Foundations on Expansive Soils Sudan Experience -Xx

8/15/2019 Foundations on Expansive Soils Sudan Experience -Xx

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Foundations on Expansive Soils: Sudan Experience

Ahmed M Elsharief, Building and Road Research Institute, University of Khartoum

Abst rac t

This paper presents an overview of the different foundation options on expansive clay soils in

Sudan and the criteria for their selection and adoption. It reviews the research projects on

foundations on expansive soils performed by graduate students at Building and Road Research

Institute. The basic research outcomes have been outlined and summarized. Design and

construction issues aimed at improving the existing practice have been discussed.

Recommendations have been given for the parameters needed for the design of bored concrete

piles in expansive soils.

Introduction

Expansive soils are soils that have potential for swelling and shrinkage under changing moisture

conditions. The volume change resulting from swelling and shrinking causes damages to structures

founded on them. The expansive soil area includes nearly all the agricultural schemes and most of

the development projects in the country (Figure 1) and covers about 40% of the total area of Sudan

(Osman and Charlie 1983).

Damages of structures caused by expansive soils have been reported from different locations in

the clay plain (Lates et al 1983). The damages include buildings, roads, factories, hydraulic

structures etc. and were attributed to lack of proper identification and classification of expansivesoils and improper design of the foundations of the damaged structures.

Several attempts have been made by graduate students at Building and Road Research Institute

(BRRI) of the University of Khartoum, since the seventies of the last century to identify and classify

expansive soils and to study the various factors affecting their swelling and shrinkage

characteristics (Hamadto, 1985; Elturabi 1985, Elsharief,1987; Elhag and Gasmelseed, 1984;

Ahmed 2002; Rahmatalla, 2007). These attempts resulted in better understanding of the factors

affecting swelling and shrinkage of Sudanese expansive soils and hence sound guides for their

identification and classification. Parallel research continued to study the causes of damages of

structures founded on expansive soils, the performance of foundations on these soils and

guidelines for their selection (Elsharief, 1987; Abu Sara, 1994; Saeed, 2004; Omer, 2003; Ahmed

2006).

This paper discusses the different foundation options for light structures founded on expansive

soils, guidelines for their selection, design and construction with special reference to the

experience in Sudan.


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Figure 1: Expansive soil plains in Sudan (Osman and Charlie 1983)

Factors Affecting the Selection of Foundations on Expansive Soils

The types of foundations commonly used worldwide to support structural loads in expansive soil

environment are: shallow individual or continuous footings, rigid or stiffened raft and bored

concrete piles.

Shallow footings are preferred where the expansive soil stratum is relatively thin to allow placing

the footing on a low expansive or low swelling stratum. Strip footings are used for load bearing

structures but lack the three dimensional rigidity needed to resist small movements. Isolated or pad

footings offer some structural rigidity needed to resist small movements therefore perform better

than strip footings. Stiffened raft foundation consists of thin concrete slab stiffened with cross

beams to provide additional stiffness of the slab. They are applicable with good performance in

areas where soils possess large amounts of movements (Zeitlen and Komornik, 1980). However,

bored concrete piles have been found to perform satisfactorily in expansive soils with high to very

high potential (Mohan, 1955;Chen, 1975; Poulos and Davis, 1973; Ahmed 2006). They are favored

in expansive soils mainly because of their ability to resist uplift forces when properly installed. Piles


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with sufficient length develop upholding forces which can resist the uplifting forces due to swelling

of clay layers in the upper zones.

The performance and selection of a technically viable foundation type for a certain structure

founded on expansive soil will depend on:

The swelling characteristics of the encountered soils, i.e., the swelling potential of the

expansive clay layer(s), depth and layering sequence of the encountered deposits within

the influence range of the foundation system

The environmental conditions; these include the moisture content of the swelling soil

layers, depth of water table, rainfall intensity, temperature and vegetation cover. These

factors affect and control the depth of the active clay zone. The utilization of water in

certain buildings (e.g. factories) is detrimental and affects future performance of

foundations. The type of the structure, its shape, rigidity/flexibility and tolerance to movements

Constructional considerations such as availability of certain construction tools (e.g. piling

rigs) and the experience of local contractors and home owners.

Geotechnical Properties of Expansive Soils in Sudan

Several researchers investigated and compiled the data on the geotechnical properties of

expansive soils in Sudan (Osman and Charlie 1983; Suhad 2003; Mohamed (2004); Saeed 2004;

Rahmtalla 2007). Their work was based mainly on data collected from service reports carried out at

BRRI and at other engineering firms. Osman and Charlie (1983) concluded, based on informationfrom various sources, that soils from the clay plains of Sudan are potentially very expansive and

that the mean plasticity index and swelling pressure are 45 and 265 KN/m2, respectively.

Suhad (2003) tested samples from 16 sources from the Central, Eastern and Southern clay plains.

She measured the physical and chemical properties which could affect their swelling

characteristics. A statistical analysis was performed on the data. The average liquid limit, plasticity

index, cation exchange capacity (CEC) and free swell were 70%, 44%, 25 meq./gram and 124%,

respectively.

Saeed (2004) measured the physical properties of the upper clay blanket at 13 locations in the

area covering the Gezira Scheme in Central Sudan. The upper clay was found to be potentially

expansive. The depth of the clay blanket ranges from 1.5 m to more than 10.0 m. Summary of theaverage basic physical properties of the upper 3.0 meters of the clay cover is given in Table 1. The

Table shows dominance of clays having high to very high swelling potential.

Mohamed (2001) studied data from 78 sites in Khartoum city and found that the city is covered by

a blanket of low/highly plastic montmorillonitic clay. The clay depth exceeds 10 m close to the Blue

Nile at Burri power station and Friendship Hall. The average liquid Limit and Plasticity Index for the

CH clays are 73% and 44%, respectively. Values higher than 159% were measured for liquid limit

and higher than 119% for plasticity index.

Table 1: Average physical properties of the upper 3.0 m in Gezira


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Location Plasticity

Index

Fines

Content

%

Clay

Content %

Barakat 35 93 33

Elmasalamia 32 82 29

Wad

Haboba

33 87 22

Turis 39 75 43

Elmaseed 34 --- ---

Alfatagoba 10 61 22

Elhuda 31 64 33

Umshadida 24 75 21

Matoog 48 79 ---

Elmaturi 43 81 52

Gaboga 32 79 27

Elmanagil 31 55 35

Elroof 33 86 23

The research and experience from service jobs at BRRI have shown that:

Expansive soils are predominantly found within the upper horizon of the clay plains and

extend from the ground surface down to more than twenty meters in some locations (e.g.

Singa in Blue Nile State; Fao in Gedarif state; Tharjath in Wuhda state). However, there areexceptions, e.g. in greater Khartoum (south of Khartoum international Airport and Alfaiha

east of the Blue Nile) where swelling soils are covered by a thin layer, up to 4.0m in depth,

of non-expansive soil.

Expansive soils in Sudan have high to very high potential for swelling and are mostly

classified as CH soils. Very high values of LL and PI have been measured, as stated above.

Swelling pressures exceeding 1000 KN/m2 were measured (Elsharief, 1987).

The dominant clay mineral common for all the clays in Central, Southern and Eastern clay

plains is montmorillonite (Suhad 2003), whereas kaolinite is always found with

montmorillonite but in lesser amounts. Illite was found with montmorillioniite in the clays of

the Southern and Central plains close to the White Nile.

Cracks and Damages Caused by Expansive Soils

Several development projects constructed in the seventies in the Central and Eastern clay plains of

Sudan experienced severe problems and distresses caused by the expansive soils on which they

were built. Examples are the civil works of Rahad Scheme, Gezira University buildings in

Nisheshiba, Asalaya Sugar Factory etc..


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Hamadto (1985) carried out field survey of damages caused by expansive soils in different parts of

the clay plain. The surveyed buildings included those founded on short piles and isolated footings

in Rahad Scheme and reinforced strip foundations in Nisheshiba, Gezira University. The surveyedsites are dominated with clays having high to very high potential for swelling. The reinforced strip

foundations used in Gezira University at Nisheshiba were 0.5 m wide and 0.8 m deep. The

surveyed buildings showed moderate to severe cracking. It was observed that houses built with

flexible mortars in Wademedani showed lesser damage compared to those built with cement

mortars. Observations and investigations of buildings founded on strip foundation in areas covered

by expansive soils in Greater Khartoum have shown that cracking of the walls is very common for

light buildings founded on strip foundation. Buildings founded on hard-core perform better than

those founded on reinforced concrete strip foundation.

Isolated footings with columns and ground beams were surveyed in Village 10 of Rahad Scheme

(Hamadto 1985). The footings were 2.0 to 4.0 m2 in size and were placed at 3.0 m depth. All thesurveyed houses showed light cracks except three houses which showed moderate to severe

damages. The author is aware of several two to four story buildings founded on pad footings in

Khartoum which experienced severe cracking of the walls and beams mainly because the grade

beams were placed directly on the expansive soils. Movements of the grade beams increase the

upward forces and hence cause heaving of the foundations. A house in Erkewit south of Khartoum

experienced severe cracks of the walls and hogging of the grade beams. The grade beams were

isolated from the ground but the walls covering the beam were bonded with it by very strong

mortar. The heaving of the cover walls exerted upward pressures on the grade beams and caused

the distresses. Experience has shown that severe damages of building founded on isolated

footings are often connected to uncontrolled wetting and failures of the drainage systems.

The bored piles used in Fao town of Rahad Scheme were 3.0 m long and 0.30 diameter originally

designed as fully reinforced. For economical reasons partially reinforced (upper 3.0 meters) piles

were executed. The piles failed in tension at the bottom of the reinforcement due to vertical pull

forces of the swelling clays and the separated upper part moved resulting in severe distresses. The

writer has experience with distresses of a two story building founded on bored piles in expansive

soil media in Omdurman city. The piles were extended to satisfactory length but grade beams were

placed directly on the expansive soil. Swelling forces acting on the grade beam caused hogging of

the beam and pulled out the piles. The result was severe cracks on the walls and crushing of the

columns.

Foundations on Expansive Soils – Review of Local Research

The important factors which contribute to the selection and performance of foundations on

expansive soils in Sudan are: i) the swelling potential is generally high to very high; ii) expansive

soils extend to depths usually much greater than the depth of seasonal moisture variations; iii) the

expansive plains in the central and eastern Sudan are found in dry arid climate, i.e. with long dry

season and evaporation much higher than precipitation and ground water table is very deep. The


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clays are therefore desiccated having low moisture content and high affinity to water "suction

potential".

Shallow Foundation

The tradition of building on strip foundation is very common mainly for the following reasons, given

the fact that conditions are very unfavorable for their selection and use (Elsharief and Saeed,

2002):-

- Homeowners, local builders and contractors lack the proper understanding of the expansive

soil problem and the knowledge of identification and classification of these soils

- Limited budgets are usually available for construction and the strip option is thought to be

economical

- Other technically viable alternatives e.g. bored piles and stiffened raft need specialized

equipments and contractors and expensive imported materials.

To minimize the deleterious effects of expansive soils on structures when the swelling

potential is high improvement techniques have been suggested and practiced by local

builders. Those involve using intercepting layers of earthen materials between the natural soil

and the foundation material and/or strengthening the structure. These techniques are detailed

as follows:

- Placing rubble masonry "hardcore" on the native soil in the strip trench and then building on it.

The practice in some areas is to construct the hardcore foundation before commencement of

the rainy season and leave it exposed during the rainy season and then resume construction.

-

Placing well compacted cohesive non-expansive soil (CNS) or sand between the native soil

and foundation material. The idea was borrowed from India and is advocated to be technically

viable for soils with high swelling potential.

Research was carried out at BRRI to evaluate several intercepting layers as effective measures to

absorb heave of foundations on expansive soils (Saeed, 2004 ). Laboratory and field experiments

were performed. The field experiments were performed in Barakat, the headquarter of the Gazira

Scheme. The intercepting layers used for the two experiments included plain concrete, plain

concrete with 30% voids (honeycomb foundation), CNS, sand and expansive soil treated with lime.

Typical brick walls 1.2 m long and 1.9 m high were built for each treatment option (Figure 2).

Trenches were excavated parallel to the walls on each side to allow for even wetting of thefoundation soil. Steel rods were placed at four locations of the each wall for heave measurements.

Heave measurements were taken by a precise level for a total testing time of 18 months. This time

was divided into two cycles, 261 days of wetting, 99 days of drying and then 127 days of wetting

and 50 days of drying. The result of the experiment are shown in Figure 3. The laboratory and field

experiments showed consistent trends. The CNS and hardcore were found to be the best options

for absorbing and minimizing heaving of strip foundations on expansive soil. The hardcore was

very effective when subjected to cycles of wetting and drying.


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Figure 2: Set-up of field experiment of different intercepting media


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Figure 3: Results of field experiment of different intercepting media

Raft Foundation

Omer (2003) reported a case study in Barakat, Gezira State, where several labor houses built in

1964 were founded on stiffened raft foundation. The house is comprised of a single room founded

on reinforced concrete raft 20 cm thick. The house is built with brick walls bonded with sand-

cement mortar and covered with 12 cm reinforced concrete slab. The houses experiencedconsiderable tilting without any signs of cracks on the walls or the other structural elements (Figure

4). The house first heaved in the side where the roof downspouts were located; the residents were

obliged to change the location of the downspouts in order to allow drainage of the water from the

roof. After a while the house tilted in the other direction and therefore acted like a boat floating on

the expansive clay. However, the rooms were intact and did not show signs of distresses and

cracks.


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Omer (2003) evaluated the different stiffened rafts design methods (Walsh 1974; Wray PTI 1978;

"Swinburne" Fraser and Wardle 1975) and applied them to

Figure 4: A labor house founded on stiffened raft in Barakat

conditions in Barakat. He concluded that the existing methods are of limited use and may not suit

conditions in Sudan where the maximum heave exceeds the maximum heave value for the three

methods which is 101 mm, Only finite element programs could suit the very high swelling soils of

Sudan.

Pile Foundation

Piles in expansive soils are designed to act as anchors against uplift forces generated by these

soils. They should develop sufficient capacity to carry structural loads and the movement of piles

due to the net effect of uplift forces and structural loads should be less than a prescribed limit

[Poulos and Davis 1980]. The ultimate bearing capacity Qu is the summation of the ultimate skin

friction Qs and ultimate base resistance Qb. For a circular pile Qs is obtained using the followingequation:

Qs = п d L α Cu (1)

Where:

d: is the pile diameter

Cu: is the undrained shear strength of the soil along the pile shaft

L: is the pile length, and


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α: is the adhesion factor

The undrained strength of the soil around the pile shaft is often obtained through laboratory testingof representative soil samples around the pile, whereas the adhesion factor varies according to the

soil type, pile type and method of pile installation. The ultimate base resistance is obtained using

the following equation:

Qb = Ab Nc Cub (2)

Ab: is the pile base area

Nc: is end bearing capacity factor, and

Cub: is the undrained shear strength of the soil below the pile base

The end bearing capacity factor is a function of the soil type and its friction angle. A factor of safety

of about 3 is adopted for the computation of the allowable pile capacity.

The design for uplift generally follows the simplified Chen (1975) method. Figure 5 shows the upliftforces within the active zone and the withholding (resisting) forces within the anchorage zone. For

a safe pile the withholding forces will resist the uplift forces. The method assumes that uplift force

is a function of the soil swelling pressure within the active zone. The unit uplift pressure f u is the soil

swelling pressure (SP) multiplied by an uplift factor (β). The uplift force along the active zone is

obtained using the following equation:

Fu = п d Za β SP (3)

Here Za is the depth of active zone. Therefore, to compute the uplift force, the designer needs to

know the uplift factor, soil swelling pressure within the active zone depth and active zone depth.

The resistance to uplift (W) is offered by the adhesion resistance of the withholding part of the pile

(L-Za) in Figure 5 and by the allowable load from the superstructure (Qd). It is given by the

following equation:

W = п d α Cu (L – Za) + Qd (4)

A safe design requires that the uplift force (Fu) should be less than or equal to the withholding force

or resistance (W). Equations (3) and (4) are equated and solved for the safe pile length.

From the above, the parameters needed for the design of piles in expansive soils are: the adhesion

factor α, bearing capacity factor Nc, uplift factor β and active zone depth Za.

Research was carried out at BRRI (Elsharief, 1987; Abu Sara 1991; Ahmed 2006) to find the

geotechnical parameters of interest for the design of piles in expansive soils. Laboratoryexperiments showed that the adhesion factor is 0.45 for moisture content below the plastic limit

and linearly increased with moisture above the plastic limit. The adhesion factor may be obtained

using the following equation for moisture content (m.c. in percentage) above the plastic limit

α = 0.045 m.c. - 0.407 (5)


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Figure 5: Forces acting on a pile in expansive soils

The end bearing capacity factor was back calculated from instrumented full scale load tests and a

value of 9 was attained. Uplift factor was found to be 0.2 from model pile tests. Two piles 0.3 m

diameter and 3.0 m long were casted in a site south of Khartoum. One of the piles was load testedto failure under natural field conditions. The area around the piles was flooded with water for 3

months and then the two piles were load tested to failure. The results are given in Figure 6. The

pile load tests showed significant drop in ultimate capacity caused by wetting. This resulted in

recommending higher factor of safety (minimum 4.0).

Active Zone

Anchorage Zone

Fu

Fa

Za

L

Qd

Ground Level


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0

10

20

30

40

50

60

0 5 10 15 20 25 30 35 40 45 50 55

Load (Ton)

C u m u l a t i v e S e t t l m e n t ( m m )

Before Wetting (P2)

After Wetting (P1)

After Wetting (P2)

Figure 6: Load settlement curves for Test Piles (P1 & P2) before and after wetting.

Steel pipe piles have been extensively used to support light structures and pipe racks in the clay

plain of southern Sudan by petroleum companies. They are driven into the clay by driving

hammers. They have been chosen mainly because of the fast installation and non-availability of

construction materials, i.e. aggregates and sand in these areas. Their performance is doubtful

mainly because of the expected large drop in capacity (skin resistance) of these piles on wetting ofthe surrounding clay. Research is going on at BRRI to study their performance.

Conclusions

This paper reviewed and summarized the outcome of the research carried out by graduate

students from Building and road Research Institute in Sudan on the design and performance of

different foundation systems on expansive soils in Sudan. It covers the local experience and also

the suitability of worldwide existing design and construction methods to conditions in Sudan. The

topic is broad and diverse, therefore the author could not cover the experience of other

researchers and practitioners from Sudan. For more detailed evaluation the reader is directed to

review/go through the cited references.

References

Abusara A. (1994) "Laboratory and Field Studies on Bored Pile Foundations in Expansive soils"

M.Sc. Thesis, Building and Road Research Institute, University of Khartoum, Khartoum, Sudan

Ahmed, E. O (2005)” Guidelines for the Design of Piles in Expansive Soils”, M.Sc in Building

Technology (Geotechnical), Building and Road Research Institute, University of Khartoum

Chen, F. H. (1975) ”Foundation on Expansive Soils”, Elsevier Co. Amsterdam


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Elsharief A. M. (1987) “Foundation on Expansive Soils: A laboratory and Field Investigation of

Swelling Potential and Performance of Short piles in Expansive Soils”, M.Sc. Thesis in Civil

Engineering at Building and Road Research Institute, University of KhartoumElturabi M. A. D. (1985) "A Study on Expansive Soils in Sudan" M.Sc. Thesis, Building and Road

Research Institute, University of Khartoum, Khartoum, Sudan

Fraser R. A. and Wardle L. J. (1975) "The Analysis of Stiffened Raft Foundation on Expansive

Soils" Proceeding of Sympseum on Recent Development in the Analysis of Soil Behavior and Their

application in Geotechnical Structures; University of N.S.W., Australia

Elhag H. A. and Gasmelsid K. M. (1984) "Use of Static Cone Penetration Machine in Sudanese

Expansive Soils" ist National Conference on Science and Technology of Buildings, Sudanese

Engineering Society.

Hamadto M. E. (1985) "performance Evaluation and Prediction of Behavior of Some Expansive

Soils in Sudan" M.Sc. Thesis, Building and Road Research Institute, University of Khartoum,Khartoum, Sudan

Lates E. M., Elmonshid B. E. F. and Abbo S. H. (1983) “Review of Problems Generated by

Expansive Sols in Sudan” Proc. Of Seminar on Expansive Clay Soils Problems in Sudan, Wad

Medani, January 1983, Hydraulic Research Station, HRS

Mohamed E. A. (2004) "Subsoil Analysis of Khartoum City" M.Sc. Thesis, Building Technology,

Building and Road Research Institute, University of Khartoum

Mohan D. (1955) "Under-reamed pile Foundations in black Cotton Soils" 25th Annual Research

Committee meeting of the Central Board of Irrigation and Power India

Omer O. G. (2003) "Analysis and Design of Stiffened Raft Foundation on Highly Expansive Soils"

M.Sc in Building Technology (Geotechnical), Building and Road Research Institute, University of

Khartoum

Poulos H. G. and Davis E. H. (1980) "Pile Foundation Analysis and Design" John Wiley and Sons

Rahmatalla H. H. (2007) "Shrinkage Behavior of Expansive Clays" M.Sc. Thesis, Building

Technology, Building and Road Research Institute, University of Khartoum

Suhad E. M. Ali (2003) "Intrinsic Swelling and Physiochemical Properties of Expansive Soils from

Sudan" M.Sc. Thesis, Building Technology, Building and Road Research Institute, University of

Khartoum

Walsh (1974) "Design of Residential Slab-on-Ground" Division of building Research, technical

paper # 5, Commonwealth Sientific and Industrial Research Organization, Highett, Victoria, Australia

Wray W. K. (1978) "Development of a Design procedure for residential and Light Commercial

Slab-on-Ground over Expansive Soils" Texas A & M University, Ph.D. Thesis

Zeitlin J. G. and Komornik A.(1980) "A Foundation Code for Expansive Soil Conditions"

proceeding of the Fourth International Conference on Expansive Soils, Denvor, Colorado Vol. 1 pp.

609-616.

Foundations on Expansive Soils Sudan Experience -Xx

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