Landslide and Slope StabilityJune-2010

n.k.agarwal

geo-consultant

Geological, Geotechnical, Environmental & Allied ServicesHydro Power, Environment, Mineral Resources, Disaster Damage Reduction, Integrated Watershed

Management, Technical Auditing & Site Inspection

Phone: +91 135 2650373/2654487, Mob.:+91 9760099015, Fax: +91 135 2657748

Landslides Impact and Remediation

Landslides Cause Accidents and Disasters

Identification

Processes

Remediation

Landslide Management

Types Studies

Academic/Documentation

Inventory

Type and Mechanism

R & D

Disaster

Rescue & Relief

Zonation

Mitigation

Management

Prevention, Stabilisation

Active Landslides

Inactive Landslides

Palaeo Landslides

Landslide and Slope Failure are a part of Mass Wasting

Processes

All objects have a tendency to acquire lowest potential

energy level

Landslide is a Generic term for a wide variety of

perceptible downslope and outward movement of

rocks and soil.

Usually Landslide occurs on steep slopes

Can also occur on low relief area

Landslides can be triggered by natural as well as

anthropogenic changes in the environment.

Natural changes include reduction in shear strength

and increase in weight of the rock/soil.

Anthropogenic changes include modification of slope

profile and surface-groundwater regime.

Many types of Mass Movements are included in Landslide

Landslide - Generic

Mass Wasting

Mass wasting is the down slope movement

of materials under the influence of gravity.

Mass wasting can occur at rates of only a

few cm/year and cause little damage and

no loss of life, or it can occur at over 100

150km/hr and result in great loss of life and

property damage.

Type of Movement BedrockSoils/Overburden

Classification of Landslides

Types of Landslides and Movements

S.No. Type Movement

1 Falls Abrupt outward and downslope movement of detached material from cliffs, steep slopes- free fall, rolling, bouncing.

2 Topple Block of Rock tilts-rotates outward and falls- rolls down slope, bounces.

3 Slide 1) Rotational Landslide : movement along an upward concave surface of rupture, the moved mass is tilted backward

2) Translational Landslide : movement is essentially outward a and no backward tilt.

4 Lateral Spread

Nearly horizontal movement on very gentle slopes mainly due to saturation and liquefaction.

5 Flows 1) Creep : Slow downslope movement of rock debris and soil.2) Debris flow : Rock debris and soil moves down slope as a slurry

mass along narrow gullies and gorges.3) Debris Avalanche : An extremely rapid movement of Debris

flow downslope.4) Mud Flow : Rapid movement of wet mud (50% sand, silt, clay)

downslope5) Lahar Flow ; Rock or Mud flow downslope of Volcanoes.

Slump Flow

NKA-2010

Types

Root Cause of Mass Wasting

Root Cause of Mass Wasting is the Force of Gravity.

There are other Factors that increase the risk of Slope Failure.

When One or more of these risk factors is high, Mass Wasting is

likely to occur

No Movement

Safety Factor = Shear Strength/Shear Stress

If Safety Factor becomes <1, Slope Failure is Expected

Slope Failure

F = Shearing Strength

mg sinə = Shearing Stress

Landslide and Slope Failure

Site characteristics

(1) Groundwater and seepage conditions.

(2) Lithology, stratigraphy, and geologic details disclosed by borings and

geologic interpretations.

(3) Maximum past overburden at the site as deduced from geological

evidence.

(4) Structure, including bedding, folding, and faulting

(5) Alteration of materials by faulting

(6) Joints and joint systems.

(7) Weathering.

(8) Cementation.

(9) Slickenside.

(10) Field evidence relating to slides, earthquake activity, movement along

existing faults, and tension jointing.

Causes of Slope Instability/Landslides

Geological causes

Weak or sensitive materials

Sheared, jointed, or fissured materials

Adversely oriented discontinuity (bedding, schistosity, fault,

unconformity, contact, and so forth)

Contrast in permeability and/or stiffness of materials

Morphological causes

Tectonic or volcanic uplift

Fluvial, wave, or glacial erosion of slope toe or lateral margins

Subterranean erosion (solution, piping)

Deposition loading slope or its crest

Vegetation removal (by fire, drought)

Thawing

Freeze-and-thaw weathering

Shrink-and-swell weathering

Anthropogenic causes

Excavation of slope or its toe

Loading of slope or its crest

Drawdown (of reservoirs)

Change in land use

Drain seepage

Deforestation

Irrigation

Mining

Artificial vibration

Water leakage from utilities

Vibrations due to explosions

Causes of Slope Instability/Landslides (contd.)

Important Triggers for Slope Failure/ Landslides

Water

Slope saturation by water due to intense rainfall,

snowmelt, changes in ground-water levels, and water-

level changes along coastlines, earth dams, and the

banks of lakes, reservoirs, canals, and rivers. Toe removal

Slope Modification by cut slopes, river erosion,

Head loading

Adding material a slide mass increases its driving

force.

Strength reduction

Weathering ultimately weakens all slope materials; slow

creep causes restructuring of clays stressed within

slopes; slow processes eventually reach critical points.

Cyclic and temporarily increased stresses may cause

soil restructuring or rock fracturing. Earthquakes

Earthquakes, generally of magnitude 6 and above,

inducing ground intensities of greater than or equal to VI,

trigger landslides in the critical reaches of hill slopes. Vibration

Artificial vibrations, as from heavy road traffic.

Important Triggers for Slope Failure/ Landslides (contd.)

Steep High Cut Slopes in Debris fan - Cut Slope Failed during the next rains

Inviting problem

Distant View of Sonapur Landslide

(Jaintia Hills, Meghalaya

Creeping landslide

• The trained eye of a geologist can discover

• Clues if a land is currently moving.

• Clues if the land has moved in the past.

• Evidence that the land is moving includes cracks

appearing where the land is being slowly pulled apart

from the hillside.

• These cracks are called tension cracks.

• With time, the ground on one side of the tension crack

may slide downhill forming a scarp and leaves a scar.

• A fresh scar has a lighter color and no/sparse

vegetation in comparison to adjoining slopes.

• Bulging- lobate landforms-soil profiles indicate old

slides.

Telltale marks of landslide

Hummocky Landforms indicate landslide or soliflucttion

Initiation of detachment

1. Date and time of occurrences:

2. Location:

i) Village / Town

ii) Tehsil / Taluka

iii) District

iv) State

v) Co-ordinates and Toposheet No.

3. Dimensions:

i) Length

ii) Breadth

iii) Height

4. Type:

i) Rock Slide / Fall

ii) Overburden Slide / Fall

5. History

i) Old

ii) New

6. Material Involved:

i)Rock / Overburden

7. Whether in road cutting / river bank / natural hill slopes

8. Structure Affected:

i) Road / Railway line

ii) Houses

iii) Land

a) Barren

b) Cultivated

c) Forest

iv) Civil Projects

9. Casualties:

i) Human

ii) Livestock

First Information Report

10. Rain fall:

i) During last 24 hours

ii) During the week prior to slide

iii) If rainfall data are not available, local

assessment may be given

11. Causes of Slide:

i) As per local assessment

12. Existing Remedial Measures:

and recommendations if any

Signature:

Name:

Address:

Email:

Ph. No.

First Information Report (contd.)

Geological Survey of India as a

Nodal Agency is responsible

for:-

Coordinating and undertaking

geological studies for landslide

Hazard Mitigation

Carrying out landslide hazard

zonation

Monitoring landslides and

avalanches

Studying the factors responsible

for sliding and suggesting

precautionary as well as

Preventive measures

Is there a landslide in your locality?

Send the First Information to Nodal Officer, GSI

(Fax: 011-29051328/26053532, 033-22861656) or email :dg@gsi.gov.in

Formulation Uniform Methodologies for LHZ on Macro and Meso

Scales

LHZ of Vulnerable Areas on Macro Scales

LHZ of identified areas on Meso Scales

Real Time Monitoring of Slides

Development and Validation of Early Warning System

Guidelines and training for Scientists, Engineers, Planners &

Decision makers

Inventory/ Database on LHZ

Formulation Strategy for Prevention and Reduction Loss

Environmental

Life, Livestock & Property

• Awareness Generation

Scope for Action

Massive Landslide – La Conchita-California (1995)

Physical weathering

NKA-2010

Planar Failure

Wedge Failure

When the rock mass is cut across by two or

more joint planes such that they cut each other,

with the minimum requirement of four planes

WITH either two free surfaces and two joint

planes or at least one free space with three

unfavorably oriented joint or discontinuity

planes ,

constituting a tetrahedral or trapezoidal wedge,

failure may initiate either along their line of

intersection or on one of the gentler dipping

planes.

Such type of failures are called 'wedge failure'.

Landslide Management

Reduction in Shearing Stress

Enhancing Shearing Strength

Strategy

Geotechnical + Bio-engineering Treatment

Reduction in Shear Stress

• Removal of Head Load of the Slide

• Grading of Slope

• Benching of Slopes

• Removal of Loose and Unstable Material

Enhancement of Shearing Strength

• Over burden laden Slope treatment by geotextile/geogrid

• Grading of over burden laden slopes

• Dressing of

• Sealing of Cracks

• Rock bolting/anchors/cable anchors

• Shotcreting

• Consolidation grouting and pile shafts in the weathered rock mass

• Drainage

Landslide Hazard Zonation

Macro Scale – 1:50,000 to 25,000 scale

Meso Scale - 1:25,000 to 1:15,000 scale

Micro Scale – 1:15,000 to 1:5,000 Scale

Zonation Map Validation

Field - Slope Instability Incidences

Statistical - Frequency Correlation

Varunavat Landslide Treatment

Initiation of varunavat Landslide

Lateral Cracks

Lateral Cracks

July-August 2003

GSI officer Noticed these cracks (21-24th August 2003)

24-25 Sep 2003

24th-25th September varunavat Slope

Failed and huge quantities of debris

rolled down on residential and

commercial buildings in Uttarkashi

• Proterozoic Garhwal Group rocks comprising

• quartzite, phyllite and metabasics are exposed.

• Rocks dip at 15o-45o in N010o-040o direction i.e. into the hill.

• The rocks are highly weathered, destressed, decomposed, jointed and

fractured.

• The hill slopes are steep 45o-60o in general, the top of the hill has

gentler slopes.

• The top varunavat hill is laden by unconsolidated, overburden mass.

• The top is at El± 1800m and the toe of the hill is at EL ± 1100m m.s.l.

• The overburden/rock contact is exposed at El ± 1660m.

Geological Set Up

Geological-Geotechnical Investigations

Contouring of Landslide Area on 1:1000 scale

Geological Mapping on 1:1000 scale

Identification of Structural Discontinuities/Lineaments

Geophysical Surveys – depth of OB

Geomechanical Characterisation of OB for

stabilisation analysis.

Monitoring of Slope movement

Major Works at Varunavat Area

S.No. Description of Work s

1 Varunawat Parvat Treatment Works

2 Additional and post Treatment Works

3 Miscellaneous Works

4 Removal of Muck from Masjid Mohalla

5 Barethi- Tekhla bye-pass road

6 Gyansu Nala Treatment

7 Relocation of Govt. Offices

8 Relocation of Private Property

9 Relocation of Commercial Property

10 Relocation of Shops

11 Rehabilitation of landslide affected villages of Bhatwari/Dunda

12 Rehabilitation of landslide affected villages of Naitwar & Fafrala

13 Construction of Roads & Bridges in Mori Tehsil

Treatment works at the Crown

DEVELOPMENT OF PLATFORMS AT EL ± 1670 AND ±1710 IN THE ENTIRE LENGTH ACROSS THE AFFECTED ZONE.

A TOTAL OF 1.51 LAC CUM LOOSE MATERIAL REMOVED FROM THE CROWN AREA AND 1.87 LAC CUM MUCK HAS BEEN REMOVED TILL 19 NOV. 2004.

CONSTRUCTION OF EARTHEN DYKE ON THE EDGE OF THE PLATFORM 3-4 M HIGH.

REMOVAL OF DEBRIS LYING ON THE HIGHWAY AT THE TOE AND CONSTRUCTION OF PROTECTION DYKES BY WAY OF WIRE CRATES AND CONCRETE BLOCKS.

STRENGTHENING OF PROTECTION MEASURES IN THE BUFFER ZONE.

CHANNELISING THE SURFACE RUN-OFF IN THE CROWN AREA TO AVOID ITS ENTRY INTO THE SURFACE CRACKS.

• Grading of Slopes to ,300 between EL 1660m and EL

1800 with berms

• Development of Platform at the contact of OB/Rock at

EL 1660m

• Weathered Rock mass strengthing at the Platform by

Pile shafts and consolidation grouting

• Strengthening of rock mass with rock bolting, cable

anchoring and reinforced shotcreting

• Laying geogrid at EL 1660m

• Development of contour drain on rocky slopes

between EL 1650m and toe of hill connected with

chute drains

• Main toe drain

• Retaining structure at the toe of the hill

Treatment (contd.)

Varunavat Landslides

Landslides along Chutes

Chutes due to landslides

varunavat Parvat - Pre September 2003 Landslide

Uttarkashi Township

Stabilisation by Geojute between EL 1790m to 1780m

Stabilisation of Overburden by Geogrid

Treatment at Crown

Geogrid treatment at Crown

NKA-2010

Cable Anchors at Crown

Stabilisation of Rocky Slopes by Rock bolts and shotcreting

Stabilisation by Cable Anchors

Excavation of Platform at Crown Area

Cellular Wall at Ram Lila Ground (settlement)

nk. agarwal

Geo-Consultant & Advisor

HIGEOS India