the use of rock bolts, steel fiber shotcrete, pre-injection
grouting, and the use of steel ribs.
Table 8: Designed Tunnel Rock Support Class and Respective Rock Support
Rock Mass Quality
Description
Rock
Support
(RS) Class
Assigned Tunnel Rock Support
Good Rock RS II Fully grouted 25 mm diameter and 2 m long rock bolts spaced at
4.5 m circumferentially and 4.5 mnlongitudinally plus 100 mm thick
shotcrete with single layer of wire mesh reinforcement (4 mm dia
welded in mesh size 100 mm x 100 mm) or steel fibercrete.
Fair to good rock mass RS III 25 mm diameter 2 m long systematic grouted rock bolts at a spacing
of 1.5 m x 1.5 m and 10cm thick steel fiber shot crete in all tunnels
and at settling basin cavern 25 mm diameter 4 m long systematic
grouted rock bolts at a spacing of 1.5 m x 1.5 m 15 cm thick steel
fiber shotcrete at the settling basin cavern.
Poor rock mass RS IV 25 mm diameter 2 m long systematic grouted rock bolts at a spacing
of 1.3 m x 1.5 m and 15 cm thick steel fiber shot crete.
Very poor rock mass RS V 25 mm diameter 2 m long systematic grouted rock bolts at a spacing
of 1.3 m x 1.3 m and 15 cm thick steel fiber shot crete.
Extremely poor rock
mass
RS VI 25mm diameter 2 m long systematic grouted rock bolts at a spacing
of 1.2 m x 1.2 m and 20 cm thick steel fiber shotcrete. Steel ribs at a
spacing of 1 meter to control plastic deformation. Advance pre-
injection grouting is provisioned to control water inflow into the
tunnel.
Exceptionally poor
rock mass
RS VII 25 mm diameter 2 m long systematic grouted rock bolts at a spacing
of 1.1 m x 1.1 m and 20 cm thick steel fiber shot crete. Steel ribs at a
spacing of 1 meter to control plastic deformation.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar43
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The headrace tunnel will be in hydrostatic condition during its operation. Since the
designed rock support in the table is not water tight, the concept of pre-injection
grouting should be applied at the required length of headrace tunnel to control
possible water leakage during operation. The BOQ and respective drawings
illustrates the given rock support for all underground works. In Table 9 the rock
support assigned for headrace tunnel has been presented since the ground condition
changes at different tunnel segment.
Table 9: Assigned Rock Support in Respect with Rock Mass and Rock Support Class
Location
Rock
Mass
Class
Rock
Support
Class
Assigned rock support measures
Ch. 0+000-Ch. 0+725 Class III RS III
25 mm diameter 2 m long systematic grouted rock bolts at a
spacing of 1.5 m x 1.5 m and 10cm thick steel fiber shot crete
in all tunnels and at settling basin cavern 25 mm diameter 4 m
long systematic grouted rock bolts at a spacing of 1.5 m x 1.5
m 15 cm thick steel fiber shotcrete at the settling basin cavern.
Ch. 0+725-Ch. 2+250 Class III RS III
25 mm diameter 2 m long systematic grouted rock bolts at a
spacing of 1.5 m x 1.5 m and 10cm thick steel fiber shot crete
in all tunnels and at settling basin cavern 25 mm diameter 4 m
long systematic grouted rock bolts at a spacing of 1.5 m x 1.5
m 15 cm thick steel fiber shotcrete at the settling basin cavern.
Ch. 2+250-Ch. 4+125 Class II RS II
Fully grouted 25 mm diameter and 2 m long rock bolts spaced
at 4.5 m circumferentially and 4.5 mnlongitudinally plus 100
mm thick shotcrete with single layer of wire mesh reinforcement
(4 mm dia welded in mesh size 100 mm x 100 mm) or steel
fibercrete.
Ch. 4+125-Ch. 4+850 Class III RS III
25 mm diameter 2 m long systematic grouted rock bolts at a
spacing of 1.5 m x 1.5 m and 10cm thick steel fiber shot crete
in all tunnels and at settling basin cavern 25 mm diameter 4 m
long systematic grouted rock bolts at a spacing of 1.5 m x 1.5
m 15 cm thick steel fiber shotcrete at the settling basin cavern.
Ch. 4+850-Ch. 5+178 Class IV RS IV
25 mm diameter 2 m long systematic grouted rock bolts at a
spacing of 1.3 m x 1.5 m and 15 cm thick steel fiber shot crete.
Steel ribs at a spacing of 1 meter to control plastic deformation.
Advance pre-injection grouting is provisioned to control water
inflow into the tunnel due to the very weak and fractured zone.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar44
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The rock mass (Table 9) from Ch. 0+000 to Ch. 2+250 requires the III support type
whereas of tunnel from Ch. 2+250 to Ch. 4+125 needs of II support types. Similarly,
from Ch. 4 + 125 to Ch. 4+850 there is required of support III. From Ch. 4+850 to Ch.
5+178 (0.50 km) requires the IV support system. Details about the tunnel and
supports are given in Table 10.
Out of 5178 m of the length, the alignment covers 4,850 m of the tunnel length which
is equal to 93.86% of the total length of the tunnel alignment needs of the support II
and III and remain of 6.14% (e.g., 328 m of the tunnel length) needs of support IV to
V.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar45
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar46
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Table 10: Assigned Rock Support in Respect with Rock Mass and Rock Support Class
Description/Structures Tunnel AlignmentChainage CH. 0+000- CH. 0+725 CH. 0+725-CH. 2+250 CH. 2+250-CH. 4+125 CH. 4+125-CH.4+850 CH. 4+850-CH.5+178
Rock typeGneiss/Schistose
gneissGneiss/Schistose
gneiss Gneiss/Schist Slate/Schist Schist/sandstone/mudstoneWeathering Fresh-slightly Fresh-slightly Fresh-slightly Fresh-slightly Fresh-moderatelyStrike and Dip 242-062/38S 234-054/35S 226-046/36S 030-210/22S 050-230/40SDip Direction/Dip Amount 066/60 084/60 080/50 045/62 054/62
RMR Value 61 58 65 62 40-37Rock Classification III III II III IVDefinition Good-Fair Good-Fair Good Good-Fair Poor to very poor rockQ values 6.5 7.46 10.88 9.64 3.29Rock Classification III III II III IVDefinition Good-Fair Good-Fair Good Good-Fair Poor to very poor rockSupport system III III II III IV-V
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
7. 6 Surge Tank/Penstock Alignment Area
The hill slope of surge tank face is moderate. The exposed rock beds are less
competent and are favourably dipping against slope face direction. The attitude of the
bedrock is 075/34 (dip direction/dip). One major (345/70) and two other minor joint
sets (228/70 and 172/79) are observed in the exposed area. The surge tank as well
as the penstock alignment passes though sandstone of the Lower Siwalik Formation.
The joint surfaces are slightly to altered with average joint spacing of 1 to 2 m. The
joint surfaces are rough and have silty sand fillings in the exposed areas. The
measured discontinuities are given in Table 4.
7.6.1 Rock Classification
Geotechnical classification for rock mass of the alignment using CSIR classification
was carried out based on the detailed surface discontinuity measurements on
exposed rock outcrops in the alignment. The results of the geotechnical classification
are presented in Tables 5 and 6.
7.6.2 Weathering and Strength
Rock mass is fresh to slightly weathered, and has less competent rock.
7.6.3 Slope Stability
The stability of surge tank/penstock has been analyzed on the basis of geotechnical
and geological observations on the surface of the hill slopes. Analysis of all the
observed bedding plane attitudes and their conditions at different locations has been
presented in Figures 28 and 29. In general, the stability condition is good and some
of the wedges formed by joints may unstable.
Figure 28: Stereographic Projection of Rock Mass at Surge Tank Area
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 29: Stereographic Projection of Rock Mass at Surge Tank Area
7.7 Powerhouse and Tailrace Area
The powerhouse and tailrace area is situated in the rocks of the Lower Siwalik, is
situated at the low land depressed valley. The powerhouse area is composed of
superficially alluvial deposits and the bedrock is encountered at uphill side.
7.7.1 Rock Classification
Geotechnical classification for rock mass powerhouse using CSIR classification was
carried out based on the detailed surface discontinuity measurements on exposed
rock outcrops around the powerhouse area. The results of the geotechnical
classification are presented in Table 4 was also carried out based on the detailed
surface discontinuity measurements on exposed rock outcrops. Generally poor rocks
are exposed around the powerhouse area.
7.7.2 Slope Stability
An analysis of foliations to determine the stability of the rock mass due to the
presence and orientations of the foliations in the rock mass is done using Lower
Hemisphere Projection of the bedding planes in Schmidt’s equal area net. Analysis of
all the observed foliation plane attitudes and their conditions at different locations has
been presented in Figures 30 and 31.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar82
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 30: Contour Density map of the Powerhouse area
Figure 31: Stereographic Projection of Rock Mass at Powerhouse Area
8. SEISMICITY
This chapter deals with the preliminary investigation of maximum credible earthquake
and peak ground acceleration for an assessment of the proposed Deumai Khola
Hydroelectric Project (Figures 36, 37 and 38). The analysis is basically made by
deterministic evaluation of earthquake sources in the vicinity with the state of art
consideration of attenuation for the Himalayan terrain. It should be acknowledged that
the problems of seismo-tectonic events of Himalaya are not fully understood and the
knowledge is increasing with more and more accumulation of research results and
data analysis. The study has considered the latest results of seismo-tectonic study of
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar83
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
the Himalaya and the vicinity. For comparison purpose, both deterministic and
probabilistic assessments of seismic hazards have been considered.
8.1 Seismo-tectonic Model
The Himalaya seismicity, in general, owes its origin to the continued northward
movement of Indian plate after the continental collision between Indian plate and
Eurasian plate. The magnitude, recurrence and the mechanism of continental
collision depend upon the geometry and plate velocity of Indian plate in relation to
southern Tibet (Eurasian Plate). Recent results suggest that the convergence rate is
about 20 mm/year and the Indian plate is sub-horizontal below the Sub-Himalaya and
the Lesser Himalaya.
The result of micro seismic investigation, geodetic monitoring and morphotectonic
study of the Central Nepal has depicted that the more frequent medium sized
earthquakes of 6 to 7 magnitude are confined either to flat decollment beneath the
Lesser Himalayas or the upper part of the middle crustal ramp. The ramp is occurring
at about 15 km depth below the foothills of the Higher Himalaya in the south of MCT
surface exposures. Big events of magnitude greater than eight are nucleated near the
ramp flat transition and rupture the whole ramp-flat system up to the blind thrust
(MBT) of the Sub-Himalaya (Pandey et. al. 1995).
This general model worked out for the Western Nepal can be applied to other parts of
the Himalaya with the evaluation of further subsequent ramping towards more south
in the Lesser Himalaya and the associated seismicity. This structural variation along
Himalayan arc is responsible for the segmentation of potential ruptures along the arc
i.e. along the longitudinal direction. For deterministic assessment of seismogenic
sources, the local structural environment magnifying the general model near the
project site is considered.
8.2 Deterministic Assessment
Considering the above interpretation, the deterministic design earthquake can be
taken as a sub- horizontal thrust of rupture extent of about 30 km occurring at a depth
of 15 km within a plan distance of a few km, (e.g., 5 km) from the site. The width of
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar84
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
the rupture is proposed to be about 25 km. A magnitude of 7.0 is estimated from
rupture area of 750 km2 with Ms = 4.15 + Log A (Wyss, 1979). Actually there has
been an earthquake of M = 7 at a distance of about 15 km from the site in 27 May
1936. However the epicentre may be closed to the site considering the uncertainty of
location. It should be noted that a similar environment exists in the Uttarkashi area of
Garhawal Himalaya where an event of magnitude Mb 6.5, Ms 7.1 occurred in 19
October 1991. Its moment magnitude was Mw 6.8 with moment equal to (0.8- 1.8)* 10
E 19 N.m. and the mechanism was a low angle thrust. The rupture length is reported
to be about 25 km with maximum slip of 2.5 m. The deterministic assessment of
maximum credible earthquake can be considered to be the big earthquake rupturing
the entire detachment of the Indian plate as discussed in the model and therefore
considered to be of magnitude 8.3 -8.6 like other great earthquakes of the Himalaya.
8.3 Horizontal Acceleration
Deterministic Approach
Evaluation of peak ground acceleration is carried out by applying the mostly used
formula of McGuire (1968), Katayama (1975), Oliveira (1984) and Kawashima (1984)
for the above earthquakes concluded deterministically from seismo-tectonic models.
Log A = 3.090 + 0.347 M -2 lag (R + 25) (C. Oliveira)
Log A = 2.308 + 0.411 M- 1.637lag-(R + 30) (T. Katayama)
Log A = 2.674 +0.278 M – 1.30 1 log (R + 25) (R. K. McGuire)
A = 1.006* 10E (0.216*M)*(R+30) E-l.218 (Kawashima)
R = hypo central distance in kilometre
The recorded peak acceleration data in Uttarkashi earthquake of Ms = 7.1 of 1991 is
0.21 9 at a distance of about 28 km. from the epicentre. Katayama's relation gives an
estimate of 0.20g; Kawashima's estimate is 0.23g while McGuire's relation estimates
as 0.24g. Oliveira's relation underestimates the acceleration.
8.4 Probabilistic Approach
Preliminary seismic hazard assessment of the country using Gumbel's third
asymptotic extremes with the instrumental seismicity database of ISC is carried out
by Bajracharya (1994) for different return periods 50, l00, 200 and 300 years,
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar85
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Attenuation model with mean value of McGuire and Oliveira ". (see above) is used for
horizontal acceleration.
Return period (years) Peak horizontal acceleration (g)
50 0.10
100 0.15
200 0.20
300 0.25
8.5 Recurrence Period
The best estimate of b value for the project area is 0.84 as shown by the analysis of
micro seismic events. The 1991 event of Uttarkashi is considered by many
investigators to be the repetition of 1833 event, which gives a basis for a recurrence
of 158 years for 7.1 magnitude event in similar geological setting. Moreover the
observed slip of about 2.5m in Uttarkashi earthquake also is consistent with 178
years of recurrence considering 70% contribution of 20 mm/year plate convergence
rate to seismic strain.
8.6 Historical Seismic Activity
The Nepal Himalaya has experienced several large earthquakes over the past
centuries. The earthquakes of larger magnitudes that have occurred in Nepal
Himalaya are summarized below in Table 11.
Table 11: Larger Magnitude of Earthquake Occurred in Nepal Himalaya
S.N Location of Earthquake Year Magnitudes
1 Udayapur, Eastern Nepal 1988 6.6
2 Chainpur, Eastern Nepal 1934 8.3
3 Dolakha, Central Nepal 1934 8.0
4 Sindhupalchowk, Central Nepal 1833 8.0
5 Kaski, Western Nepal 1954 6.4
6 Myagdi, Western Nepal 1936 7.0
7 Bajhang, Far Western Nepal 1980 6.5
8 Dharchula, Far Western Nepal 1966 6.1
9 Dharchula, Far Western Nepal 1966 6.3
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar86
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
10 Dharchula, Far Western Nepal 1916 7.3
8.7 Earthquake Catalogue
The National Building Code Development Project (BCDP, 1994) has developed an
earthquake catalogue using earthquake data catalogues of the US Geological
Survey, The National Earthquake Information Centre (NEIC), National Oceanic and
Atmospheric Administration and National Geological Data Centre (NGDC). The
complete earthquake catalogue for the magnitudes M 4.5 and greater is given in
Table 12.
Table 12: Instrumentally Recorded Earthquake
S.N Magnitudes Catalogue Year
1 M 6.0 and grater than M 6.0 Catalogue complete for the period 1911 to 1992
2 M 5.5 and greater than M 5.5 Catalogue complete for the period 1925 to 1992
3 M 5.9 and greater than M 5.9 Catalogue complete for the period early 1960 to 1992
4 M 4.5 and greater than M 4.5 Catalogue complete for the period late 1970 to 1980s
The largest event reported in the catalogue is the magnitude 8.3 Bihar–Nepal
earthquake (Chainpur), which appears to have occurred in1934.
Figure 36: Epicenter of the Earthquake in the Nepal Himalaya
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
Project Area
Project Area
87
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 37: Probabilistic Seismic Hazard Assessment Map of the Nepal Himalaya
Figure 38: Seismic Risk Zone of the Nepal Himalaya
For the minimum acceleration of 150 gal (Figure 37), reduction factor of 0.5 the
calculated effective design seismic coefficient is approximately 0.09.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
Project Area
88
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
For the maximum acceleration of 200 gal (Figure 37), reduction factor of 0.65 the
calculated effective design seismic coefficient is approximately 0.13.
Hence, the design horizontal seismic coefficient ranges from 0.09 to 0.13 (calculated
values).
Based on above results the design seismic coefficient for the project can be taken
range of 0.09 to 0.13 which is more or less same value represent from the return
period of the earthquake. The project area lies seismically in zone 2.
8.8 Seismic Zoning
The Seismic Hazard Mapping and Risk Assessment component of the NBCDP
carried out detailed analysis of the earth activity and the tectonic structure of Nepal,
and had identified groups of earthquakes with major tectonic features leading to the
identification of seismic zones of assumed uniform seismicity. The three seismic
zones thus identified in Nepal are shown Figure 38.
8.9 Seismic Design Acceleration Coefficient
On the basis of the MHSP studies on Seismic Hazard Assessment and the
derivation of the basic design earthquake accelerations, and on the basis of the
earthquake design coefficient used in other major hydroelectric projects in Nepal,
e.g. the Kali Gandaki ”A” Hydroelectric Project and the Arun-III Hydroelectric Project,
the following earthquake coefficients were recommended and used in the design of
major and minor structures of the LIVHEP are as given in Table 13.
Table 13: Design Earthquake Acceleration Coefficients
S.N StructureBasic Horizontal Acceleration
Coefficient (αH)
Vertical Acceleration Coefficient
(αv)
1 Major Structure 0.25 67% of(αH)
2 Minor Structure 0.20 67% of(αH)
9. CONSTRUCTION MATERIALS SURVEY
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar89
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
A construction material investigation was conducted in the vicinity of the headworks
and powerhouse site as well as along the Deumai HEP area. The investigation is
focused on locating prospective borrow areas of non-cohesive and cohesive
materials, which are to be used mainly as an ingredient of concrete as well as clay.
The prospective borrow sites were identified as sources of coarse aggregates from
either boulders of gneiss or bedrocks. So, the materials should be collected either
from the Kankai Khola for the powerhouse area and the Deumai Khola for the intake
and weir axis area. The area for the materials for the project is located entire the
project area.
The requisite quantities of construction material like boulders, cobble, gravel and
sand are generally available in and around the project. Point bar and braided bar
deposits of the Deumai Khola and Kankai Khola and excavated materials are the
main source of construction material. These deposits predominantly consist of gneiss
boulder, cobble and gravel including some gneiss and quartzite. The boulder, gravel
and sand deposits in the point bars in and around the powerhouse site along the
Kankai Khola and Deumai Khola can be used as construction materials.
9.1 Borrow Area
At the intake site, construction material can be extracted from the alluvial bar deposit
on the both banks as well as upstream of the river from the Kankai Khola. Material
excavated during the construction period from Deumai Khola between
Gajurmukhidham and Weir axis area. Similarly the red clay can be extracted from the
area around Gajurmukhidham area and surrounding area whereas around the
powerhouse area the construction materials can be extracted from either the Kankai
Khola or the Deumai Khola. The location of the materials extraction is shown in the
Figure 39. The excavated materials can be used as for the construction materials.
Four samples of construction material including aggregates and sands from the
project area have been collected and tested in the laboratory to determine the
physico-mechanical properties.
The location as well as the expected volume and composition of the materials are
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar90
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
presented in Table 13 and Figure 33. The materials for the core and filter for the dam
is more than sufficient quantity.
Table 13: Location and Tentative Volume of the Construction Materials
Location Volume Stability Hydrology Distance Materials
S1 Weir axis and Gajurmukhidam
Intake Area
Aggregate and sands
Unlimited Stable Dry Maximum 2 km from
Intake area
Aggregate of
gneiss, quartzite
S2 Upstream from the Intake Area
Sands and aggregates
Unlimited Stable Dry 2 km km from intake
area
Riverbed
materials
S3 Kankai Khola just down from
tailrace
Sands and aggregate
Unlimited Stable Dry 0.3 km from intake Riverbed
materials
S4 Iban and Jitpur as well as
Dhuseni area
Red soil clay
Unlimited Stable Wet-dry 5km from the
headworks area
Hill slope
Total Volume Unlimited
Three rock samples from the quarry sites to measure the point load test as well as the
petrographic analysis. The following physic-mechnical properties of the collected
samples shall be done:
9.2 Sampling of Construction Material
Sampling of aggregate for concrete including packing and marking of samples shall be undertaken
following IS: 2430 – 1986.
Coarse Aggregate
The following tests shall be carried out to test the material for use as coarse aggregate:
S.No. Name of Test Reference Standard to be Followed
1 Specific Gravity IS: 2386 (Part 3)-1963
2 Water Absorption IS: 2386 (Part 3)-1963
3 Aggregate Abrasion Value (Los-Angeles) IS: 2386 (Part 4)-1963
4 Aggregate Crushing Value IS: 2386 (Part 4)-1963
5 Aggregate Impact Value IS: 2386 (Part 4)-1963
6 Soundness ( 5 cycles) (Sodium Sulphate) IS: 2386 (Part 5)-1963
7 Flakiness Index IS: 2386 (Part 1)-1963
8 Elongation Index IS: 2386 (Part 1)-1963
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar91
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
S.No. Name of Test Reference Standard to be Followed
9 Petrographic Examination IS: 2386 (Part 8)-1963
10Potential Alkali Reactivity of Aggregate (Chemical Method)
IS: 2386 (Part 7)-1963
11Alkali Aggregate Reactivity by Mortar Bar Method (Accelerated Technique)
ASTM Designation: C 1260-01
Fine Aggregate
The following tests shall be carried out to test the material for use as fine aggregate:
S.No. Name of Test Reference Standard to be Followed
1 Gradation and Fineness Modulus IS: 2386 (Part 1)-1963
2 Silt and Clay Content IS: 2386 (Part 2)-1963
3 Specific Gravity IS: 2386 (Part 3)-1963
4 Organic Impurities IS: 2386 (Part 2)-1963
5 Mortar making properties (7 & 28 days) IS: 2386 (Part 6)-1963
6 Soundness (5 cycles) (Sodium Sulphate) IS: 2386 (Part 5)-1963
7Petrographic Examination (for natural river sand samples only)
IS: 2386 (Part 8)-1963
8Potential Alkali Reactivity (Chemical Method)
IS: 2386 (Part 7)-1963
9Alkali Aggregate Reactivity by Mortar Bar Method (Accelerated Technique)
ASTM Designation: C 1260-01
10 Water absorption IS 2386 ( Part 3) – 1963
11 Clay lump (%) IS 2386 (Part 2) – 1963
12 Soundness ( 5 cycles) IS: 2386 (Part 5) – 1963
Table 14: Laboratory tests on the soil, aggregate/rock samples
Sample Location Rock Type Test
Aggregate/ sands
S1 Weir axis and
Gajurmukhidam
Intake Area
Aggregate/Sands ACV, PTG, LAA, SSS, AIV, AAR,
SPG, UWT
S2 Intake Area
Sands
Aggregate/Sands GAC, SPG/ABS, ABL, SST,
NMC, OIM, DDT, CT
S3 Kankai Khola just down from
tailrace
Aggregate/Sands ACV, PTG, LAA, SSS, AIV, AAR,
SPG, UWT
S4 Hill slope around the Red clay GAC, SPG/ABS, ABL, SST,
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar92
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Reservoir area NMC, OIM, DDT, CT
Rock Samples
RX-1 Intake Gneiss Point Load Test (PLT)
RX-2 Tunnel, Schist Point Load Test (PLT)
RX-3 Powerhouse Sandstone Point Load Test (PLT)
GAC-Grain Size Analysis, SPG/ABS-Specific Gravity/Absorption, ABL-Atterberg Limit Test, NMC-Natural Moisture
Content, OIM-Organic Impurities, DDT-Dry Density Test, CT-Compact Test, ACV-Aggregate Crushing Value, PTG-
Petrographic Test, LAA-Los Angeles Abrasion Test, SSS-Sulphate Soundness Test, AIV-Aggregate Impact Value, AAR,
SPG-Specific Gravity, UWT-Unit Weight Test, PLT-Point Load Test, ART-Alkali Reactivity Test
9.3 Laboratory Test
All laboratory tests were carried out at well-equipped Vishwa drilling Geotechnical
laboratory in Kathmandu.
Sieve Analysis
The grain size analysis (gradation test) was carried out according to AASHTO T 27 –
82 standard procedures by receiving the sample through a stack of sieve from 75mm
to 0.075mm in diameter. The mass of material retained in each individual sieve was
determined and the cumulative percentage was calculated. The grain size curve was
plotted on the basis of obtained data.
Atterberg Limits
The Atterberg Limits namely, the liquid limit and plastic limit were determined
according to the standard procedure outlined in BS 1377: 1975, Test 2(A) and Test 3.
Plasticity Index was obtained after the test. However, depending the nature of the
materials Indian Standard (IS) was also adopted for the test.
Specific Gravity and Absorption Test
The specific gravity and absorption test were carried out for fine and coarse
aggregate in accordance to BS 812: Part 2: 1975 standard. The absorption value is
also in the range of 1%. Usually aggregate with absorption value of greater than 2%
are considered as unsuitable for construction material. Specific gravity of the
materials has range of 2.64 and 2.69.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar93
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Los Angeles Abrasion Test
The Los Angeles Abrasion test was carried out according to the standard procedures
outlined by AASHTO T 96 – 77 (1982). The percentage of abrasion was calculated on
the basis of the tests. Samples collected for construction material were subjected to
the Los Angeles Abrasion Test. The Los Angeles Abrasion tests were performed in
the samples for coarse aggregates S1 and S2 the abrasion values are 31.0% and
35.0% respectively. Usually for ordinary concrete, the LAA value of aggregate should
not exceed 45%.
Sulphate Soundness Test
The soundness test was carried out on the construction material to determine the
durability of aggregates against physical weathering. The test was done as per the
standard procedures of determining the sulphate soundness of aggregates as
recommended by AASHTO T 104 – 77 (1982). The sulphate soundness tests were
performed on the samples S1 and S2 and the sulphate soundness values are 2.1%
and 2.7%, respectively. The test results indicate that the value obtained does not
exceed the limiting value of 10%.
Loose Density Determination
The loose density determination test was carried out on foundation materials as per
the standard procedures outlined by the AASHTO T 19 – 80.
Compaction Test
The Compaction test namely, the moisture/density relationship were determined
according to the standard procedure outlined in IS 2720 (part VIII 1983).
Point Load Test
The point load test was carried out on the core sample collected from the bore hole
according to the suggested method for determining load strength by point load tester
model PIL – 5 of rock test.
Aggregate Crushing Value Test
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar94
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The aggregate crushing test was carried out according to British Standard of BS 812:
Part 110: 1990. The test was performed on the samples S1 and S2 and the
aggregate crushing values are 14.9% and 16.9%, respectively.
Table 14 Summary of the Pithole
Test Pit Location Material Depth Remarks
S1 Deumai Khola Aggregate/
Sand
> 2 m Construction
material
S2 Deumai Khola Aggregate/
Sand
> 2 m Construction
material
S3 Kankai Khola Aggregate/
Sand
> 2 m Construction
material
S4 Headwork area Red clay > 2 m Cohesive material
The test pit S1 and S2 were excavated from Deumai Khola near Gajurmukhidham
whereas the S3 and S4 were also excavated from the Kankai Khola near the
powerhouse area of the proposed Deumai HEP and Iban village area. The samples
are collected for both fine and coarse aggregates as well as the red soil for laboratory
testing.
9.4 Results of the Laboratory Tests
The results laboratory test and summary of the results of the construction materials
are given in Table 15. Distribution of the construction materials are shown in Figure
15. Details results of the laboratory is shown in Annex A.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar95
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Table 15: Summary of Results of the Construction Materials
Sam
ple
Specific
gravity
Absor
ption
Crushing
value
Sulphate
Soundness
Los
Angeles
Aggregate
Reactivity
Unit
weight
Density Elongation
Index
Moisture
content
Grain
size
Organic
matter
Point
Load
Flakiness
Index
Plastic
Limit
Petrograp
hy
S1 2.69 18.54
0.74
34.2
38.43
1.75 1.86 10.59 0.74 Mediu
m
sand
0.21 20.12
S2 2.67 0.41 20.210.34
36.639.37
8.97 19.27
S3 2.59 1.98 19.60
0.45
30.74
34.67
1.54 1.87 8.83 0.52 Mediu
m
sand
0.26 17.56
S4 1.64 17.5 34.36
RX1 6.99 Gneiss
RX2 7.95
RX3 4.35 Sandstone
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
S1/S2
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 39: Location Map of the
Construction Materials
10. DISPOSAL AREA
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
S1/S2
S3
S4
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
The excavated materials from the tunnel cannot be disposed on the both banks of the
Deumai Khola and Kankai Khola because of the wide valley and available wide
space. The proposed disposal area at Mahaguna and Dumre villages. The proposed
area is presently used as the barren land. The tentative area available for the
disposal of the excavated materials is shown in Table 15 and Figure 40. The
proposed area for mucking has sufficient space and easy to connect access road to
proposed area.
Table 15: Location of the Disposal Area
Location Area (m2) Geomorphology Land use Stability
MD1 Just
downstream from the
weir axis area
600x100 Flat, River valley cultivated/barren Seems to be stable
MD2 Below Besi
village along the
Deumai Khola
500x30 Flat, River valley cultivated/barren Seems to be stable
MD3 Kankai Khola
near Dumre village
1000x100 Flat River valley Barren Stable
Total Area 175000
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar82
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
Figure 40: Location Map of the Muck Disposal area
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
MD1
MD2
MD3
83
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
10. GEOPHYSICAL SURVEY
10.1 2D-Electrical Resistivity
Eight lines of the 2D-Electrical Resistivity Tomogram covers 1,500 m has been
conducted in the reservoir and the intake as well as the weir axis area. Details of the
data is given in Annex B.
Lines Length (m) Summary of Results
L1, Weir axis-2 300 Thin layer of overburden, irregular pattern of the
resistivity below the overburden
L2, Weir axis-2 240 Right bank of the Deumai Khola, bedrocks can be found
at shallow depth
L3, weir axis-2 135 Left bank of the Deumai Khola, bedrocks are met at
shallow depth
L4, Weir axis-1 165 Left bank of the Deumai Khola, 90% fresh rocks at
shallow depth
L5, Weir axis-1 165 Thin layer of overburden with slightly weathered rocks,
weak zones are not seen
L6, Weir axis-1 120 Right bank, thin layers of overburden and fresh rocks
L7, Reservoir area
Phawa Khola
135 Left bank of Phawa Khola irregular pattern of distribution
of the rocks
L8, Reservoir area
Phawa Khola
225 Right bank of Phawa Khola, thick overburden materials
Total 1485
The results of the 2D-Electrical resistivity Survey shows that geologically the best
option for the weir axis is suitable is weir -1 (lines 4, 5 and 6). Fresh and thick
bedrocks can be met at shallow depth but the option weir-2 has the irregular pattern of
the overburden distribution. However, the option weir-1 shall be better than the option
weir-2. The area for the diversion tunnel alignment, bedrocks exposed on both banks
and narrow width are the best way to choose the location for the weir axis.
The reservoir area where only two ERT lines have been conducted at the both banks
of the Phawa Khola. The results show that irregular distribution of the overburden. The
main causes for finding the irregular pattern is presence of the big boulders along with
the alluvial deposits.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
10.2 Core Drillings
Based on the ERT report the core drilling at the weir axis shall be conducted.
Location Depth (m) Required testsWeir axisLeft bank of the weir axis at proposed location 20 m
VerticalLugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Left bank of weir axis 50 m upstream from the proposed weir axis
25 mVertical
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Left bank of weir axis 50 m downstream from the proposed weir axis
25 mVertical
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Right bank of the weir axis at proposed location 20 mVertical
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Right bank of weir axis 50 m upstream from the proposed weir axis
25 mVertical
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Right bank of weir axis 50 m downstream from the proposed weir axis
25 mVertical
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Central part of the weir axis at proposed location 20 mVertical
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Central part of weir axis 50 m upstream from the proposed weir axis
30 m Vertical
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Central part of weir axis 50 m downstream from the proposed weir axis
30 mVertical
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Intake areaCentral part of intake area 30 m
45 degree inclined
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Surge Tank and Powerhouse areaSurge tank area 30 m
VerticalLugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Powerhouse area 30 mVertical
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Powerhouse area 30 mVertical
Lugeon test, permeability test, Petrography, Uniaxial Compressive strength test, hydraulic strength test
Total depth 340 mSeismic RefractionReservoir area left bank 1 kmReservoir area, right bank 1 km
2 kmDriftingOn right bank of the weir axis 20 mOn left bank of the weir axis 20 mOn the intake 30 mSurge tank 30 mTotal 100 m
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar2
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
11. CONCLUSIONS AND RECOMMENNDATIONS
11. 1 Conclusions
1. Geologically, the project lies in partly in the gneiss and schistose gneiss of the
Sarung Khola Formation and Shiprin Formation, Kathmandu Group Seti
Formation and Takure Formation of the Midland Group (93%), of the Lesser
Himalaya, Eastern Nepal and 7% of lies in the rocks of the Siwailk Group.
2. Headworks is located the rocks of Sarung Khola Formation, tunnel alignment
passes on the rocks of the Sarung Khola, Shiprin Khola, Seti and Takure
formations and powerhouse component is located in the rocks of the Lower
Siwalik. Headwoksarea comprises gneiss and tunnel alignment composes the
gneiss and slate, schist. The powerhouse is comprised of sandstone and
mudstone.
3. Structurally, the project is located south of the MCT and MBT. The foliation and
bedding plane of the rocks extends northeast. The tunnel alignment crosses
the MBT and MCT zones.
4. The alluvial deposits are found around the powerhouse and weir area.
Thickness of the alluvial deposits is more than 5 m. Thin layers of colluvial
deposits are found along the tunnel alignment.
5. The slope stability of the project area is good. The dipping of the foliation and
bedding plane is opposite to oblique to the natural hill slope and thick bedded
so favorable condition for the tunneling. Stability condition of the reservoir area
and powerhouse are more or less fair
6. The RMR and Q values of the rock mass exposed along the tunnel alignment
range from 37 to 65 whereas the Q values 3 to 10.
7. The tunnel alignment has fair to good rock mass and last portion of the tunnel
has poor to very poor rock mass. The support system of the tunnel varies from
III to V types.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar3
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
8. Only one adit is recommended which has the length about 500 m and situated
at central part of the tunnel alignment.
9. The exposed rocks mainly gneiss and schist are fresh to slightly weathered
condition and the spacing of the discontinuities range from 1 to 3 m whereas
the sandstone has low to moderate spacing in the rock mass.
10.Sufficient quantity of the construction materials are found along the Deumai
Khola and Kankai Khola riverbed and the excavated materials can be used as
materials.
11.The disposal areas for the excavated materials from the tunnel are located
along the both banks of the Deumai Khola as well as in the Kankai Khola.
11.2 Recommendations
1. Detailed geotechnical studies of the underground structures should be
carried out during the preparation of the DPR.
2. Stability condition of the reservoir area is more or less fair but there should
be controlled the influx of the sediments from the Phawa Khola and Sawa
Khola.
3. Based on the results of the ERT along the Deumai Khola at the weir axis
area 9 core drill holes recommended and two holes in the intake area. At
least one hole in the surge tank and two holes in the powerhouse area.
Total 340 m core drill is recommended.
4. At least two pitholes and sampling for the calculation of the bearing capacity
of the soil along the penstock alignment is suggested.
5. At least two km of the Seismic Refraction survey is proposed for along the
reservoir area to determine the some geological disturbance like fault and
lineaments.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar4
Feasibility Report of Deumai Khola HEP Geological and Geotechnical Studies
6. At least 6 holes in weir axis and 2 holes in 2 holes in the powerhouse area
should be done to clarify the geotechnical properties of the rocks.
7. The following parameter should be done in the core samples: Point Load
Test, Uniaxial Compression Strength, Petrography, Bulk Density, Porosity,
Slack Durability, Specific gravity, Direct Shear Strength Test, Modulus of
Elasticity and Poisson Ratio, Void Index, Water content saturation
8. Drifting is proposed at the weir axis as well as surge tank area.
Research and Development Group Dolakha Nirman Company (P) Ltd., Biratnagar5