FOUNDATIONS ON COLLAPSIBLE AND EXPANSIVE SOILS

Transcription

1 CHAPTER 18 FOUNDATIONS ON COLLAPSIBLE AND EXPANSIVE SOILS 18.1 GENERAL CONSIDERATIONS The structure of soils that experience large loss of strength or great increase in compressibility with comparatively 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 with the 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 been developed as a result of leaching that has left a network of resistant minerals capable of transmitting stresses around zones in which weaker minerals or voids exist. Footings on quick clays can be designed by the procedures applicable for clays as explained in Chapter 12. Very loose sands should not be used for support of footings. This chapter deals only with soils under categories 3 and 4 listed above. There are two types of soils that exhibit volume changes under constant loads with changes in water content. The possibilities are indicated in Fig which represent the result of a pair of tests in a consolidation apparatus on identical undisturbed samples. Curve a represents the e-\og p curve for a test started at the natural moisture content and to which no water is permitted access. Curves b and c, on the other hand, correspond to tests on samples to which water is allowed access under all loads until equilibrium is reached. If the resulting e-\og p curve, such as curve b, lies entirely below curve a, the soil is said to have collapsed. Under field conditions, at present overburden pressure/?, 791

2 792 Chapter 18 P\ Pi Pressure (log scale) Figure 18.1 Behavior of soil in double oedometer or paired confined compression test (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 and which experiences collapse, (c) same as (b) for sample that exhibits swelling (after Peck et al., 1974) and void ratio e Q, the addition of water at the commencement of the tests to sample 1, causes the void ratio to decrease to e v The collapsible settlement S c 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 those derived from granites under conditions of tropical weathering. On the other hand, if the addition of water to the second sample leads to curve c, located entirely above a, the soil is said to have swelled. At a given applied pressure p r the void ratio increases 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 with high plasticity indices.

3 Foundations on Collapsible and Expansive Soils 793 PART A COLLAPSIBLE SOILS 18.2 GENERAL OBSERVATIONS According to Dudley (1970), and Harden et al., (1973), four factors are needed to produce collapse in 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 be reduced, and the interaggregate or intergranular contacts to fail in shear, resulting in a reduction in total volume of the soil mass. Collapsible behavior of compacted and cohesive soils depends on the percentage of fines, the initial 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 phase material composed of air, water, soil skeleton, and contractile skin. Under the idealization, two phases can flow, that is air and water, and two phases come to equilibrium under imposed loads, that is the soil skeleton and contractile skin. Currently, regarding the behavior of compacted collapsing soils, geotechnical engineering recognized that 1. Any type of soil compacted at dry of optimum conditions and at a low dry density may develop 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 losing strength with the increase of the water content and at a critical degree of saturation, the soil structure collapses (Jennings and Knight 1957; Barden et al., 1973). Symbols Major loess deposits Reports of collapse in other type deposits Figure 18.2 Locations of major loess deposits in the United States along with other sites of reported collapsible soils (after Dudley, 1 970)

4 794 Chapter Soils have been observed to collapse 90 G, = G =2.6 Soils have not generally been observed to collapse \ 20 I 30 \ 40 Liquid limit 50 Figure 18.3 Collapsible and noncollapsible loess (after Holtz and Hilf, The soil collapse progresses as the degree of saturation increases. There is, however, a critical degree of saturation for a given soil above which negligible collapse will occur regardless 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 shear failure of the soil mass. 5. During wetting induced collapse, under a constant vertical load and under K o -oedometer conditions, a soil specimen undergoes an increase in horizontal stresses. 6. Under a triaxial stress state, the magnitude of volumetric strain resulting from a change in stress state or from wetting, depends on the mean normal total stress and is independent of the principal stress ratio. The geotechnical engineer needs to be able to identify readily the soils that are likely to collapse and 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 igneous rocks. Some soils at their natural water content will support a heavy load but when water is provided they undergo a considerable reduction in volume. The amount of collapse is a function of the relative proportions of each component including degree of saturation, initial void ratio, stress history 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 in many parts of the United States, Central Europe, China, Africa, Russia, India, Argentina and elsewhere. Figure 18.2 gives the distribution of collapsible soil in the United States.

5 Foundations on Collapsible and Expansive Soils 795 Holtz and Hilf (1961) proposed the use of the natural dry density and liquid limit as criteria for predicting collapse. Figure 18.3 shows a plot giving the relationship between liquid limit and dry unit weight of soil, such that soils that plot above the line shown in the figure are susceptible to collapse upon wetting COLLAPSE POTENTIAL AND SETTLEMENT Collapse Potential A 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 applied progressively until about 200 kpa (4 kip/ft 2 ) is reached. At this pressure the specimen is flooded with water for saturation and left for 24 hours. The consolidation test is carried on to its maximum loading. The resulting e-log p curve plotted from the data obtained is shown in Fig The collapse potential C is then expressed as in which Ae c = change in void ratio upon wetting, e o = natural void ratio. The collapse potential is also defined as (18.2a) C ~ - H (18.2b) where, A// c = change in the height upon wetting, H C - initial height. \ Pressure P Jog p Figure 18.4 Typical collapse potential test result

6 796 Chapter 18 Table 18.1 C// >20 Collapse potential values Severity of problem No problem Moderate trouble Trouble Severe trouble Very severe trouble Jennings and Knight have suggested some values for collapse potential as shown in Table These values are only qualitative to indicate the severity of the problem COMPUTATION OF COLLAPSE SETTLEMENT The double oedometer method was suggested by Jennings and Knight (1975) for determining a quantitative measure of collapse settlement. The method consists of conducting two consolidation tests. 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 moisture content curve Curve of sample soaked for 24 hrs Adjusted curve (curve 2) A> ton/fr Figure 18.5 Double consolidation test and adjustments for normally consolidated soil (Clemence and Finbarr, 1981)

7 Foundations on Collapsible and Expansive Soils 797 Soil at natural moisture content Adjusted n.m.c / curve Soaked sample for 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/m 2 (= 0.15 lb/in 2 ) for a period of 24 hours. 3. After 24 hours, saturate one specimen by flooding and keep the other at its natural moisture content 4. After the completion of 24 hour flooding, continue the consolidation tests for both the samples 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 samples as shown in Fig for normally consolidated soil. 6. Follow the same procedure for overconsolidated soil and plot the e-log p curves as shown in Fig From e-log p plots, obtain the initial void ratios of the two samples after the first 24 hour of loading. It is a fact that the two curves do not have the same initial void ratio. The total overburden pressure p Q at the depth of the sample is obtained and plotted on the e-log p curves in Figs 18.5 and The preconsolidation pressures p c are found from the soaked curves of Figs 18.5 and 18.6 and plotted.

8 798 Chapter 18 Normally Consolidated Case For the case in which p c /p Q is about unity, the soil is considered normally consolidated. In such a case, compression takes place along the virgin curve. The straight line which is tangential to the soaked e-log p curve passes through the point (e Q, p 0 ) as shown in Fig Through the point (e Q, p Q ) a curve is drawn parallel to the e-log p curve obtained from the sample tested at natural moisture content. The settlement for any increment in pressure A/? due to the foundation load may be expressed in two parts as Le n H c (18.3a) where ke n = change in void ratio due to load Ap as per the e-log p curve without change in moisture content Ae c, = change in void ratio at the same load Ap with the increase in moisture content (settlement caused due to collapse of the soil structure) = thickness of soil stratum susceptible to collapse. H c From Eqs (18. 3 a) and (18. 3b), the total settlement due to the collapse of the soil structure is ** + *e c ) (18.4) Overconsolidated Case In the case of an overconsolidated soil the ratio p c /p Q is greater than unity. Draw a curve from the point (e Q, p 0 ) on the soaked soil curve parallel to the curve which represents no change in moisture content during the consolidation stage. For any load (p Q + A/?) > p c, the settlement of the foundation may be determined by making use of the same Eq. (18.4). The changes in void ratios /\e n and Ae c are defined in Fig Example 18.1 A footing of size 10 x 10 ft is founded at a depth of 5 ft below ground level in collapsible soil of the loessial type. The thickness of the stratum susceptible to collapse is 30 ft. The soil at the site is normally consolidated. In order to determine the collapse settlement, double oedometer tests were conducted on two undisturbed soil samples as per the procedure explained in Section The e- log p curves of the two samples are given in Fig The average unit weight of soil y = lb/ ft 3 and the induced stress A/?, at the middle of the stratum due to the foundation pressure, is 4,400 lb/ft 2 (= 2.20 t/ft 2 ). Estimate the collapse settlement of the footing under a soaked condition. Solution Double consolidation test results of the soil samples are given in Fig Curve 1 was obtained with natural moisture content. Curve 3 was obtained from the soaked sample after 24 hours. The virgin curve is drawn in the same way as for a normally loaded clay soil (Fig. 7.9a). The effective overburden pressure p 0 at the middle of the collapsible layer is p Q = 15 x = 1,599 lb/ft 2 or 0.8 ton/ft 2

9 Foundations on Collapsible and Expansive Soils 799 A vertical line is drawn in Fig atp 0 = 0.8 ton/ft 2. Point A is the intersection of the vertical line and the virgin curve giving the value of e Q = p Q + Ap = = 3.0 t/ft 2. At (p 0 + Ap) = 3 ton/ft 2, we have (from Fig. 18.5) &e n = = 0.06 Ae c = = 0.14 From Eq. (18.3) A A x30x12 _ nn. = - = in Total settlement S c = in. The total settlement would be reduced if the thickness of the collapsible layer is less or the foundation pressure is less. Example 18.2 Refer to Example Determine the expected collapse settlement under wetted conditions if the soil stratum below the footing is overconsolidated. Double oedometer test results are given in Fig In this case/? 0 = 0.5 ton/ft 2, Ap = 2 ton/ft 2, and/? c = 1.5 ton/ft 2. Solution The virgin curve for the soaked sample can be determined in the same way as for an overconsolidated clay (Fig. 7.9b). Double oedometer test results are given in Fig From this figure: e Q = 0.6, &e n = = 0.05, Ae c = = 0.07 As in Ex ^H 005X30X Total S = in FOUNDATION DESIGN Foundation design in collapsible soil is a very difficult task. The results from laboratory or field tests can be used to predict the likely settlement that may occur under severe conditions. In many cases, deep foundations, such as piles, piers etc, may be used to transmit foundation loads to deeper bearing strata below the collapsible soil deposit. In cases where it is feasible to support the structure on shallow foundations in or above the collapsing soils, the use of continuous strip footings may provide a more economical and safer foundation than isolated footings (Clemence and Finbarr, 1981). Differential settlements between columns can be minimized, and a more equitable distribution of stresses may be achieved with the use of strip footing design as shown in Fig (Clemence and Finbarr, 1981).

10 800 Chapter 18 Load-bearing beams Figure 18.7 Continuous footing design with load-bearing beams for collapsible soil (after Clemence and Finbarr, 1981) 18.6 TREATMENT METHODS FOR COLLAPSIBLE SOILS On some sites, it may be feasible to apply a pretreatment technique either to stabilize the soil or cause collapse of the soil deposit prior to construction of a specific structure. A great variety of treatment methods have been used in the past. Moistening and compaction techniques, with either conventional impact, or vibratory rollers may be used for shallow depths up to about 1.5 m. For deeper depths, vibroflotation, stone columns, and displacement piles may be tried. Heat treatment to solidify the soil in place has also been used in some countries such as Russia. Chemical stabilization 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 treatment provides the most effective means of reducing the settlement of collapsible soils to tolerable limits. Prewetting, in combination with dynamic compaction, offers the potential for increasing compaction efficiency and uniformity, while increasing vibration attenuation. Prewetting with a 2 percent 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 be completely ineffective in reducing collapse potential for shallow foundations. Prewetting must be accompanied by preloading, surcharging or excavation in order to be effective. PART B EXPANSIVE SOILS 18.7 DISTRIBUTION OF EXPANSIVE SOILS The problem of expansive soils is widespread throughout world. The countries that are facing problems with expansive soils are Australia, the United States, Canada, China, Israel, India, and Egypt. The clay mineral that is mostly responsible for expansiveness belongs to the montmorillonite group. Fig shows the distribution of the montmorillonite group of minerals in the United States. The major concern with expansive soils exists generally in the western part of the United States. In the northern and central United States, the expansive soil problems are primarily related to highly overconsolidated shales. This includes the Dakotas, Montana, Wyoming and Colorado (Chen, 1988). In Minneapolis, the expansive soil problem exists in the Cretaceous

11 Foundations on Collapsible and Expansive Soils 801 Figure 18.8 General abundance of montmorillonite in near outcrop bedrock formations in the United States (Chen, 1988) deposits along the Mississippi River and a shrinkage/swelling problem exists in the lacustrine deposits in the Great Lakes Area. In general, expansive soils are not encountered regularly in the eastern parts of the central United States. In eastern Oklahoma and Texas, the problems encompass both shrinking and swelling. In the Los Angeles area, the problem is primarily one of desiccated alluvial and colluvial soils. The weathered volcanic material in the Denver formation commonly swells when wetted and is a cause of major engineering problems in the Denver area. The six major natural hazards are earthquakes, landslides, expansive soils, hurricane, tornado and flood. A study points out that expansive soils tie with hurricane wind/storm surge for second place 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 would exceed 4.5 billion dollars annually (Chen, 1988) GENERAL CHARACTERISTICS OF SWELLING SOILS Swelling soils, which are clayey soils, are also called expansive soils. When these soils are partially saturated, they increase in volume with the addition of water. They shrink greatly on drying and develop cracks on the surface. These soils possess a high plasticity index. Black cotton soils found in many parts of India belong to this category. Their color varies from dark grey to black. It is easy to recognize these soils in the field during either dry or wet seasons. Shrinkage cracks are visible on the ground surface during dry seasons. The maximum width of these cracks may be up to 20 mm or more and they travel deep into the ground. A lump of dry black cotton soil requires a hammer to break. 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. The depths of these soils in some regions may be up to 6 m or more. Normally the water table is met at great depths in these regions. As such the soils become wet only during rainy seasons and are dry or

12 802 Chapter 18 Increasing moisture content of soil Ground surface D, D,,< = Unstable zone Moisture variation during dry season Stable zone \ Equilibrium moisture content (covered area) Desiccated moisture content (uncovered natural conditions) Wet season moisture content (seasonal variation) Depth of seasonal moisture content fluctuation Depth of desiccation or unstable zone Figure 18.9 Moisture content variation with depth below ground surface (Chen, 1988) partially saturated during the dry seasons. In regions which have well-defined, alternately wet and dry seasons, these soils swell and shrink in regular cycles. Since moisture change in the soils bring about severe movements of the mass, any structure built on such soils experiences recurring cracking and progressive damage. If one measures the water content of the expansive soils with respect to depth during dry and wet seasons, the variation is similar to the one shown in Fig During dry seasons, the natural water content is practically zero on the surface and the volume of the soil reaches the shrinkage limit. The water content increases with depth and reaches a value w n at a depth D, beyond which it remains almost constant. During the wet season the water content increases and reaches a maximum at the surface. The water content decreases with depth from a maximum of w n at the surface to a constant value of w n at almost the same depth D us. This indicates that the intake of water by the expansive soil into its lattice structure is a maximum at the surface and nil at depth D us. This means that the soil lying within this depth D us is subjected to drying and wetting and hence cause considerable movements in the soil. The movements are considerable close to the ground surface and decrease with depth. The cracks that are developed in the dry seasons close due to lateral movements during the wet seasons.

13 Foundations on Collapsible and Expansive Soils 803 The zone which lies within the depth D us may be called the unstable zone (or active zone) and the one below this the stable zone. Structures built within this unstable zone are likely to move up and down according to seasons and hence suffer damage if differential movements are considerable. 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 starts taking in water during the wet season. This swelling pressure is due to constraints offered by the foundation for free swelling. The maximum swelling pressure may be as high as 2 MPa (20 tsf). If the 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. If the imposed bearing pressure is greater than the swelling pressure, there will not be any problem for the structure. If on the other hand, the structure is built during the wet season, it will definitely experience settlement as the dry season approaches, whether the imposed bearing pressure is high or low. However, the imposed bearing pressure during the wet season should be within the allowable bearing pressure of the soil. The better practice is to construct a structure during the dry season and complete it before the wet season. In covered areas below a building there will be very little change in the moisture content except due to lateral migration of water from uncovered areas. The moisture profile is depicted by curve 1 in Fig CLAY MINERALOGY AND MECHANISM OF SWELLING Clays can be divided into three general groups on the basis of their crystalline arrangement. They are: 1. Kaolinite group 2. 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 kaolinite mineral is formed by the stacking of the crystalline layers of about 7 A thick one above the other with 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 sheet stackings that are difficult to dislodge. The mineral is, therefore, stable and water cannot enter between the sheets to expand the unit cells. The structural arrangement of the montmorillonite mineral is composed of units made of two silica tetrahedral sheets with a central alumina-octahedral sheet. The silica and gibbsite sheets are combined in such a way that the tips of the tetrahedrons of each silica sheet and one of the hydroxyl layers of the octahedral sheet form a common layer. The atoms common to both the silica and gibbsite layers are oxygen instead of hydroxyls. The thickness of the silica-gibbsite-silica unit is about 10 A. In stacking of these combined units one above the other, oxygen layers of each unit are adjacent to oxygen of the neighboring units, with a consequence that there is a weak bond and excellent cleavage between them. Water can enter between the sheets causing them to expand significantly and thus the structure can break into 10 A thick structural units. The soils containing a considerable amount of montmorillonite minerals will exhibit high swelling and shrinkage characteristics. The illite group of minerals has the same structural arrangement as the montmorillonite group. The presence of potassium as the bonding materials between units makes the illite minerals swell less.

14 804 Chapter DEFINITION OF SOME PARAMETERS Expansive soils can be classified on the basis of certain inherent characteristics of the soil. It is first necessary 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 an oedometer test which is soaked under a surcharge load of 7 kpa (1 lb/in 2 ) after being compacted to maximum dry density at optimum moisture content according to the AASHTO compaction test. Swelling Pressure The swelling pressure /? 5, is defined as the pressure required for preventing volume expansion in soil in contact with water. It should be noted here that the swelling pressure measured in a laboratory oedometer is different from that in the field. The actual field swelling pressure is always less than the one measured in the laboratory. Free Swell Free swell 5, is defined as V f -V. S f =-^ xm (18.5) where V { = initial dry volume of poured soil Vr - final volume of poured soil According toholtz and Gibbs (1956), 10 cm 3 (V.) of dry soil passing thorough a No. 40 sieve is poured into a 100 cm 3 graduated cylinder filled with water. The volume of settled soil is measured after 24 hours which gives the value of V~ Bentonite-clay is supposed to have a free swell value ranging from 1200 to 2000 percent. The free swell value increases with plasticity index. Holtz and Gibbs suggested that soils having a free-swell value as low as 100 percent can cause considerable damage to lightly loaded structures and soils heaving a free swell value below 50 percent seldom exhibit appreciable volume change even under light loadings EVALUATION OF THE SWELLING POTENTIAL OF EXPANSIVE 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 investigation of building sites in those areas having expansive soil. These tests are 1. Atterberg limits tests 2. Linear shrinkage tests 3. Free swell tests 4. Colloid content tests Atterberg Limits Holtz and Gibbs (1956) demonstrated that the plasticity index, I p, and the liquid limit, w /5 are useful indices for determining the swelling characteristics of most clays. Since the liquid limit and the

15 Foundations on Collapsible and Expansive Soils 805 Note: Percent swell measured under 1 psi surcharge for sample compacted of optimum 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/bentonite I I Commercial illite 1:1 Commercial illite/kaolinite Commercial kaolinite Percent clay sizes (finer than mm) Figure Relationship between percentage of swell and percentage of clay sizes 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 are related. The relation between the swelling potential of clays and the plasticity index has been established as given in Table 18.2 Linear Shrinkage The swell potential is presumed to be related to the opposite property of linear shrinkage measured in a very simple test. Altmeyer (1955) suggested the values given in Table 18.3 as a guide to the determination 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 (Chen, 1988). For a given clay type, the amount of swell will increase with the amount of clay present in the soil. Table 18.2 Relation between swelling potential and plasticity index, / Plasticity index l p (%) and above Swelling potential Low Medium High Very high

16 806 Chapter 18 Table 18.3 Relation between swelling potential, shrinkage limits, and linear shrinkage Shrinkage limit % Linear shrinkage % Degree of expansion > 12 > Critical Marginal Non-critical CLASSIFICATION OF SWELLING SOILS BY INDIRECT MEASUREMENT By utilizing the various parameters as explained in Section 18.11, the swelling potential can be evaluated without resorting to direct measurement (Chen, 1988). USBR Method Holtz and Gibbs (1956) developed this method which is based on the simultaneous consideration of several soil properties. The typical relationships of these properties with swelling potential are shown in Fig Table 18.4 has been prepared based on the curves presented in Fig by Holtz and Gibbs (1956). The relationship between the swell potential and the plasticity index can be expressed as follows (Chen, 1988) where, A = B = / = plasticity index. Volume change in % H K> U> * O OO CT\ - > N> C f J -"$ 1 / / / 1 i i i t '^ i r * i* i / /', 1. ;:/.. -;4 ^? _^ / : / 4 / /" 0 f i \ \ / 1 \ \\ v\ i \«^ : V- \ \ ^ v ) ' Colloid content Plasticity index Shrinkage limit (%) (% less than mm) (a) (b) (c) (18.6) Figure 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) f, X CJJ s 3 T3 OJ S O

17 Foundations on Collapsible and Expansive Soils 807 Table 18.4 Data for making estimates of probable volume changes for expansive soils (Source: Chen, 1988) Data from index tests* Colloid content, percent minus mm Plasticity index Shrinkage limit Probable expansion, percent total vol. change Degree of expansion > < 15 > <18 < >15 *Based on vertical loading of 1.0 psi. (after Holtz and Gibbs, 1956) > <10 Very high High Medium Low 10 a 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 Relationships of volume change to plasticity index (Source: Chen, 1988)

18 808 Chapter 18 Figure shows that with an increase in plasticity index, the increase of swelling potential is much less than predicted by Holtz and Gibbs. The curves given by Chen (1988) are based on thousands of tests performed over a period of 30 years and as such are more realistic. Activity Method Skempton (1953) defined activity by the following expression A = ~^ (18.7) where / = plasticity index C = percentage of clay size finer than mm by weight. The activity method as proposed by Seed, Woodward, and Lundgren, (1962) was based on remolded, artificially prepared soils comprising of mixtures of bentonite, illite, kaolinite and fine sand 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 This method appears to be an improvement over the USER method Percent clay sizes (finer than mm) Swelling potential = 25% Swelling potential = 5% Swelling potential = 1.5% Figure Classification chart for swelling potential (after Seed, Woodward, and Lundgren, 1962)

19 Foundations on Collapsible and Expansive Soils 809 The Potential Volume Change Method (PVC) A determination of soil volume change was developed by Lambe under the auspices of the Federal Housing Administration (Source: Chen, 1988). Remolded samples were specified. The procedure is as given below. The sample is first compacted in a fixed ring consolidometer with a compaction effect of 55,000 ft-lb per cu ft. Then an initial pressure of 200 psi is applied, and water added to the sample which is partially restrained by vertical expansion by a proving ring. The proving ring reading is taken at the end of 2 hours. The reading is converted to pressure and is designated as the swell index. From Fig , the swell index can be converted to potential volume change. Lambe established the categories of PVC rating as shown in Table The PVC method has been widely used by the Federal Housing Administration as well as the Colorado State Highway Department (Chen, 1988). ouuu I *7nnn / \j\j\j I 1 / < H CT / / ' 4000 _c ^ // '/ I 2000 /,/ / ^/ / 1000 / */ / ^ / 200 ^ ^ <*. () Non critical Marginal Critical Very Critical / / / Potential Volume Change (PVC) Figure Swell index versus potential volume change (from 'FHA soil PVC meter publication,' Federal Housing Administration Publication no. 701) (Source: Chen, 1988)

20 810 Chapter 18 (Source: Chen, 1988) Table 18.5 PVC rating Less than 2 2^ 4-6 >6 Potential volume change rating (PVC) Category Non-critical Marginal Critical Very critical Figure 18.15(a) shows a soil volume change meter (ELE International Inc). This meter measures both shrinkage and swelling of soils, ideal for measuring swelling of clay soils, and fast and 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 unified method to measure the characteristics of swelling soils. It is claimed that the El is a basic index property 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 is between 49 and 51 percent. The sample is then compacted into a 4 inch diameter mold in two layers to give a total compacted depth of approximates 2 inches. Each layer is compacted by 15 blows of 5.5 Ib hammer dropping from a height of 12 inches. The prepared specimen is allowed to consolidate under 1 lb/in 2 pressure for a period of 10 minutes, then inundated with water until the rate of expansion ceases. The expansion index is expressed as / = xlooo (189) h i 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 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 expansion index of soils. Table 18.6 Classification of potentially expansive soil Expansion Index, El Expansion potential 0-20 Very low Low Medium High > 130 Very high (Source: Chen, 1988)

21 Foundations on Collapsible and Expansive Soils 811 Swell Index Vijayvergiya and Gazzaly (1973) suggested a simple way of identifying the swell potential of clays, based on the concept of the swell index. They defined the swell index, I s, as follows - *~~ (18-10) where w n = natural moisture content in percent \v l = liquid limit in percent The relationship between I s and swell potential for a wide range of liquid limit is shown in Fig Swell index is widely used for the design of post-tensioned slabs on expansive soils. Prediction of Swelling Potential Plasticity index and shrinkage limit can be used to indicate the swelling characteristics of expansive soils. According to Seed at al., (1962), the swelling potential is given as a function of the plasticity index by the formula (18.11) Figure (a) Soil volume change meter, and (b) Expansion index test apparatus (Courtesy: Soiltest)

22 812 Chapter 18 u. / {' 0.4 < j s ^ Percent swell: < 1 Swell pressure: < ^^1 ^ 0.3 ton/sq ft - Perc ent swell: 1 to 4 Swe11 pressure: 0.3 to 1.25 ton/sq ft L 4- Percent swell: 4 to 10 Swell pressure: 1.25 to 3 ton/sq ft -~ ^~~"1 ^MPerce nt swell: > 10 Swell pressure: ; > 3 ton/sq fl ** ( Liquid limit Figure Relationship between swell index and liquid limit for expansive clays (Source: Chen, 1988) where S = swelling potential in percent / = plasticity index in percent k = 3.6 xlo~ 5, a factor for clay content between 8 and 65 percent SWELLING PRESSURE BY DIRECT MEASUREMENT ASTM defines swelling pressure which prevents the specimen from swelling or that pressure which 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 strain controlled (Chen, 1988). In the stress controlled method, the conventional oedometer is used. The samples are placed in the consolidation ring trimmed to a height of 0.75 to 1 inch. The samples are subjected to a vertical 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 sample has ceased the vertical stress is increased in increments until it has been compressed to its original height. The stress required to compress the sample to its original height is commonly termed the zero volume change swelling pressure. A typical consolidation curve is shown in Fig

23 Foundations on Collapsible and Expansive Soils 813 I I ^^ xj "X Placement conditions Dry density = 76.3 pcf Moisture content \A. n ofc Atterberg limits: Liquid 1imit = 68% Plasticity index = 17% N >\s s \ ^s '-3 10 ta O U N T 0.8% expansion at 100 psf F>ressure when wetted \ N ) -10 Figure Applied pressure (psf) Typical stress controlled swell-consolidation curve Prediction of Swelling Pressure Komornik et al., (1969) have given an equation for predicting swelling pressure as \ogp s = W, ^ w n where p s = swelling pressure in kg/cm 2 w ; = liquid limit (%) w n = natural moisture content (%) y d = dry density of soil in kg/cm 3 (18.12) EFFECT OF INITIAL MOISTURE CONTENT AND INITIAL DRY DENSITY ON SWELLING PRESSURE The capability of swelling decreases with an increase of the initial water content of a given soil because its capacity to absorb water decreases with the increase of its degree of saturation. It was found from swelling tests on black cotton soil samples, that the initial water content has a small effect on swelling pressure until it reaches the shrinkage limit, then its effect increases (Abouleid, 1982). This is depicted in Fig (a). The effect of initial dry density on the swelling percent and the swelling pressure increases with an increase of the dry density because the dense soil contains more clay particles in a unit volume and consequently greater movement will occur in a dense soil than in a loose soil upon

24 814 Chapter on H 1? 10 9 f. A 0 *-> S.L. \> I \ ^ \ \ NV n ^» Water content % Black cotton soil w. = 90, w p = 30, w, = 10 r ș 0 j*t 2.0 C/3 a L0 D. z S.I, Dry density (t/m 3 ) Mineralogy: Largely sodium, Montmorillonite Figure (a) effect of initial water content on swelling pressure of black cotton soil, and (b) effect of initial dry density on swelling pressure of black cotton soil (Source: Abouleid, 1982) (b) wetting (Abouleid, 1982). The effect of initial dry density on swelling pressure is shown in Fig (b) ESTIMATING THE MAGNITUDE OF SWELLING When footings are built in expansive soil, they experience lifting due to the swelling or heaving of the soil. The amount of total heave and the rate of heave of the expansive soil on which a structure is founded are very complex. The heave estimate depends on many factors which cannot be readily determined. Some of the major factors that contribute to heaving are: 1. Climatic conditions involving precipitation, evaporation, and transpiration affect the moisture in the soil. The depth and degree of desiccation affect the amount of swell in a given soil horizon. 2. The thickness of the expansive soil stratum is another factor. The thickness of the stratum is controlled by the depth to the water table. 3. The depth to the water table is responsible for the change in moisture of the expansive soil lying above the water table. No swelling of soil takes place when it lies below the water table. 4. The predicted amount of heave depends on the nature and degree of desiccation of the soil immediately after construction of a foundation. 5. The single most important element controlling the swelling pressure as well as the swell potential is the in-situ density of the soil. On the completion of excavation, the stress condition in the soil mass undergoes changes, such as the release of stresses due to elastic

25 Foundations on Collapsible and Expansive Soils 815 rebound of the soil. If construction proceeds without delay, the structural load compensates for the stress release. 6. The permeability of the soil determines the rate of ingress of water into the soil either by gravitational 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 given structural 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 of depth. The samples are to be obtained during the dry season when the moisture contents are at their lowest. 2. Load specimens (at natural moisture content) in a consolidometer under a pressure equal to the ultimate value of the overburden plus the weight of the structure. Add water to saturate the 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 Undercut The 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 depth at that height. Total swell at any depth equals area under the curve integrated upward from the depth of zero swell. 2. For a given allowable value of swell, read the amount of undercut necessary from the total swell versus depth curve. Van Der Merwe Method (1964) Probably the nearest practical approach to the problem of estimating swell is that of Van Der Merwe. This method starts by classifying the swell potential of soil into very high to low categories as shown in Fig Then assign potential expansion (PE) expressed in in./ft of thickness based on Table Swell potential Table 18.7 Potential expansion Potential expansion (PE) in./ft Very high 1 High 1/2 Medium 1/4 Low 0

26 816 Chapter 18 Pu u Q I Reduction factor, F 0.2 I 0.4 I 0.6 \ 0.8./ /, / > ' ix^" Clay fraction of whole sample (% < 2 micron) (a) -18 on -ZU ^n - / (b) Figure Relationships for using Van Der Merwe's prediction method: (a) potential expansiveness, and (b) reduction factor (Van der Merwe, 1964) 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 A// = i=n A (18.13) where A// e = total expansion (in.) A. =(P ).(AD) I.(F). (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 (b) gives the reduction factor plotted against depth.

27 Foundations on Collapsible and Expansive Soils 817 Outside brick wall Cement concrete apron with light reinforcement RCC foundation ;^r -, ~A '» /W\//^\^Sv -- ' /#<Ns Granular fill Figure Foundation in expansive soil DESIGN OF FOUNDATIONS IN SWELLING SOILS It is necessary to note that all parts of a building will not equally be affected by the swelling potential of the soil. Beneath the center of a building where the soil is protected from sun and rain the moisture changes are small and the soil movements the least. Beneath outside walls, the movement 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 soils 2. Designing of foundations that will remain undamaged in spite of swelling 3. Elimination of swelling potential of soil. All three methods are in use either singly or in combination, but the first is by far the most widespread. Fig show a typical type of foundation under an outside wall. The granular fill provided around the shallow foundation mitigates the effects of expansion of the soils DRILLED PIER FOUNDATIONS Drilled 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 base are 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 considerations which are derived from regional experience". They further stated that "when surface conditions are well established and are relatively uniform, and the performance of past constructions well documented, 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 the structural loads down to stable material or to a stable zone where moisture changes are improbable.

28 818 Chapter 18 There should be no direct contact between the soil and the structure with the exception of the soils supporting the piers. Straight-shaft Piers in Expansive Soils Figure (a) shows a straight-shaft drilled pier embedded in expansive soil. The following notations are used. Lj = length of shaft in the unstable zone (active zone) affected by wetting. L 2 = length of shaft in the stable zone unaffected by wetting d = diameter of shaft Q - structural dead load = qa b q - unit dead load pressure and A b = base area of pier When 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 result there will be an upward force developed on the surface of the shaft which tries to pull the pile out of its position. The upward force can be resisted in the following ways. 1. The downward dead load Q acting on the pier top 2. The resisting force provided by the shaft length L 2 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 condition a 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 Q up Chen (1988) suggested the following equation for estimating the uplift force Q Qu P =7lda u P s L i (18.15) where d - diameter of pier shaft a = coefficient of uplift between concrete and soil = 0.15 p s = swelling pressure = 10,000 psf (480 kn/m 2 ) for soil with high degree of expansion = 5,000 (240 kn/m 2 ) 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 is possible for the wetting zone to extend beyond feet in some countries and limiting the depth of unstable zone to a such a low value of 5 ft may lead to unsafe conditions for the stability of structures. However, it is for designers to decide this depth Lj according to local conditions. With regards to swelling pressure p s, it is unrealistic to fix any definite value of 10,000 or 5,000 psf for all types of expansive soils under all conditions of wetting. It is also not definitely known if the results obtained from laboratory tests truly represent the in situ swelling pressure. Possibly one way of overcoming this complex problem is to relate the uplift resistance to undrained cohesive strength