CHAPTER 18civilwares.free.fr/Geotechnical engineering - Principles and... · CHAPTER 18 FOUNDATIONS...

CHAPTER 18FOUNDATIONS ON COLLAPSIBLE ANDEXPANSIVE SOILS

18.1 GENERAL CONSIDERATIONSThe structure of soils that experience large loss of strength or great increase in compressibility withcomparatively small changes in stress or deformations is said to be metastable (Peck et al., 1974).Metastable soils include (Peck et al., 1974):

1. Extra-sensitive clays such as quick clays,2. Loose saturated sands susceptible to liquefaction,3. Unsaturated primarily granular soils in which a loose state is maintained by apparent

cohesion, cohesion due to clays at the intergranular contacts or cohesion associated withthe accumulation of soluble salts as a binder, and

4. Some saprolites either above or below the water table in which a high void ratio has beendeveloped as a result of leaching that has left a network of resistant minerals capable oftransmitting stresses around zones in which weaker minerals or voids exist.

Footings on quick clays can be designed by the procedures applicable for clays as explainedin Chapter 12. Very loose sands should not be used for support of footings. This chapter deals onlywith soils under categories 3 and 4 listed above.

There are two types of soils that exhibit volume changes under constant loads with changes inwater content. The possibilities are indicated in Fig. 18.1 which represent the result of a pair of testsin a consolidation apparatus on identical undisturbed samples. Curve a represents the e-\og p curvefor a test started at the natural moisture content and to which no water is permitted access. Curvesb and c, on the other hand, correspond to tests on samples to which water is allowed access under allloads until equilibrium is reached. If the resulting e-\og p curve, such as curve b, lies entirely belowcurve a, the soil is said to have collapsed. Under field conditions, at present overburden pressure/?,

791

792 Chapter 18

P\ PiPressure (log scale)

Figure 18.1 Behavior of soil in double oedometer or paired confined compressiontest (a) relation between void ratio and total pressure for sample to which no water

is added, (b) relation for identical sample to which water is allowed access andwhich experiences collapse, (c) same as (b) for sample that exhibits swelling

(after Peck et al., 1974)

and void ratio eQ, the addition of water at the commencement of the tests to sample 1, causes thevoid ratio to decrease to ev The collapsible settlement Sc may be expressed as

S = (IS.la)

where H = the thickness of the stratum in the field.Soils exhibiting this behavior include true loess, clayey loose sands in which the clay serves

merely as a binder, loose sands cemented by soluble salts, and certain residual soils such as thosederived from granites under conditions of tropical weathering.

On the other hand, if the addition of water to the second sample leads to curve c, locatedentirely above a, the soil is said to have swelled. At a given applied pressure pr the void ratioincreases to e',, and the corresponding rise of the ground is expressed as

S = (18.Ib)

Soils exhibiting this behavior to a marked degree are usually montmorillonitic clays withhigh plasticity indices.

Foundations on Collapsible and Expansive Soils 793

PART A—COLLAPSIBLE SOILS

18.2 GENERAL OBSERVATIONSAccording to Dudley (1970), and Harden et al., (1973), four factors are needed to produce collapsein a soil structure:

1. An open, partially unstable, unsaturated fabric

2. A high enough net total stress that will cause the structure to be metastable

3. A bonding or cementing agent that stabilizes the soil in the unsaturated condition

4. The addition of water to the soil which causes the bonding or cementing agent to bereduced, and the interaggregate or intergranular contacts to fail in shear, resulting in areduction in total volume of the soil mass.

Collapsible behavior of compacted and cohesive soils depends on the percentage of fines, theinitial water content, the initial dry density and the energy and the process used in compaction.

Current practice in geotechnical engineering recognizes an unsaturated soil as a four phasematerial composed of air, water, soil skeleton, and contractile skin. Under the idealization, twophases can flow, that is air and water, and two phases come to equilibrium under imposed loads, thatis the soil skeleton and contractile skin. Currently, regarding the behavior of compacted collapsingsoils, geotechnical engineering recognized that

1. Any type of soil compacted at dry of optimum conditions and at a low dry density maydevelop a collapsible fabric or metastable structure (Barden et al., 1973).

2. A compacted and metastable soil structure is supported by microforces of shear strength,that is bonds, that are highly dependent upon capillary action. The bonds start losingstrength with the increase of the water content and at a critical degree of saturation, the soilstructure collapses (Jennings and Knight 1957; Barden et al., 1973).

Figure 18.2

Symbols

Major loessdeposits

Reports of collapsein other type deposits

Locations of major loess deposits in the United States along with othersites of reported collapsible soils (after Dudley, 1 970)

794 Chapter 18

50

60-

70-

90

100-

110

Soils have beenobserved to collapse

G =2.6

G, = 2.7

Soils have notgenerally beenobserved tocollapse

\ I \10 20 30 40

Liquid limit50

Figure 18.3 Collapsible and noncollapsible loess (after Holtz and Hilf, 1961

3. The soil collapse progresses as the degree of saturation increases. There is, however, acritical degree of saturation for a given soil above which negligible collapse will occurregardless of the magnitude of the prewetting overburden pressure (Jennings and Burland,1962; Houston et al., 1989).

4. The collapse of a soil is associated with localized shear failures rather than an overall shearfailure of the soil mass.

5. During wetting induced collapse, under a constant vertical load and under Ko-oedometerconditions, a soil specimen undergoes an increase in horizontal stresses.

6. Under a triaxial stress state, the magnitude of volumetric strain resulting from a change instress state or from wetting, depends on the mean normal total stress and is independent ofthe principal stress ratio.

The geotechnical engineer needs to be able to identify readily the soils that are likely to collapseand to determine the amount of collapse that may occur. Soils that are likely to collapse are loose fills,altered windblown sands, hillwash of loose consistency, and decomposed granites and acid igneousrocks.

Some soils at their natural water content will support a heavy load but when water is providedthey undergo a considerable reduction in volume. The amount of collapse is a function of therelative proportions of each component including degree of saturation, initial void ratio, stresshistory of the materials, thickness of the collapsible strata and the amount of added load.

Collapsing soils of the loessial type are found in many parts of the world. Loess is found inmany parts of the United States, Central Europe, China, Africa, Russia, India, Argentina andelsewhere. Figure 18.2 gives the distribution of collapsible soil in the United States.


Holtz and Hilf (1961) proposed the use of the natural dry density and liquid limit as criteriafor predicting collapse. Figure 18.3 shows a plot giving the relationship between liquid limit anddry unit weight of soil, such that soils that plot above the line shown in the figure are susceptible tocollapse upon wetting.

18.3 COLLAPSE POTENTIAL AND SETTLEMENT

Collapse PotentialA procedure for determining the collapse potential of a soil was suggested by Jennings and Knight(1975). The procedure is as follows:

A sample of an undisturbed soil is cut and fit into a consolidometer ring and loads are appliedprogressively until about 200 kPa (4 kip/ft2) is reached. At this pressure the specimen is floodedwith water for saturation and left for 24 hours. The consolidation test is carried on to its maximumloading. The resulting e-log p curve plotted from the data obtained is shown in Fig. 18.4.

The collapse potential C is then expressed as

(18.2a)

in which Aec = change in void ratio upon wetting, eo = natural void ratio.The collapse potential is also defined as

C -C ~H (18.2b)

where, A//c = change in the height upon wetting, HC - initial height.

\

Pressure P Jog p

Figure 18.4 Typical collapse potential test result

796 Chapter 18

Table 18.1 Collapse potential values

C//0

0-1

1-5

3-1010-20

>20

Severity of problem

No problemModerate troubleTroubleSevere troubleVery severe trouble

Jennings and Knight have suggested some values for collapse potential as shown inTable 18.1. These values are only qualitative to indicate the severity of the problem.

18.4 COMPUTATION OF COLLAPSE SETTLEMENTThe double oedometer method was suggested by Jennings and Knight (1975) for determining aquantitative measure of collapse settlement. The method consists of conducting two consolidationtests. Two identical undisturbed soil samples are used in the tests. The procedure is as follows:

1. Insert two identical undisturbed samples into the rings of two oedometers.

Natural moisturecontent curve

Adjusted curve(curve 2)Curve of sample

soaked for 24 hrs

1 0.2 0.4 0.6 A> 1.00. 6 10 20ton/fr

Figure 18.5 Double consolidation test and adjustments for normally consolidatedsoil (Clemence and Finbarr, 1981)


Soil at naturalmoisture content

Adjusted n.m.c/ curve

Soaked samplefor 24 hours

0.1

Figure 18.6 Double consolidation test and adjustments for overconsolidated soil(Clemence and Finbarr, 1981)

2. Keep both the specimens under a pressure of 1 kN/m2 (= 0.15 lb/in2) for a period of 24hours.

3. After 24 hours, saturate one specimen by flooding and keep the other at its naturalmoisture content

4. After the completion of 24 hour flooding, continue the consolidation tests for both thesamples by doubling the loads. Follow the standard procedure for the consolidation test.

5. Obtain the necessary data from the two tests, and plot e-log p curves for both the samplesas shown in Fig. 18.5 for normally consolidated soil.

6. Follow the same procedure for overconsolidated soil and plot the e-log p curves as shownin Fig. 18.6.

From e-log p plots, obtain the initial void ratios of the two samples after the first 24 hour ofloading. It is a fact that the two curves do not have the same initial void ratio. The total overburdenpressure pQ at the depth of the sample is obtained and plotted on the e-log p curves in Figs 18.5 and18.6. The preconsolidation pressures pc are found from the soaked curves of Figs 18.5 and 18.6 andplotted.

798 Chapter 18

Normally Consolidated Case

For the case in which pc/pQ is about unity, the soil is considered normally consolidated. In such acase, compression takes place along the virgin curve. The straight line which is tangential to thesoaked e-log p curve passes through the point (eQ, p0) as shown in Fig. 18.5. Through the point(eQ, pQ) a curve is drawn parallel to the e-log p curve obtained from the sample tested at naturalmoisture content. The settlement for any increment in pressure A/? due to the foundation load maybe expressed in two parts as

Le H— n c

(18.3a)

where ken = change in void ratio due to load Ap as per the e-log p curve without change inmoisture content

Aec, = change in void ratio at the same load Ap with the increase in moisture content(settlement caused due to collapse of the soil structure)

Hc = thickness of soil stratum susceptible to collapse.

From Eqs (18. 3 a) and (18. 3b), the total settlement due to the collapse of the soil structure is

**„+ *ec) (18.4)

Overconsolidated CaseIn the case of an overconsolidated soil the ratio pc/pQ is greater than unity. Draw a curve from thepoint (eQ, p0) on the soaked soil curve parallel to the curve which represents no change in moisturecontent during the consolidation stage. For any load (pQ + A/?) > pc, the settlement of the foundationmay be determined by making use of the same Eq. (18.4). The changes in void ratios /\en and Aec

are defined in Fig. 18.6.

Example 18.1A footing of size 10 x 10 ft is founded at a depth of 5 ft below ground level in collapsible soil of theloessial type. The thickness of the stratum susceptible to collapse is 30 ft. The soil at the site isnormally consolidated. In order to determine the collapse settlement, double oedometer tests wereconducted on two undisturbed soil samples as per the procedure explained in Section 18.4. The e-log p curves of the two samples are given in Fig. 18.5. The average unit weight of soil y = 106.6 lb/ft3 and the induced stress A/?, at the middle of the stratum due to the foundation pressure, is 4,400lb/ft2 (= 2.20 t/ft2). Estimate the collapse settlement of the footing under a soaked condition.

SolutionDouble consolidation test results of the soil samples are given in Fig. 18.5. Curve 1 was obtainedwith natural moisture content. Curve 3 was obtained from the soaked sample after 24 hours. Thevirgin curve is drawn in the same way as for a normally loaded clay soil (Fig. 7.9a).

The effective overburden pressure p0 at the middle of the collapsible layer is

pQ = 15 x 106.6 = 1,599 lb/ft2 or 0.8 ton/ft2


A vertical line is drawn in Fig. 18.5 atp0 = 0.8 ton/ft2. Point A is the intersection of the verticalline and the virgin curve giving the value of eQ = 0.68. pQ + Ap = 0.8 + 2.2 = 3.0 t/ft2. At(p0 + Ap) = 3 ton/ft2, we have (from Fig. 18.5)

&en = 0.68 - 0.62 = 0.06

Aec = 0.62 -0.48 = 0.14

From Eq. (18.3)

A£A1 + 0.68

0.14x30x12 _n n .= - = 30.00 in.

1 + 0.68

Total settlement Sc = 42.86 in.

The total settlement would be reduced if the thickness of the collapsible layer is less or thefoundation pressure is less.

Example 18.2Refer to Example 18.1. Determine the expected collapse settlement under wetted conditions if thesoil stratum below the footing is overconsolidated. Double oedometer test results are given inFig. 18.6. In this case/?0 = 0.5 ton/ft2, Ap = 2 ton/ft2, and/?c = 1.5 ton/ft2.

Solution

The virgin curve for the soaked sample can be determined in the same way as for an overconsolidatedclay (Fig. 7.9b). Double oedometer test results are given in Fig. 18.6. From this figure:

eQ = 0.6, &en = 0.6 - 0.55 = 0.05, Aec = 0.55 - 0.48 = 0.07

As in Ex. 18.1

^H 005X30X121 + 0.6

Total S = 27.00 in.

18.5 FOUNDATION DESIGNFoundation design in collapsible soil is a very difficult task. The results from laboratory or fieldtests can be used to predict the likely settlement that may occur under severe conditions. In manycases, deep foundations, such as piles, piers etc, may be used to transmit foundation loads to deeperbearing strata below the collapsible soil deposit. In cases where it is feasible to support the structureon shallow foundations in or above the collapsing soils, the use of continuous strip footings mayprovide a more economical and safer foundation than isolated footings (Clemence and Finbarr,1981). Differential settlements between columns can be minimized, and a more equitabledistribution of stresses may be achieved with the use of strip footing design as shown in Fig. 18.7(Clemence and Finbarr, 1981).

800 Chapter 18

Load-bearingbeams

Figure 18.7 Continuous footing design with load-bearing beams for collapsible soil(after Clemence and Finbarr, 1981)

18.6 TREATMENT METHODS FOR COLLAPSIBLE SOILSOn some sites, it may be feasible to apply a pretreatment technique either to stabilize the soil orcause collapse of the soil deposit prior to construction of a specific structure. A great variety oftreatment methods have been used in the past. Moistening and compaction techniques, with eitherconventional impact, or vibratory rollers may be used for shallow depths up to about 1.5 m. Fordeeper depths, vibroflotation, stone columns, and displacement piles may be tried. Heat treatmentto solidify the soil in place has also been used in some countries such as Russia. Chemicalstabilization with the use of sodium silicate and injection of carbon dioxide have been suggested(Semkin et al., 1986).

Field tests conducted by Rollins et al., (1990) indicate that dynamic compaction treatmentprovides the most effective means of reducing the settlement of collapsible soils to tolerable limits.Prewetting, in combination with dynamic compaction, offers the potential for increasingcompaction efficiency and uniformity, while increasing vibration attenuation. Prewetting with a 2percent solution of sodium silicate provides cementation that reduces the potential for settlement.Prewetting with water was found to be the easiest and least costly treatment, but it proved to becompletely ineffective in reducing collapse potential for shallow foundations. Prewetting must beaccompanied by preloading, surcharging or excavation in order to be effective.

PART B—EXPANSIVE SOILS

18.7 DISTRIBUTION OF EXPANSIVE SOILSThe problem of expansive soils is widespread throughout world. The countries that are facingproblems with expansive soils are Australia, the United States, Canada, China, Israel, India, andEgypt. The clay mineral that is mostly responsible for expansiveness belongs to themontmorillonite group. Fig. 18.8 shows the distribution of the montmorillonite group of mineralsin the United States. The major concern with expansive soils exists generally in the western part ofthe United States. In the northern and central United States, the expansive soil problems areprimarily related to highly overconsolidated shales. This includes the Dakotas, Montana, Wyomingand Colorado (Chen, 1988). In Minneapolis, the expansive soil problem exists in the Cretaceous


Figure 18.8 General abundance of montmorillonite in near outcrop bedrockformations in the United States (Chen, 1988)

deposits along the Mississippi River and a shrinkage/swelling problem exists in the lacustrinedeposits in the Great Lakes Area. In general, expansive soils are not encountered regularly in theeastern parts of the central United States.

In eastern Oklahoma and Texas, the problems encompass both shrinking and swelling. In theLos Angeles area, the problem is primarily one of desiccated alluvial and colluvial soils. Theweathered volcanic material in the Denver formation commonly swells when wetted and is a causeof major engineering problems in the Denver area.

The six major natural hazards are earthquakes, landslides, expansive soils, hurricane, tornadoand flood. A study points out that expansive soils tie with hurricane wind/storm surge for secondplace among America's most destructive natural hazards in terms of dollar losses to buildings.According to the study, it was projected that by the year 2000, losses due to expansive soil wouldexceed 4.5 billion dollars annually (Chen, 1988).

18.8 GENERAL CHARACTERISTICS OF SWELLING SOILSSwelling soils, which are clayey soils, are also called expansive soils. When these soils are partiallysaturated, they increase in volume with the addition of water. They shrink greatly on drying anddevelop cracks on the surface. These soils possess a high plasticity index. Black cotton soils foundin many parts of India belong to this category. Their color varies from dark grey to black. It is easyto recognize these soils in the field during either dry or wet seasons. Shrinkage cracks are visible onthe ground surface during dry seasons. The maximum width of these cracks may be up to 20 mm ormore and they travel deep into the ground. A lump of dry black cotton soil requires a hammer tobreak. During rainy seasons, these soils become very sticky and very difficult to traverse.

Expansive soils are residual soils which are the result of weathering of the parent rock. Thedepths of these soils in some regions may be up to 6 m or more. Normally the water table is met atgreat depths in these regions. As such the soils become wet only during rainy seasons and are dry or

802 Chapter 18

Increasing moisture content of soil

Ground surface

D,

D,,< = Unstable zone

Moisture variationduring dry season

Figure 18.9

Stable zone \

Equilibrium moisture content (covered area)Desiccated moisture content (uncovered natural conditions)Wet season moisture content (seasonal variation)Depth of seasonal moisture content fluctuationDepth of desiccation or unstable zone

Moisture content variation with depth below ground surface(Chen, 1988)

partially saturated during the dry seasons. In regions which have well-defined, alternately wet anddry seasons, these soils swell and shrink in regular cycles. Since moisture change in the soils bringabout severe movements of the mass, any structure built on such soils experiences recurringcracking and progressive damage. If one measures the water content of the expansive soils withrespect to depth during dry and wet seasons, the variation is similar to the one shown in Fig. 18.9.

During dry seasons, the natural water content is practically zero on the surface and thevolume of the soil reaches the shrinkage limit. The water content increases with depth and reachesa value wn at a depth D , beyond which it remains almost constant. During the wet season the watercontent increases and reaches a maximum at the surface. The water content decreases with depthfrom a maximum of wn at the surface to a constant value of wn at almost the same depth Dus. Thisindicates that the intake of water by the expansive soil into its lattice structure is a maximum at thesurface and nil at depth Dus. This means that the soil lying within this depth Dus is subjected todrying and wetting and hence cause considerable movements in the soil. The movements areconsiderable close to the ground surface and decrease with depth. The cracks that are developed inthe dry seasons close due to lateral movements during the wet seasons.


The zone which lies within the depth Dus may be called the unstable zone (or active zone) andthe one below this the stable zone. Structures built within this unstable zone are likely to move upand down according to seasons and hence suffer damage if differential movements areconsiderable.

If a structure is built during the dry season with the foundation lying within the unstable zone,the base of the foundation experiences a swelling pressure as the partially saturated soil startstaking in water during the wet season. This swelling pressure is due to constraints offered by thefoundation for free swelling. The maximum swelling pressure may be as high as 2 MPa (20 tsf). Ifthe imposed bearing pressure on the foundation by the structure is less than the swelling pressure,the structure is likely to be lifted up at least locally which would lead to cracks in the structure. Ifthe imposed bearing pressure is greater than the swelling pressure, there will not be any problem forthe structure. If on the other hand, the structure is built during the wet season, it will definitelyexperience settlement as the dry season approaches, whether the imposed bearing pressure is highor low. However, the imposed bearing pressure during the wet season should be within theallowable bearing pressure of the soil. The better practice is to construct a structure during the dryseason and complete it before the wet season.

In covered areas below a building there will be very little change in the moisture contentexcept due to lateral migration of water from uncovered areas. The moisture profile is depicted bycurve 1 in Fig. 18.9.

18.9 CLAY MINERALOGY AND MECHANISM OF SWELLINGClays can be divided into three general groups on the basis of their crystalline arrangement. Theyare:

1. Kaolinite group2. Montmorillonite group (also called the smectite group)3. Illite group.

The kaolinite group of minerals are the most stable of the groups of minerals. The kaolinitemineral is formed by the stacking of the crystalline layers of about 7 A thick one above the otherwith the base of the silica sheet bonding to hydroxyls of the gibbsite sheet by hydrogen bonds.Since hydrogen bonds are comparatively strong, the kaolinite crystals consists of many sheetstackings that are difficult to dislodge. The mineral is, therefore, stable and water cannot enterbetween the sheets to expand the unit cells.

The structural arrangement of the montmorillonite mineral is composed of units made of twosilica tetrahedral sheets with a central alumina-octahedral sheet. The silica and gibbsite sheets arecombined in such a way that the tips of the tetrahedrons of each silica sheet and one of the hydroxyllayers of the octahedral sheet form a common layer. The atoms common to both the silica andgibbsite layers are oxygen instead of hydroxyls. The thickness of the silica-gibbsite-silica unit isabout 10 A. In stacking of these combined units one above the other, oxygen layers of each unit areadjacent to oxygen of the neighboring units, with a consequence that there is a weak bond andexcellent cleavage between them. Water can enter between the sheets causing them to expandsignificantly and thus the structure can break into 10 A thick structural units. The soils containing aconsiderable amount of montmorillonite minerals will exhibit high swelling and shrinkagecharacteristics. The illite group of minerals has the same structural arrangement as themontmorillonite group. The presence of potassium as the bonding materials between units makesthe illite minerals swell less.

804 Chapter 18

18.10 DEFINITION OF SOME PARAMETERSExpansive soils can be classified on the basis of certain inherent characteristics of the soil. It is firstnecessary to understand certain basic parameters used in the classification.

Swelling Potential

Swelling potential is defined as the percentage of swell of a laterally confined sample in anoedometer test which is soaked under a surcharge load of 7 kPa (1 lb/in2) after being compacted tomaximum dry density at optimum moisture content according to the AASHTO compaction test.

Swelling PressureThe swelling pressure /?5, is defined as the pressure required for preventing volume expansion insoil in contact with water. It should be noted here that the swelling pressure measured in alaboratory oedometer is different from that in the field. The actual field swelling pressure is alwaysless than the one measured in the laboratory.

Free Swell

Free swell 5, is defined as

Vf-V.Sf=-^—xm (18.5)

where V{ = initial dry volume of poured soilVr - final volume of poured soil

According toHoltz and Gibbs (1956), 10 cm3 (V.) of dry soil passing thorough a No. 40sieve is poured into a 100 cm3 graduated cylinder filled with water. The volume of settled soilis measured after 24 hours which gives the value of V~ Bentonite-clay is supposed to have afree swell value ranging from 1200 to 2000 percent. The free swell value increases withplasticity index. Holtz and Gibbs suggested that soils having a free-swell value as low as 100percent can cause considerable damage to lightly loaded structures and soils heaving a freeswell value below 50 percent seldom exhibit appreciable volume change even under lightloadings.

18.11 EVALUATION OF THE SWELLING POTENTIAL OFEXPANSIVE SOILS BY SINGLE INDEX METHOD (CHEN, 1988)Simple soil property tests can be used for the evaluation of the swelling potential of expansive soils(Chen, 1988). Such tests are easy to perform and should be used as routine tests in the investigationof building sites in those areas having expansive soil. These tests are

1. Atterberg limits tests2. Linear shrinkage tests

3. Free swell tests4. Colloid content tests

Atterberg LimitsHoltz and Gibbs (1956) demonstrated that the plasticity index, Ip, and the liquid limit, w/5 are usefulindices for determining the swelling characteristics of most clays. Since the liquid limit and the


Note: Percent swell measured under 1 psi surcharge for sample compacted ofoptimum water content to maximum density in standard RASHO test

Clay component: commercial bentonite

1:1 Commercial illite/bentonite

6:1 Commercial kaolinite/bentonite

3:1 Commercial illite/bentoniteI I

•— Commercial illite

1:1 Commercial illite/kaolinite

Commercial kaolinite

30 40 50 60 70Percent clay sizes (finer than 0.002 mm)

80 90 100

Figure 18.10 Relationship between percentage of swell and percentage of claysizes for experimental soils (after Seed et al., 1962)

swelling of clays both depend on the amount of water a clay tries to absorb, it is natural that they arerelated. The relation between the swelling potential of clays and the plasticity index has beenestablished as given in Table 18.2

Linear ShrinkageThe swell potential is presumed to be related to the opposite property of linear shrinkage measuredin a very simple test. Altmeyer (1955) suggested the values given in Table 18.3 as a guide to thedetermination of potential expansiveness based on shrinkage limits and linear shrinkage.

Colloid Content

There is a direct relationship between colloid content and swelling potential as shown in Fig. 18.10(Chen, 1988). For a given clay type, the amount of swell will increase with the amount of claypresent in the soil.

Table 18.2 Relation between swelling potential and plasticity index, /

Plasticity index lp (%) Swelling potential

0-15

10-35

20-5535 and above

Low

MediumHighVery high

806 Chapter 18

Table 18.3 Relation between swelling potential, shrinkage limits, and linear shrinkage

Shrinkage limit % Linear shrinkage % Degree of expansion

10-12

> 12

>8

5-8

0-5

CriticalMarginalNon-critical

18.12 CLASSIFICATION OF SWELLING SOILS BY INDIRECTMEASUREMENTBy utilizing the various parameters as explained in Section 18.11, the swelling potential can beevaluated without resorting to direct measurement (Chen, 1988).

USBR MethodHoltz and Gibbs (1956) developed this method which is based on the simultaneous consideration ofseveral soil properties. The typical relationships of these properties with swelling potential areshown in Fig. 18.11. Table 18.4 has been prepared based on the curves presented in Fig. 18.11 byHoltz and Gibbs (1956).

The relationship between the swell potential and the plasticity index can be expressed asfollows (Chen, 1988)

(18.6)

where, A = 0.0838B = 0.2558/ = plasticity index.

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chan

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) 20 40 0' 20 40 0 8 16 24Colloid content Plasticity index Shrinkage limit (%)

(% less than 0.001 mm)

(a) (b) (c)

Figure 18.11 Relation of volume change to (a) colloid content, (b) plasticity index,and (c) shrinkage limit (air-dry to saturated condition under a load of 1 Ib per sq in)

(Holtz and Gibbs, 1956)


Table 18.4 Data for making estimates of probable volume changes for expansivesoils (Source: Chen, 1988)

Data from index tests*

Colloid content, per-

cent minus 0.001 mm

>28

20-1313-23< 15

Plasticity

index

>35

25-4115-28<18

Shrinkage

limit

<11

7-1210-16>15

Probable expansion,

percent total

vol. change

>30

20-30

10-30<10

Degree of

expansion

Very high

High

Medium

Low

*Based on vertical loading of 1.0 psi. (after Holtz and Gibbs, 1956)

10

a 5

030 35 4015 20 25

Plasticity index (%)1. Holtz and Gibbs (Surcharge pressure 1 psi)2. Seed, Woodward and Lundgren (Surcharge pressure 1 psi)3. Chen (Surcharge pressure 1 psi)4. Chen (Surcharge pressure 6.94 psi)

Figure 18.12 Relationships of volume change to plasticity index(Source: Chen, 1988)

808 Chapter 18

Figure 18.12 shows that with an increase in plasticity index, the increase of swelling potentialis much less than predicted by Holtz and Gibbs. The curves given by Chen (1988) are based onthousands of tests performed over a period of 30 years and as such are more realistic.

Activity MethodSkempton (1953) defined activity by the following expression

A = ~^ (18.7)

where / = plasticity indexC = percentage of clay size finer than 0.002 mm by weight.

The activity method as proposed by Seed, Woodward, and Lundgren, (1962) was based onremolded, artificially prepared soils comprising of mixtures of bentonite, illite, kaolinite and finesand in different proportions. The activity for the artificially prepared sample was defined as

activity A = (18.9)C-n

where n - 5 for natural soils and, n = 10 for artificial mixtures.The proposed classification chart is shown in Fig. 18.13. This method appears to be an

improvement over the USER method.

Swelling potential = 25%Swelling potential = 5%Swelling potential = 1.5%

Figure 18.13

20 30 40 50 60 70Percent clay sizes (finer than 0.002 mm)

Classification chart for swelling potential (after Seed, Woodward, andLundgren, 1962)


The Potential Volume Change Method (PVC)A determination of soil volume change was developed by Lambe under the auspices of the FederalHousing Administration (Source: Chen, 1988). Remolded samples were specified. The procedureis as given below.

The sample is first compacted in a fixed ring consolidometer with a compaction effect of55,000 ft-lb per cu ft. Then an initial pressure of 200 psi is applied, and water added to the samplewhich is partially restrained by vertical expansion by a proving ring. The proving ring reading istaken at the end of 2 hours. The reading is converted to pressure and is designated as the swell index.From Fig. 18.14, the swell index can be converted to potential volume change. Lambe establishedthe categories of PVC rating as shown in Table 18.5.

The PVC method has been widely used by the Federal Housing Administration as well as theColorado State Highway Department (Chen, 1988).

ouuu

*7nnn/ \j\j\j

<£H

CT

| 4000_c

I

2000

1000

200

(

/

^

,

/

^

/

/

. — —

^/

/

— <*

•^//

/

/̂

/

'/

*/

/

'

/

I

/

/

/

I

1

) 1 2 3 4 5 6 7 8 9 1 0 1 1 1Non

critical Marginal Critical Very Critical

Potential Volume Change (PVC)

Figure 18.14 Swell index versus potential volume change (from 'FHA soil PVCmeter publication,' Federal Housing Administration Publication no. 701) (Source:

Chen, 1988)

810 Chapter 18

Table 18.5 Potential volume change rating (PVC)

PVC rating

Less than 2

2^

4-6

> 6

Category

Non-critical

Marginal

CriticalVery critical

(Source: Chen, 1988)

Figure 18.15(a) shows a soil volume change meter (ELE International Inc). This metermeasures both shrinkage and swelling of soils, ideal for measuring swelling of clay soils, and fastand easy to operate.

Expansion Index (El)-Chen (1988)The ASTM Committee on Soil and Rock suggested the use of an Expansion Index (El) as a unifiedmethod to measure the characteristics of swelling soils. It is claimed that the El is a basic indexproperty of soil such as the liquid limit, the plastic limit and the plasticity index of the soil.

The sample is sieved through a No 4 sieve. Water is added so that the degree of saturation isbetween 49 and 51 percent. The sample is then compacted into a 4 inch diameter mold in two layersto give a total compacted depth of approximates 2 inches. Each layer is compacted by 15 blows of5.5 Ib hammer dropping from a height of 12 inches. The prepared specimen is allowed toconsolidate under 1 lb/in2 pressure for a period of 10 minutes, then inundated with water until therate of expansion ceases.

The expansion index is expressed as

£/ = — xlOOO (189)hi

where A/I = change in thickness of sample, in.

h. = initial thickness of sample, in.The classification of a potentially expansive soil is based on Table 18.6.This method offers a simple testing procedure for comparing expansive soil characteristics.Figure 18.15(b) shows an ASTMD-829 expansion index test apparatus (ELE International

Inc). This is a completely self-contained apparatus designed for use in determining the expansionindex of soils.

Table 18.6 Classification of potentially expansive soil

Expansion Index, El Expansion potential

0-20 Very low21-50 Low

51-90 Medium

91-130 High

> 130 Very high

(Source: Chen, 1988)


Swell IndexVijayvergiya and Gazzaly (1973) suggested a simple way of identifying the swell potential ofclays, based on the concept of the swell index. They defined the swell index, Is, as follows

-*~~ (18-10)

where wn = natural moisture content in percent\vl = liquid limit in percent

The relationship between Is and swell potential for a wide range of liquid limit is shown inFig 18.16. Swell index is widely used for the design of post-tensioned slabs on expansive soils.

Prediction of Swelling PotentialPlasticity index and shrinkage limit can be used to indicate the swelling characteristics of expansivesoils. According to Seed at al., (1962), the swelling potential is given as a function of the plasticityindex by the formula

(18.11)

Figure 18.15 (a) Soil volume change meter, and (b) Expansion index test apparatus(Courtesy: Soiltest)

812 Chapter 18

u. /

0.6

0.5

{' 0.4<j

s 0.3

0.2

0.1

0.03

^

PercSwe

Percent swell: < 1Swell pressure: <

^ 1̂ ^ent swell: 1 to 411 pressure: 0.3 to 1.25

0.3 ton/sq f

-

ton/sq ft

— L 4- •Percent swell: 4 to 10Swell pressure: 1 .25 to 3 ton/sq ft

^MPerce

Swell

^~~"1nt swell: >pressure: ;

-~

10> 3 ton/sq fl

t

** —

— • — •

0 40 50 60 70 8(Liquid limit

Figure 18.16 Relationship between swell index and liquid limit for expansive clays(Source: Chen, 1988)

where S = swelling potential in percent/ = plasticity index in percentk = 3.6 xlO~5, a factor for clay content between 8 and 65 percent.

18.13 SWELLING PRESSURE BY DIRECT MEASUREMENTASTM defines swelling pressure which prevents the specimen from swelling or that pressurewhich is required to return the specimen to its original state (void ratio, height) after swelling.Essentially, the methods of measuring swelling pressure can be either stress controlled or straincontrolled (Chen, 1988).

In the stress controlled method, the conventional oedometer is used. The samples are placedin the consolidation ring trimmed to a height of 0.75 to 1 inch. The samples are subjected to avertical pressure ranging from 500 psf to 2000 psf depending upon the expected field conditions.On the completion of consolidation, water is added to the sample. When the swelling of the samplehas ceased the vertical stress is increased in increments until it has been compressed to its originalheight. The stress required to compress the sample to its original height is commonly termed thezero volume change swelling pressure. A typical consolidation curve is shown in Fig 18.17.


II

30

20

'-3 10ta

OU

-10

9^^•xj

Placement conditioDry densityMoisture contentAtterberg limits:

"X

T

F

N> \ss

\^

ns= 76.3 p

\A. n oLiquid 1Plasticit

s

°N

cffcimit = 68%y index = 17%

\N

0.8% expansion at 100 psf>ressure when wetted

)

100 1000 10000Applied pressure (psf)

Figure 18.17 Typical stress controlled swell-consolidation curve

Prediction of Swelling Pressure

Komornik et al., (1969) have given an equation for predicting swelling pressure as

\ogps = 2.132 + 0.0208W, +0.00065^ -0.0269wn

where ps = swelling pressure in kg/cm2

w; = liquid limit (%)wn = natural moisture content (%)yd = dry density of soil in kg/cm3

(18.12)

18.14 EFFECT OF INITIAL MOISTURE CONTENT AND INITIALDRY DENSITY ON SWELLING PRESSUREThe capability of swelling decreases with an increase of the initial water content of a given soilbecause its capacity to absorb water decreases with the increase of its degree of saturation. It wasfound from swelling tests on black cotton soil samples, that the initial water content has a smalleffect on swelling pressure until it reaches the shrinkage limit, then its effect increases (Abouleid,1982). This is depicted in Fig. 18.18(a).

The effect of initial dry density on the swelling percent and the swelling pressure increaseswith an increase of the dry density because the dense soil contains more clay particles in a unitvolume and consequently greater movement will occur in a dense soil than in a loose soil upon

814 Chapter 18

00

on

18

16

H

1?

10

9

f.

A

0

n

• *->

\>

S.L.

I

\

^

\\NV

^»0 2 4 6 8 10 12 14 16 18 20 22 24

Water content %

Black cotton soilw. = 90, wp = 30, w, = 10

Mineralogy: Largely sodium, Montmorillonite

2.0

rs,0j*t

|C/3

a L0D.

0.0

zS.I,

1.2 .4 1.6 1.8 2.0 2.2

Dry density (t/m3)

(b)

Figure 18.18 (a) effect of initial water content on swelling pressure of black cottonsoil, and (b) effect of initial dry density on swelling pressure of black cotton soil

(Source: Abouleid, 1982)

wetting (Abouleid, 1982). The effect of initial dry density on swelling pressure is shown inFig. 18.18(b).

18.15 ESTIMATING THE MAGNITUDE OF SWELLINGWhen footings are built in expansive soil, they experience lifting due to the swelling or heaving ofthe soil. The amount of total heave and the rate of heave of the expansive soil on which a structureis founded are very complex. The heave estimate depends on many factors which cannot be readilydetermined. Some of the major factors that contribute to heaving are:

1. Climatic conditions involving precipitation, evaporation, and transpiration affect themoisture in the soil. The depth and degree of desiccation affect the amount of swell in agiven soil horizon.

2. The thickness of the expansive soil stratum is another factor. The thickness of the stratum iscontrolled by the depth to the water table.

3. The depth to the water table is responsible for the change in moisture of the expansive soillying above the water table. No swelling of soil takes place when it lies below the watertable.

4. The predicted amount of heave depends on the nature and degree of desiccation of the soilimmediately after construction of a foundation.

5. The single most important element controlling the swelling pressure as well as the swellpotential is the in-situ density of the soil. On the completion of excavation, the stresscondition in the soil mass undergoes changes, such as the release of stresses due to elastic


rebound of the soil. If construction proceeds without delay, the structural load compensatesfor the stress release.

6. The permeability of the soil determines the rate of ingress of water into the soil either bygravitational flow or diffusion, and this in turn determines the rate of heave.

Various methods have been proposed to predict the amount of total heave under a givenstructural load. The following methods, however, are described here.

1. The Department of Navy method (1982)2. The South African method [also known as the Van Der Merwe method (1964)]

The Department of Navy Method

Procedure for Estimating Total Swell under Structural Load

1. Obtain representative undisturbed samples of soil below the foundation level at intervals ofdepth. The samples are to be obtained during the dry season when the moisture contentsare at their lowest.

2. Load specimens (at natural moisture content) in a consolidometer under a pressure equal tothe ultimate value of the overburden plus the weight of the structure. Add water to saturatethe specimen. Measure the swell.

3. Compute the final swell in terms of percent of original sample height.4. Plot swell versus depth.5. Compute the total swell which is equal to the area under the percent swell versus depth

curve.

Procedure for Estimating UndercutThe procedure for estimating undercut to reduce swell to an allowable value is as follows:

1. From the percent swell versus depth curve, plot the relationship of total swell versus depthat that height. Total swell at any depth equals area under the curve integrated upward fromthe depth of zero swell.

2. For a given allowable value of swell, read the amount of undercut necessary from the totalswell versus depth curve.

Van Der Merwe Method (1964)Probably the nearest practical approach to the problem of estimating swell is that of Van DerMerwe. This method starts by classifying the swell potential of soil into very high to low categoriesas shown in Fig. 18.19. Then assign potential expansion (PE) expressed in in./ft of thickness basedon Table 18.7.

Table 18.7 Potential expansion

Swell potential Potential expansion (PE) in./ft

Very high 1

High 1/2

Medium 1/4

Low 0

816 Chapter 18

Reduction factor, F0.2 0.4 0.6 0.8

PuQ -16 -

10 20 30 40 50 60 70

Clay fraction of whole sample (% < 2 micron)

(a) (b)

Figure 18.19 Relationships for using Van Der Merwe's prediction method:(a) potential expansiveness, and (b) reduction factor (Van der Merwe, 1964)

u

-2

-4

-6

-8

-10

-12

-16

-18on-ZU

-22

-24

-26

-28

^n

I

-

-

-

-

-

-

/

I

,/

I

>

/

\

./

'

ix^"1.0

Procedure for Estimating Swell

1 . Assume the thickness of an expansive soil layer or the lowest level of ground water.2. Divide this thickness (z) into several soil layers with variable swell potential.

-3. The total expansion is expressed as

i=n

A// = A

where A//e = total expansion (in.)

A. =(P£).(AD)I.(F).

(18.13)

(18.14)

"1

(F),. = log" -- '- - reduction factor for layer /.

z = total thickness of expansive soil layer (ft)

D{ = depth to midpoint of i th layer (ft)

(AD). = thickness of i th layer (ft)

Fig. 18.19(b) gives the reduction factor plotted against depth.


Outside brick wall

Cement concreteapron with lightreinforcement

RCCfoundation

•;^r -, ~A • •'„» •„-- •'/W\//^\^Sv /#<Ns

Granular fill

Figure 18.20 Foundation in expansive soil

18.16 DESIGN OF FOUNDATIONS IN SWELLING SOILSIt is necessary to note that all parts of a building will not equally be affected by the swellingpotential of the soil. Beneath the center of a building where the soil is protected from sun and rainthe moisture changes are small and the soil movements the least. Beneath outside walls, themovement are greater. Damage to buildings is greatest on the outside walls due to soil movements.

Three general types of foundations can be considered in expansive soils. They are

1. Structures that can be kept isolated from the swelling effects of the soils2. Designing of foundations that will remain undamaged in spite of swelling3. Elimination of swelling potential of soil.

All three methods are in use either singly or in combination, but the first is by far the mostwidespread. Fig. 18.20 show a typical type of foundation under an outside wall. The granular fillprovided around the shallow foundation mitigates the effects of expansion of the soils.

18.17 DRILLED PIER FOUNDATIONSDrilled piers are commonly used to resist uplift forces caused by the swelling of soils. Drilled piers,when made with an enlarged base, are called, belled piers and when made without an enlarged baseare referred to as straight-shaft piers.

Woodward, et al., (1972) commented on the empirical design of piers: "Many piers,particularly where rock bearing is used, have been designed using strictly empirical considerationswhich are derived from regional experience". They further stated that "when surface conditions arewell established and are relatively uniform, and the performance of past constructions welldocumented, the design by experience approach is usually found to be satisfactory."

The principle of drilled piers is to provide a relatively inexpensive way of transferring thestructural loads down to stable material or to a stable zone where moisture changes are improbable.

818 Chapter 18

There should be no direct contact between the soil and the structure with the exception of the soilssupporting the piers.

Straight-shaft Piers in Expansive SoilsFigure 18.21 (a) shows a straight-shaft drilled pier embedded in expansive soil. The followingnotations are used.

Lj = length of shaft in the unstable zone (active zone) affected by wetting.L2 = length of shaft in the stable zone unaffected by wettingd = diameter of shaftQ - structural dead load = qAb

q - unit dead load pressure andAb = base area of pierWhen the soil in the unstable zone takes water during the wet season, the soil tries to expand

which is partially or wholly prevented by the rough surface of the pile shaft of length Lj. As a resultthere will be an upward force developed on the surface of the shaft which tries to pull the pile out ofits position. The upward force can be resisted in the following ways.

1. The downward dead load Q acting on the pier top2. The resisting force provided by the shaft length L2 embedded in the stable zone.

Two approaches for solving this problem may be considered. They are

1. The method suggested by Chen (1988)2. The O'Neill (1988) method with belled pier.

Two cases may be considered. They are

1. The stability of the pier when no downward load Q is acting on the top. For this conditiona factor of safety of 1.2 is normally found sufficient.

2. The stability of the pier when Q is acting on the top. For this a value of F? = 2.0 is used.

Equations for Uplift Force Qup

Chen (1988) suggested the following equation for estimating the uplift force Q

QuP=7lda

uPsLi (18.15)

where d - diameter of pier shafta = coefficient of uplift between concrete and soil = 0.15ps = swelling pressure

= 10,000 psf (480 kN/m2) for soil with high degree of expansion= 5,000 (240 kN/m2) for soil with medium degree of expansion

The depth (Lj) of the unstable zone (wetting zone) varies with the environmental conditions.According to Chen (1988) the wetting zone is limited only to the upper 5 feet of the pier. It ispossible for the wetting zone to extend beyond 10-15 feet in some countries and limiting the depthof unstable zone to a such a low value of 5 ft may lead to unsafe conditions for the stability ofstructures. However, it is for designers to decide this depth Lj according to local conditions. Withregards to swelling pressure ps, it is unrealistic to fix any definite value of 10,000 or 5,000 psf forall types of expansive soils under all conditions of wetting. It is also not definitely known if theresults obtained from laboratory tests truly represent the in situ swelling pressure. Possibly one wayof overcoming this complex problem is to relate the uplift resistance to undrained cohesive strength


of soil just as in the case of friction piers under compressive loading. Equation (18.15) may bewritten as

/") — 'TT/y/V s* f / 1 O 1 /^ \^«p ~ i w 1 (18.lu)

where c^ = adhesion factor between concrete and soil under a swelling conditioncu = unit cohesion under undrained conditionsIt is possible that the value of a? may be equal to 1.0 or more according to the swelling type

and environmental conditions of the soil. Local experience will help to determine the value of ocThis approach is simple and pragmatic.

Resisting Force

The length of pier embedded in the stable zone should be sufficient to keep the pier being pulled outof the ground with a suitable factor of safety. If L2 is the length of the pier in the stable zone, theresisting force QR is the frictional resistance offered by the surface of the pier within the stablezone. We may write

QR=7tiL2acu (18.17)

where a = adhesion factor under compression loading

cu = undrained unit cohesion of soilThe value of a may be obtained from Fig. 17.15.Two cases of stability may be considered:

1. Without taking into account the dead load Q acting on the pier top, and using F = 1.2

QUP =ff (18.18a)

2. By taking into account the dead load Q and using Fs = 2.0

QK(<2M/7 - 0 = yf (18.18b)

For a given shaft diameter d equations (18.18a) and (18.18b) help to determine the length L2

of the pier in the stable zone. The one that gives the maximum length L2 should be used.

Belled PiersPiers with a belled bottom are normally used when large uplift forces have to be resisted.Fig. 18.21(b) shows a belled pier with all the forces acting.

The uplift force for a belled pier is the same as that applicable for a straight shaft. Theresisting force equation for the pier in the stable zone may be written as (O'Neill, 1988)

QRl=xdL2acu (18.19a)

7T r ~ ~ i r ^(18.19b)

where<N - bearing capacity factord, = diameter of the underream

D

820 Chapter 18

L =

L2

Q Q

/Ws\ //X\\

"

, 1

d

//x6\ //x\\ //X^\ /XA\

QuP UnstableUnstable zone

zone

QRStablezone i i

H

J

/

L_

f'!

it1iiii//

d

\

\\

\1

!1lllls\

'/WS //XS\ ;

f~ up L

QR\

O

I I

^

j

L\

(a) (b)

Figure 18.21 Drilled pier in expansive soil

c = unit cohesion under undrained condition7 = unit weight of soilThe values of NC are given in Table 18.8 (O'Neill, 1988)Two cases of stability may be written as before.

1. Without taking the dead load Q and using F - 1.2

2. By taking into account the dead load and Fs = 2.0

Table 18.8 Values of N.

1.7

2.5

>5.0


Dead load Dead load

•3 ^g1) c; JD

2r

Reinforcementfor tension

Grade beam

Air space beneath grade beam

Swellingpressure -J

(X

Skin friction /s

kr Reinforcementfor tension

Uplifting pressure

Resisting force

2^? = ^fc

Bell-pier foundationStraight shaft-pier foundation

Figure 18.22 Grade beam and pier system (Chen, 1988)

For a given shaft diameter d and base diameter db, the above equations help to determine thevalue of LT The one that gives the maximum value for L2 has to be used in the design.

Fig. 18.22 gives a typical foundation design with grade beams and drilled piers (Chen,1988). The piers should be taken sufficiently below the unstable zone of wetting in order to resistthe uplift forces.

Example 18.3

A footing founded at a depth of 1 ft below ground level in expansive soil was subjected to loadsfrom the superstructure. Site investigation revealed that the expansive soil extended to a depth of 8ft below the base of the foundation, and the moisture contents in the soil during the constructionperiod were at their lowest. In order to determine the percent swell, three undisturbed samples atdepths of 2,4 and 6 ft were collected and swell tests were conducted per the procedure described inSection 18.16. Fig. Ex. 18.3a shows the results of the swell tests plotted against depth. A linepassing through the points is drawn. The line indicates that the swell is zero at 8 ft depth andmaximum at a base level equal to 3%. Determine (a) the total swell, and (b) the depth of undercutnecessary for an allowable swell of 0.03 ft.

Solution

(a) The total swell is equal to the area under the percent swell versus depth curve inFig. Ex. 18.3a.Total swell = 1/2 x 8 x 3 x 1/100 = 0.12 ft

822 Chapter 18

Base of structure Base of structure

®Swell determined from test

1 2Percent swell

(a) Estimation of total swell

0.04 0.120.08Total swell, ft

(b) Estimation of depth of undercut

0.16

Figure Ex. 18.3

(b) Depth of undercut

From the percent swell versus depth relationship given by the curve in Fig. 18.3a, totalswell at different depths are calculated and plotted against depth in Fig. 18.3b. For examplethe total swell at depth 2 ft below the foundation base is

Total swell = 1/2 (8 - 2) x 2.25 x 1/100 = 0.067 ft plotted against depth 2 ft. Similarly totalswell at other depths can be calculated and plotted. Point B on curve in Fig. 18.3b gives theallowable swell of 0.03 ft at a depth of 4 ft below foundation base. That is, the undercutnecessary in clay is 4 ft which may be replaced by an equivalent thickness of nonswellingcompacted fill.

Example 18.4Fig. Ex. 18.4 shows that the soil to a depth of 20 ft is an expansive type with different degrees ofswelling potential. The soil mass to a 20 ft depth is divided into four layers based on the swellpotential rating given in Table 18.7. Calculate the total swell per the Van Der Merwe method.

Solution

The procedure for calculating the total swell is explained in Section 18.16. The details of thecalculated results are tabulated below.


Potential expansionF - factor

0.25 0.5 0.75 1.0 1.5/z^/rs^?^

Layer 1 5

Layer 2 8

;Layer 3 2

i/

Layer 4 5

GWT V i

i/S^^Sx

ft Low

ft Very high

ft High

ft Very high

Lowest

(a)

The

U

5

u<£•B 10o,uQ

15

20

Figure 1

details of c

1

»/n i£

*V0.20

-Wo. 13

^,n\ 7«r

/

F=lo

1

g'1 (-D/20)

(b)

Ex. 18.4

alculated results

Layer No. Thickness PE D F &HS ( i n . )AD(ft) ft

1 5 0 2.5 0.75

2 8 1.0 9.0 0.35

3 2 0.5 14.0 0.20

4 5 1.0 17.5 0.13

0

2.80

0.20

0.65

Total 3.65

In the table above D = depth from ground level to the mid-depth of the layer considered. F = reduction factor.

Example 18.5A drilled pier [refer to Fig. 18.21 (a)] was constructed in expansive soil. The water table was notencountered. The details of the pier and soil are:

L = 20 ft, d= 12 in., L{ = 5 ft, L2 = 15 ft,/?, = 10,000 lb/ft2, cu = 2089 lb/ft2, SPT(N) = 25 blowsper foot,

Required:

(a) total uplift capacity Qu

(b) total resisting force due to surface friction

824 Chapter 18

(c) factor of safety without taking into account the dead load Q acting on the top of the pier

(d) factor of safety with the dead load acting on the top of the pier

Assume Q = 10 kips. Calculate Qu by Chen's method (Eq. 18.15).

Solution

(a) Uplift force Q from Eq. (18.15)

. 3.14 x(l)x 0.15x10,000x5O = TtdapL, = — = 23.55 kipsup u s l 1000

(b) Resisting force QR

FromEq. (18.17)

QR = nd(L-Ll)ucu

where cu = 2089 lb/ft2 - 100 kN/m2

—£- = a 1.0 where p = atmospheric pressure =101 kPaPa 101

From Fig. 17.15, a = 0.55 for cjpa = 1.0

Now substituting the known values

QR = 3.14 x 1 x (20 - 5) x 0.55 x 2000 = 51,810 Ib = 52 kips

Q

z

L

'

y//$\/A

j\

1

'2

UnstableQUp zone

Stablezone

.t

Figure Ex. 18.5


(c) Factor of safety with Q = 0FromEq. (18.18a)

F = ®R_ = _^L = 2.2 > 1.2 required - -OK.S QUP 23.5

(d) Factor of safety with Q= 10 kipsFromEq. (18.1 8b)

= — = 3.9 > 2.0 required - -OK.(Qu-Q) (23.5-10) 13.5up

Example 18.6Solve Example 18.5 with Lj = 10 ft. All the other data remain the same.

Solution

(a) Uplift force Qup

Qup = 23.5 x (10/5) = 47.0 kips

where Q = 23.5 kips for Lj = 5 ft(b) Resisting force QR

QR = 52 x (10/15) = 34.7 kipswhere QR = 52 kips for L2 = 15 ft

(c) Factor of safety for Q = 0

347F = — — = 0.74 < 1.2 as required - - not OK.s 47.0 M

(d) Factor of safety for Q = 10 kips

34 7 34 7F. = - : - = — — = 0.94 < 2.0 as required - - not OK.s (47-10) 37

The above calculations indicate that if the wetting zone (unstable zone) is 10 ft thick thestructure will not be stable for L = 20 ft.

Example 18.7Determine the length of pier required in the stable zone for Fs=l.2 where (2 = 0 and FS = 2.0 when2=10 kips. All the other data given in Example 18.6 remain the same.

Solution

(a) Uplifting force Qup for L, (10 ft) = 47 kips(b) Resisting force for length L2 in the stable zone.

Qp = n , a c L, = 3.14 x 1 x 0.55 x 2000 L, = 3,454 L, lb/ft2*^A u U L 2. L

(c) Q = 0, minimum F = 1.2

826 Chapter 18

or L2 = ̂ = 3.454 L2

Q,v 47-ooo

solving we have L2 - 16.3 ft.

(d) Q = 10 kips. Minimum Fs = 2.0

QR 3,454 L2 3,454 L2F =2.0 =

(47,000-10,000) 37,000

Solving we have L2 = 21.4 ft.

The above calculations indicate that the minimum L9 = 21.4 ft or say 22 ft is required forthe structure to be stable with L{ = 10 ft. The total length L = 10 + 22 = 32 ft.

Example 18.8Figure Ex. 18.8 shows a drilled pier with a belled bottom constructed in expansive soil. The watertable is not encountered. The details of the pier and soil are given below:

Ll - 10 ft, L2 = 10 ft, Lb = 2.5 ft,d= 12 in., db = 3 ft, cu = 2000 lb/ft2, p^ = 10,000 lb/ft2, y= 1 10lb/ft3.

Required



(c) Factor of safety for Q = 0 at the top of the pier

(d) Factor of safety for Q = 20 kips at the top of the pier

Solution


As in Ex. 18.6£up = 47kips


QRl=ndL2acu

a = 0.55 as in Ex. 18.5

Substituting known values

QRl = 3.14 x 1 x 10 x 0.55 x 2000 = 34540 Ibs = 34.54 kips

where db = 3 ft, c = 2000 lb/ft2, NC = 7.0 from Table 18.8 for L2/db = 10/3 = 3.33

Substituting known values

QR2 = — [32 - (I)2 ] [2000 x 7.0 +110 x 10]

= 6.28 [15,100] = 94,828 Ibs = 94.8 kips

QR = QRl + QR2 = 34.54 + 94.8 = 129. 3 kips

(c) Factor of safety for Q - 0

OP 1293F - R = = 275 > 1 2 --OKA „ X—. / *J -"̂ i*^ *-S A*.

<2UP 47.0


!G

I,= 10ft

= 10 ft

Unstablezone

QU

Stable zone

QK

Figure Ex. 18.8

(d) Factor of safety for Q = 20 kips

h 129.3F = •

(47-20)= 4.79 > 2.0 - - as required OK.

The above calculations indicate that the design is over conservative. The length L2 can bereduced to provide an acceptable factor of safety.

18.18 ELIMINATION OF SWELLINGThe elimination of foundation swelling can be achieved in two ways. They are

1. Providing a granular bed and cover below and around the foundation (Fig. 18.19)2. Chemical stabilization of swelling soils

Figure 18.19 gives a typical example of the first type. In this case, the excavation is carriedout up to a depth greater than the width of the foundation by about 20 to 30 cms. Freely drainingsoil, such as a mixture of sand and gravel, is placed and compacted up to the base level of thefoundation. A Reinforced concrete footing is constructed at this level. A mixture of sand and gravelis filled up loosely over the fill. A reinforced concrete apron about 2 m wide is provided around thebuilding to prevent moisture directly entering the foundation. A cushion of granular soils below thefoundation absorbs the effect of swelling, and thereby its effect on the foundation will considerably

828 Chapter 18

be reduced. A foundation of this type should be constructed only during the dry season when thesoil has shrunk to its lowest level. Arrangements should be made to drain away the water from thegranular base during the rainy seasons.

Chemical stabilization of swelling soils by the addition of lime may be remarkably effectiveif the lime can be mixed thoroughly with the soil and compacted at about the optimum moisturecontent. The appropriate percentage usually ranges from about 3 to 8 percent. The lime content isestimated on the basis of pH tests and checked by compacting, curing and testing samples in thelaboratory. The lime has the effect of reducing the plasticity of the soil, and hence its swellingpotential.

18.19 PROBLEMS

18.1 A building was constructed in a loessial type normally consolidated collapsible soil withthe foundation at a depth of 1 m below ground level. The soil to a depth of 6 m below thefoundation was found to be collapsible on flooding. The average overburden pressure was56 kN/m2. Double consolidometer tests were conducted on two undisturbed samples takenat a depth of 4 m below ground level, one with its natural moisture content and the otherunder soaked conditions per the procedure explained in Section 18.4. The following datawere available.

Applied pressure, kN/m2 10 20 40 100 200 400 800

Void ratios at naturalmoisture content 0.80 0.79 0.78 0.75 0.725 0.68 0.61

Void ratios in thesoaked condition 0.75 0.71 0.66 0.58 0.51 0.43 0.32

Plot the e-log p curves and determine the collapsible settlement for an increase in pressureAp = 34 kN/m2 at the middle of the collapsible stratum.

18.2 Soil investigation at a site indicated overconsolidated collapsible loessial soil extending toa great depth. It is required to construct a footing at the site founded at a depth of 1.0 mbelow ground level. The site is subject to flooding. The average unit weight of the soil is19.5 kN/m3. Two oedometer tests were conducted on two undisturbed samples taken at adepth of 5 m from ground level. One test was conducted at its natural moisture content andthe other on a soaked condition per the procedure explained in Section 18.4.The following test results are available.

Applied pressure, kN/m2 10 20 40 100 200 400 800 2000

Void ratio under naturalmoisture condition 0.795 0.79 0.787 0.78 0.77 0.74 0.71 0.64

Void ratio undersoaked condition 0.775 0.77 0.757 0.730 0.68 0.63 0.54 0.37

The swell index determined from the rebound curve of the soaked sample is equal to 0.08.Required:(a) Plots of e-log p curves for both tests.(b) Determination of the average overburden pressure at the middle of the soil stratum.(c) Determination of the preconsolidation pressure based on the curve obtained from the

soaked sample(d) Total collapse settlement for an increase in pressure A/? = 710 kN/m2?


18.3

18.4

A footing for a building is founded 0.5 m below ground level in an expansive clay stratumwhich extends to a great depth. Swell tests were conducted on three undisturbed samplestaken at different depths and the details of the tests are given below.

Depth (m)below GL

123

Swell %

2.91.750.63

Required:(a) The total swell under structural loadings(b) Depth of undercut for an allowable swell of 1 cmFig. Prob. 18.4 gives the profile of an expansive soil with varying degrees of swellingpotential. Calculate the total swell per the Van Der Merwe method.

Potential expansion rating/XXVX /XXVS //XS\ //"/VS

Layer 1

Layer 2

Layer 3

Layer 4

/XA\ /X/VN/XXSN/yW, S/*\

6 ft High

4 ft Low

8 ft Very high

6 ft High

Figure Prob. 18.4

18.5 Fig. Prob. 18.5 depicts a drilled pier embedded in expansive soil. The details of the pier andsoil properties are given in the figure.Determine:(a) The total uplift capacity.(b) Total resisting force.(c) Factor of safety with no load acting on the top of the pier.(d) Factor of safety with a dead load of 100 kN on the top of the pier.Calculate Qu by Chen's method.

18.6 Solve problem 18.5 using Eq. (18.16)18.7 Fig. Prob. 18.7 shows a drilled pier with a belled bottom. All the particulars of the pier and

soil are given in the figure.Required:(a) The total uplift force.(b) The total resisting force

830 Chapter 18

1

i

\

1 I '

1

,

L

i

2

i

^_c

g= 100 kN

r — »•

1 Unstablezone

e«P

Stablezonee«

Given:L, = 3 m, L~!_ - 10 md = 40 cmcu = 75 kN/m2

= 500 kN/m2

Figure Prob. 18.5

(c) Factor of safety for <2 = 0

(d) Factor of safety for Q = 200 kN.

Use Chen's method for computing Q

18.8 Solve Prob. 18.7 by making use of Eq. (18.16) for computing Q .

Q

dh= 1.2m

Figure Prob. 18.7

Given:L| = 6 m, L^ - 4 m

LA = 0.75 m,d = 0.4m

cu = 75 kN/m2

y = 17.5 kN/m3

p, = 500 kN/m2

(2 = 200 kN


4f t

Unstablezone

Stable zone

Given:cu = 800 lb/ft2

y = 110 lb/ft3

ps = 10,000 lb/ft2

Q = 60 kips

Figure Prob. 18.9

18.9 Refer to Fig. Prob. 18.9. The following data are available:

Lj = 15 ft, L2 = 13 ft, d = 4 ft, ̂ = 8 ft and L6 = 6 ft.All the other data are given in the figure.

Required:

(a) The total uplift force

(b) The total resisting force

(c) Factor of safety for Q = 0

(d) Factor of safety for Q = 60 kips.

Use Chen's method for computing Q .

18.10 Solve Prob. 18.9 using Eq. (18.16) for computing Qup.

18.11 If the length L2 is not sufficient in Prob. 18.10, determine the required length to getF. = 3.0.

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