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792 Chapter 18, 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 eQ the addition of water at the commencement of the tests to sample 1 causes the. void ratio to decrease to ev The collapsible settlement Sc may be expressed as. 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 pr the void ratio. increases to e and the corresponding rise of the ground is expressed as. Soils exhibiting this behavior to a marked degree are usually montmorillonitic clays with. high plasticity indices, 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. Major loess, 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. 794 Chapter 18, Soils have been, observed to collapse.

G 2 6 Soils have not, 100 generally been, observed to. 10 20 30 40 50, Liquid limit, 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 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 Ko 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. 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. 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, 18 3 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 ft2 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 18 4. The collapse potential C is then expressed as, in which Aec change in void ratio upon wetting eo natural void ratio. The collapse potential is also defined as, 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 Severity of problem, 0 1 No problem, 1 5 Moderate trouble. 3 10 Trouble, 10 20 Severe trouble, 20 Very severe trouble. Jennings and Knight have suggested some values for collapse potential as shown in. Table 18 1 These values are only qualitative to indicate the severity of the problem. 18 4 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, Adjusted curve, Curve of sample curve 2. soaked for 24 hrs, 1 0 2 0 4 0 6 A 1 0 6 10 20ton fr. Figure 18 5 Double consolidation test and adjustments for normally consolidated. soil Clemence and Finbarr 1981, Foundations on Collapsible and Expansive Soils 797. Soil at natural, moisture content, Adjusted n m c, Soaked sample. for 24 hours, 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 24. 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 18 5 for normally consolidated soil. 6 Follow the same procedure for overconsolidated soil and plot the e log p curves as shown. in Fig 18 6, 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 pQ at the depth of the sample is obtained and plotted on the e log p curves in Figs 18 5 and. 18 6 The preconsolidation pressures pc are found from the soaked curves of Figs 18 5 and 18 6 and. 798 Chapter 18, Normally Consolidated Case, For the case in which pc pQ 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 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 natural. moisture content The settlement for any increment in pressure A due to the foundation load may. be expressed in two parts as, where ken change in void ratio due to load Ap as per the e log p curve without change in.

moisture 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. Overconsolidated Case, In the case of an overconsolidated soil the ratio pc pQ is greater than unity Draw a curve from the. point eQ p0 on the soaked soil curve parallel to the curve which represents no change in moisture. content during the consolidation stage For any load pQ A pc the settlement of the foundation. may 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 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 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 4 400. lb ft2 2 20 t ft 2 Estimate the collapse settlement of the footing under a soaked condition. Double consolidation test results of the soil samples are given in Fig 18 5 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 p0 at the middle of the collapsible layer is. pQ 15 x 106 6 1 599 lb ft 2 or 0 8 ton ft 2, Foundations on Collapsible and Expansive Soils 799. A vertical line is drawn in Fig 18 5 atp 0 0 8 ton ft2 Point A is the intersection of the vertical. line 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, 0 14x30x12 n n, Total settlement Sc 42 86 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 18 1 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 18 6 In this case 0 0 5 ton ft2 Ap 2 ton ft2 and c 1 5 ton ft2. 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 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 005X30X12, Total S 27 00 in, 18 5 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 18 7. Clemence and Finbarr 1981, 800 Chapter 18, Load bearing.

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. 796 Chapter 18 Table 18 1 Collapse potential values C 0 0 1 1 5 3 10 10 20 gt 20 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 18 1 These values are only qualitative to indicate the severity of the problem