Geotechnical and Geological Engineering

Geotechnical and Geological Engineering Retaining Structures in Expansive Clays --Manuscript Draft-Manuscript Number:

GEGE-D-12-01005R2

Full Title:

Retaining Structures in Expansive Clays

Article Type:

SI: Hard Soils and Weak Rocks

Keywords:

Unsaturated, earth pressure, drilled shaft, tie-back, soil-nailed, walls, soil-water characteristic curve, finite element

Corresponding Author:

A.T. Papagiannakis, Ph.D. Univeristy of Texas - San Antonio San Antonio, Texas UNITED STATES

Corresponding Author Secondary Information: Corresponding Author's Institution:

Univeristy of Texas - San Antonio

Corresponding Author's Secondary Institution: First Author:

A.T. Papagiannakis, Ph.D.

First Author Secondary Information: Order of Authors:

A.T. Papagiannakis, Ph.D. Sazzad Bin-Shaffique, PhD Robert Lytton, PhD

Order of Authors Secondary Information: Abstract:

Under increasing moisture conditions, retaining structures in expansive clays experience swelling stresses. The magnitude of these stresses varies with the amount of water increase and the extent of soil confinement imposed by the retaining structures. This paper describes a comprehensive study of this problem for two types of retaining structures, namely drilled-shaft and tie back retaining walls. Its approach involves laboratory testing to characterize the swelling properties of the high plasticity clays encountered in South Texas and finite element simulation of the structural interaction between the soil and the retaining structures due to the moisture changes observed at an instrumented wall site. The magnitude of swelling pressures was found to decrease with depth following the diminishing increase in moisture with depth. The amount of soil confinement effected by the wall significantly increased clay swelling pressures.

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Retaining Structures in Expansive Clays A.T. Papagiannakis1, S. Bin-Shafique Department of Civil and Environmental Engineering, University of Texas-San Antonio TX, USA R.L. Lytton Department of Civil Engineering, Texas A&M Univ. College Station TX, USA

ABSTRACT Under increasing moisture conditions, retaining structures in expansive clays experience swelling stresses. The magnitude of these stresses varies with the amount of water increase and the extent of soil confinement imposed by the retaining structures. This paper describes a comprehensive study of this problem for two types of retaining structures, namely drilled-shaft and tie back retaining walls. Its approach involves laboratory testing to characterize the swelling properties of the high plasticity clays encountered in South Texas and finite element simulation of the structural interaction between the soil and the retaining structures due to the moisture changes observed at an instrumented wall site. The magnitude of swelling pressures was found to decrease with depth following the diminishing increase in moisture with depth. The amount of soil confinement effected by the wall significantly increased clay swelling pressures.

Keywords: Unsaturated, earth pressure, drilled shaft, tie-back, soil-nailed, walls, soil-water characteristic curve, finite element.

1

Corresponding Author.

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1

INTRODUCTION

High sensitivity expansive clays are prevalent in many parts of the world including southern Texas [1, 2]. They exhibit significant volume changes in response to changes in water content [2, 3]. In roadway cuts, the most commonly used retaining wall types are tie-back, drilled shaft, and soilnailed [4]. The current design procedure used by the Texas Department of Transportation (TxDOT) for designing such walls is based on the lateral pressure calculations from the classical Rankine and Coulomb methods using the drained shear strength parameters. The current TxDOT design procedure does not account for the lateral pressure due to the expansion of the high plasticity clays, which may be significant [4]. Although considerable research has been carried out on the swelling potential of expansive clays, there is limited literature on accounting for these pressures in designing retaining walls. Solving this problem needs to address the interaction between the retaining wall and the soil. This paper summarizes a study addressing this problem. It involves:  Volumetric water content measurements obtained during and post construction using psychrometers at a tie-back wall site constructed at San Antonio, Texas.  Laboratory characterization of the swelling properties of the clay samples extracted from this site.  Finite element modeling (FEM) of the wall-clay system for two types of retaining walls, namely drilled-shaft and tie-back.

2.0 BACKGROUND In order to understand the performance of tie-back retaining walls in high plasticity expansive soils, a research project was undertaken involving a 6m high tie-back wall constructed on interstate highway 35 in San Antonio, Texas, USA. Undisturbed soil samples were collected from this field site and subjected to a range of laboratory tests including suction measurements for obtaining the soil-water characteristic curve (SWCC) of the soil. In addition, the change in volumetric water content profile was monitored at this site using psychrometers installed at depth intervals of 1.5 m to the depth of the base of the wall (i.e., 6 m). Numerical analysis involved the numerical simulation of this tie-back wall as well as a drilled shaft wall at the same location and of the same dimensions. This analysis captures the complex interaction between these retaining structures and the high plasticity expansive soils being supported. Two simulation approaches were taken:  The moisture measured profile was input into stress analysis  The rainfall at the site was input into a seepage model and its output moisture distribution was input into stress analysis. The following sections give a summary of the soil testing results, the numerical simulation approach and the results of the analysis.

3.0 METHODOLOGY 3.1

Soil Characterization

Undisturbed soil samples were collected at 0.3 m depth increments from each of two boreholes drilled at the site. One of the boreholes (Borehole-1) was located 2.4 m away from the tied-back wall and Borehole-2 was located 13.1 m away from the wall. Soil samples were subjected to laboratory testing to obtain the properties necessary for the FEM modeling. Testing included index properties (Attereberg’s limit, grain size distribution, density, specific gravity of soil solids etc.) as well as the properties necessary for FEM modeling (soil suction, soil suction-related volume compression index, the swelling index, the elastic modulus, the angle of friction etc.). The boring logs from the field site show that the same high plasticity soil layer is extended down to the 6m drilling depth and exhibited very little structural fracturing. The grain size distribution is uniform and the percent fines (P200) is 96%. The average in-situ unit weight is 18.2 kN/m3 and the specific gravity of the soil solid is 2.63. The liquid limit (LL) of the soil is 58% and the plasticity index (PI) is 38%. According to the USCS, the soil can be classified as high plasticity clay (CH). Similarly, the soil is classified as A-7-6 with a group index (GI) of 89 2

Table 1. Soil Properties Obtained from Laboratory Testing Strength property

Volume change property

Parameter

Value

Parameter

Value

Cohesion (kPa) Effective angle of friction ( 0 ) Poisson’s ratio Young’s modulus (kPa)

10 27.5 0.33 3.0 x 102

Saturated hydraulic conductivity (m/sec) Suction related compression index Compression index Recompression index

2.3 x 10-8 0.058 0.195 0.110

The suction-related compression index (i.e., slope of the void ratio vs. the logarithm of matric suction curve) [5], the compression index, and the recompression index were found to be 0.058, 0.195, and 0.110, respectively. In order to develop a soil water characteristic curve (SWCC), the air entry pressure and the soil suction at different volumetric water contents were measured using the filter paper [6] and the pressure plate techniques [7]. This data is shown in Fig. 1. These data points represent the average of two sets of filter paper tests and six sets of pressure plate tests. The narrow range in volumetric water content made fitting a well-established SWCC model (e.g., models by Brooks and Corey, van Genuchten, Fredlund and Xing and so on) particularly challenging. To do so, it was decided to first define the intercept using the method described in Hong (2008) for clay soil and then plot a best fit curve through the data points. This SWCC was used to estimate the soil pressure/suction at different volumetric water contents in subsequent analysis. 10000

1000

Suction (kPa)

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according to the AASHTO classification. The initial void ratio of the soil was 0.87. The strength and volume change properties [5] of the soil are shown in Table 1.

100 Filter Paper Test Pressure Plate

10

0.88 θsat

θsat

1 0

0.1

0.2

0.3

0.4

0.5

Volumetric Water Content (%)

Figure 1. SWCC of the expansive clay soil

3.2 Monitoring the Volumetric Water Content Change Thermocouple psychrometers (manufactured by Wescor, Inc., of Logan, Utah) were installed at different depths (1.5 m, 3 m, 4.5 m, and 6 m) at the field site to monitor the changes in volumetric water content of the high plasticity clays. Before installation, a calibration curve was developed at the laboratory for each of these psychrometers using the soil collected from the field site. The field installation involved embedding each psychrometer in piece of the clay core extracted and lowered into place by its lead wire. The psychrometers were monitored using a data logger at 2week intervals over a period of 18 months. These psychromrter readings provided an indirect measure of water content. Precipitation data was obtained over the same period of time and plotted along with the measured water content data to capture the effect of precipitation on moisture content changes. An example of these combined measurements is shown in Fig. 2, for measurements obtained at a depth of 3 m. Fig. 2 also indicates the timing of the excavation covering at this site. 3

Saturated Water Content

28 Precipitation (in)

26

Water Content

24

2

22

1

20

0 19-Feb

Water Content (%)

3

30 Completely covered

4

Uncovered

Precipitation (in.)

5

Partially covered

6

18 19-May

19-Aug

19-Nov

19-Feb

19-May

Time (Months)

Figure 2. Precipitation and psychrometer measurements; 3 m depth The overall volumetric water content profile over the 18-month period monitored is shown in Fig. 3. It is noted that the particular site was backfilled with about 2 meters of granular material and hence, it was decided to discount the measurements obtained from the surface-most psychrometer from future swelling pressure calculations. Fig. 3(a) shows the moisture measurements obtained including the top 2 m of the backfill. The moisture content profile shown in Fig. 3(b) was used instead. The initial volumetric water content profile was the lowest over the entire monitoring period. One of the main reasons for that was the drought experienced in 2009. Hence the soil at this site experienced wetting over the monitoring period. The wetting envelop is shown in Fig. 3(b). Clearly, it is wetting that is of interest in estimating swelling pressures on retaining structures. The wetting envelop shown in Fig. 3(b) suggests a moisture active zone of about 6 m and, as expected, decreasing moisture changes with increasing depth. The difference between the initial water content profile and the maximum water content profile observed during the monitoring period (i.e., Fig. 3(b)) was used in the FEM modeling.

Moisture Content (%) 10

15

20

Moisture Content (%)

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5ft 10ft 15ft

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Depth (ft)

Saturated Water Content

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Saturated Water Content

10

Depth (ft)

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Wetting envelope Initial water content profile

25

30

(a)

(b)

Figure 3. In-situ soil volumetric water content profile changes: (a) including the measurements of the top backfilled 2 m and (b) excluding the measurements of top backfilled 2 m.

4

3.3 FEM Software Package

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The commercially available and widely used FEM software package GeoStudio was used as the modeling tool to simulate the interaction between the retaining walls and the high plasticity expansive soil. SIGMA/W (a component of GeoStudio) is a finite element code that can be used to analyze the stress and deformation of earth structures due to change in soil water moisture [9]. The software has a provision to input the SWCC either by incorporating several points from the established SWCC or by inputting the fitting parameters of the SWCC models, such as Fredlund and Xing model, van Genuchten model etc. The SWCC combined with moisture change input allow SIGMA/W to estimate stresses and deformations. SEEP/W is another component of GeoStudio that allows estimation of the moisture content distribution, given soil permeability properties and flux boundary conditions (i.e., precipitation). Coupling SEEP/W with SIGMA/W allows estimation of swelling soil stress on retaining structures from precipitation input and hence allows a more general study of the problem [9]. 3.4 Software Validation and Soil Constitutive Model Selection Although SIGMA/W is widely used for the design and analysis of a variety of geotechnical problems, it has not been extensively used for analyzing the complex interaction between retaining structures and expansive soils. Thus, the software had to be validated to ensure that it would predict the behavior of drilled shaft and tie-back walls in expansive soils. Due to lack of field stress measurements, two situations from the literature were simulated for validating SIGMA/W. These are described next. Joshi and Katti (1980) constructed a large scale retaining wall to study the lateral pressure from black cotton expansive soils. The test tank was made up of stiffened mild steel plates with internal dimensions of 1m x 1.2 m in plan and 3.0 m in height. The box was filled with dry expansive soil and the initial suction profile of the soil was measured. Then water was added to achieve saturation and the lateral pressures at different depths were measured [10]. Similarly, Brown et al. (2011) conducted a full-scale test to determine the lateral deflection of a drilled shaft wall constructed in expansive soil. The lateral displacements of the drilled shaft wall were measured using the installed inclinometers [11]. These two structures were simulated using SIGMA/W and the pressures/displacements estimated were compared with the measured values given in the literature. In doing, so two different soil constituive models were used; a Mohr-Coulomb elasticplastic (EP) model and a modified cam-clay (MCC) model. The literature on the application of these models in describing soil stress-volumetric behavior abounds [12, 13, 14, 15, 16]. Both these soil models were used for verification purposes, which involved simulation of the walls described by Joshi and Katti (1980) and Brown et al. (2011). The results of the validation analysis performed suggested that the EP model performed slightly better than the MCC model. Using the EP model, SIGMA/W produced results within 8% of the measurements publsihed by these studies.

3.5 FEM Simulation Approach After the discretization of the wall-soil geometry, a preliminary FEM stress-deformation analysis was carried out using SIGMA/W to establish the stress distribution due to the overburden weight. Then the SIGMA/W was coupled with SEEP/W to carry out two steady state unsaturated flow analyses; one with the initial water content profile and the other with the wetting envelope measured. The SEEP/W analysis resulted in the distribution of water/suction throughout the FEM domain. The same amount of wetting of the soil measured in the retained soil was also applied to the other side at the bottom of the wall. The resulting stress from SIGMA/W and water content/suction distributions obtained from the initial water content profile using SEEP/W were imported as initial conditions into the FEM stress analysis. The water content/suction distributions obtained from the wetting envelope was used as the final condition to the FEM stress analysis model. After providing the input of the SWCC by incorporating several points from the established SWCC and the volume change properties of the soil, the volume change model predicted the distribution of the lateral pressure and deflections of the retaining structure. 3.6 Simulation of the Tie Back Wall The geometry, boundary conditions, and the finite element mesh for tie-back retaining wall is shown in Fig. 4. The geometry of the tie-back wall analyzed was modeled after the San Antonio 5

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Figure 4. Geometry, boundary conditions, and finite element mesh for tie-back retaining wall

wall site (i.e., 6 m tall wall with another 6 m embedded into the soil). The tie-back wall model included two tie-back rods configured exactly as at the field site. The length of the top tie-bar was 21.5 m and the length of the bottom one was 18.5 m. The length of the tie rods was divided into two parts, a bonded and an unbonded part, (i.e., 17/4.5 m and 14/4.5 m, resp.). The tie bars had a diameter of 25 mm. A pretension force of 355 kN was applied to the unbound part of the tie back rods. The simulation was carried out in three excavation stages. The first stage simulated the excavation to the bottom of the top tie-bar and the application of its pre-tension force. The second stage simulated the excavation to the depth of the second tie-bar, its pre-tensioning and finally the excavation to the bottom of the wall. The tie-back wall was allowed to move horizontally, being retained by the tie-back rods and the resistance of the soil along its embedded length. Both the vertical boundaries of the domain were constrained horizontally. The bottom horizontal boundary pinned in both directions while the top horizontal boundary was allowed to move in every direction. Non-contact elements were inserted between the wall and the soil to prevent tensional stresses between them. The mesh for the model was automatically generated and refined manually at locations of high stress concentration. Given the small displacements anticipated, the soil moduli on either side of the wall were considered the same, without differentiating them for active and passive conditions. The elastic modulus of the tie-back rod was assumed to be 200 GPa.

3.7 Simulation of Drilled Shaft Wall The geometry of the drilled shaft wall involved a larger embedment (i.e., 6 m tall wall with another 14 m embedded into the soil) to better reflect such wall designs currently used by the Texas Department of Transportation [4]. The overall size and outer boundary of the domain of the drilled shaft model was the same as that of the tie-back wall (Fig. 4). The drilled shaft wall was allowed to move horizontally, being retained only by the resistance of the soil along its embedded length. Similar to the tie-back wall, non-contact elements were inserted between the wall and the soil and the soil moduli on either side of the wall were assumed to be the same. The mesh for the model was automatically generated and refined manually at locations of high stress concentration. The diameter and the elastic modulus of the drilled shaft were assumed to be 1 m and 30 GPa, respectively. The simulation was carried out in one stage. First the drilled shaft was inserted into the soil from the top, and then the soil at the outer side was excavated to the bottom of the wall.

4.0 RESULTS AND DISCUSSION 4.1 Modeling of the Tie Back Wall The lateral stress and the lateral deflection of the tie back retaining wall estimated under the volumetric water changes observed in the field are shown in Fig. 5. Solutions for both a full depth clay layer and a clay layer with a 2 m granular backfill at the top are shown. For the full depth 6

Horizontal Stress (kPa) 0

100

200

300

Horizontal Deflection (mm) 400

-100

0

-80

-60

-40

-20

0

0

Depth of top tie-back rod 3

3

Depth (m)

Depth of bottom tie-back rod

Depth (m)

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clay layer and under the water changes observed at the field site (i.e., Fig. 3(b)), the increase in stress due to swelling was 80 kPa at the top and remained relatively at that level through the 6 m depth of the wall. For the wall with the granular backfill, the swelling stresses are approximately the same as for the full-depth clay wall below the 2 m granular layer.

6

9

6

9 No Swell

No Swell

Swell - 2m Fill

Swell - 2 m Fill

Swell - No Fill

Swell - No Fill

12

12

(a)

(b)

Figure 5. Results of the tie-back wall analysis: (a) Lateral stresses and (b) Lateral displacements; Swelling was due to the maximum change in suction observed at the field site (i.e., Fig. 3(b)). The lateral deflections at the top of the tie-back wall were estimated to be 40 mm and 20 mm for the full depth clay layer and the clay layer backfilled with 2 m top granular layer, respectively. These are below the deflection limit of 1% of the wall height (i.e., 0.01x6 m or 60 mm) for this 6 m tall wall and hence, this wall type is feasible under the conditions at the particular site.

4.2 Modeling of the Drilled Shaft Wall The lateral pressure on the drilled shaft wall and its lateral deflection estimated from the numerical simulation are shown in Fig. 6. This figure differentiates between a full-depth expansive soil behind the wall and one where the top 2 m are backfilled with a granular non-expansive material. Fig. 6 shows that for a site with no granular backfill, the additional lateral stress from swelling due to the maximum volumetric water changes observed at the test site vary from about 45 kPa near the surface to negligible amounts below the foot of the wall. As expected, the swelling pressures at the top 2 m of the wall for the case of the granular backfill are negligible. The estimated deflections shown in Fig. 6(b) indicate the cantilever function of this type of wall. The maximum lateral defection predicted was at the top of the wall and amounted to 105 mm and 70 mm for the full depth clay layer and the clay layer backfilled with 2 m top granular layer, respectively. Even for the 2 m granular backfilled wall, the uppermost deflection was found above the maximum acceptable limit of 60 mm. A wall embedment depth larger than the 13.7 m analyzed would have decreased the uppermost deflection making it perhaps feasible for the conditions at the particular site. Overall, as expected the stresses were higher on the tie-back wall than the drilled shaft wall (i.e., Figs. 6(a) versus 5(a)) suggesting that the higher the confinement of the expanding clay the higher is the resulting increase in stress. The opposite is true for the lateral deflections. The estimated deflections for the tie back wall were significantly lower than those of the drilled shaft wall (i.e., Figs. 5(b) versus 6(b)). Clearly, selection of one of these two wall types should consider their cost. In doing so, the cost of the embedment needed for the drilled shaft wall to bring its lateral top displacements below 60 mm should be compared to the cost of installing tie bars for the tie-bar wall.

7

Horizontal Stress (kPa) 0

100

200

300

Horizontal Deflection (mm) -125

400

0

0

5

5

Depth (m)

Depth (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

10

-50

-25

0

No Swell Swell - 2 m Fill

Swell - 2 m Fill Swell - No Fill

-75

10

No Swell

15

-100

15

Swell - No Fill

20

20

(a)

(b)

Figure 6. Results of the drilled shaft wall analysis: (a) Lateral stresses and (b) Lateral displacements; Swelling was due to the maximum change in field suction (i.e., Fig. 3(b)).

5.0 PARAMETRIC STUDY A parametric study was conducted with respect to the possible range in soil suction profiles, from as constructed to in-service. These unsaturated soil profiles were established from over 8000 insitu suction measurements obtained by Bryant et al. in Texas [17]. Their extreme ranges are highlighted in Fig. 7 as profiles “a” and “d” for the driest and wettest conditions, respectively. For the purposes of this parametric study, a number of in between soil suction profiles were identified by considering 0.5 pF increments (i.e., 0.3163 kPa) in soil surface suction and assuming no significant water changes below a depth of 6 m. These suction profiles are identified as “b”, “c” and “d” in Fig. 7. In addition, a situation where the soil becomes saturated at the surface was identified as suction profile “g”. Surface saturation could occur in cases where the soil surface is uncovered and there is an abundance of precipitation. This water could penetrate into the soil mass through cracks, despite the low permeability of the intact clay. Accordingly, a number of cases were established for conducting the parametric study:  Case-B: change in suction profile from “b” to “g”  Case-C: change in suction profile from “c” to “g”  Case-D: change in suction profile from “d” to “g”. Clearly, the higher the drop in suction between these profiles the larger the stress increases are anticipated from swelling (i.e., Case-B should generate more swelling stresses than Case-C and so on). Note that Case-A, (i.e., change in suction profile from “a” to “g” was considered too extreme and hence it was not considered in the parametric study. Examples of the results of this parametric study are shown in the form of the estimated lateral stresses and deflections on the drilled shaft wall (Fig. 8) and tie back wall (Fig. 9). As expected, the lateral stresses are the highest for Case-B, followed by Case-C and Case-D. Furthermore, the stresses for the tie back wall are higher than those of the drilled shaft wall, for each of the parametric study cases. For Case-B, the stresses near the surface of the drilled shaft wall and the tie-back wall (i.e., 300 kPa and 400 kPa, respectively), are high enough to cause passive soil failures. A granular surface backfill would alleviate this problem. As expected, the lateral deflections are higher for the drilled shaft wall (Fig. 8(b)) than for the tie back wall (Fig. 9(b)) and they increase as swelling increases that is they are highest for Case-B and lowest for Case-D. Regardless, only the tie-back wall under Case-D appears to meet the maximum deflection design limit of 60 mm. It is clearly shown that the drier the soil condition during construction the higher the wall stresses and deflections. At the extreme case of surface flooding (i.e., soil profile “g”), such walls will experience prohibitively high stresses and deflections. 8

25

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Drying

Total Suction (kPa)

Wetting 0

1.E+05

1.E+00 0.0

1.E+04

1.E+01

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1.E+02 a b 1.E+03 e c

1.E+02

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f 1.E+04 d

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g

Depth (m)

1.5 Series1

3.0

4.5

6.0 Measured By Bryant et al. (2008) Measured at Walters St. and I-35 Extrapolated (Backfill Materials) Suction Profiles at Walters St. and I-35 Initial Suction Profiles Final Suction

Figure 7. Volumetric water content profile change scenarios

Lateral Deflection (mm)

Lateral Stress (kPa) 0

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0

0

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5

Depth (m)

Depth (m)

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0

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20

(a)

(b)

Figure 8. Lateral stresses and deflections from the parametric study of the drilled shaft wall

9

Lateral Deflection (mm)

Lateral Stress (kPa) 0

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Depth of top tie-back rod

Depth of top tie-back rod

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Depth (m)

Depth (m)

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Depth of bottom tie-back rod

6

9

No Swell Case-B Case-C Case-D

12

(a)

(b)

Figure 9. Lateral stresses and deflections from the parametric study of the tie back wall

5.0

CONCLUSIONS

This paper described a numerical approach for estimating the swelling stresses exerted on retaining structures from swelling clays. A FEM stress analysis model was used. Two wall types were analyzed a drilled shaft wall and a tie-back wall. The latter was modeled after a wall built in San Antonio Texas. Soil volumetric water content was monitored at this site using psychrometers installed at 1.5 depth intervals. Material properties from this site were obtained from representative samples and tested extensively to establish the soil property input for the analysis. This included soil suction versus volumetric water content measurements using the filter paper and the pressure plate techniques to obtain the SWCC of the soil. The results of the analysis suggest that swelling soils can add substantially to the ordinary stresses exerted on retaining walls, especially when construction takes place under very dry/high suction conditions and then the surface of the soil becomes wetted. These swelling stresses follow the pattern of water content increase with depth; they are higher near the surface of the soil and diminish with depth, as dictated by the permeability of the soil. The tie-back wall design analyzed was sufficient in containing the deflections from the swelling of the soil under the volumetric water content profile changes observed in the field. The inclusion of a layer of fill material in the back of the wall is a very common provision in retaining walls whenever possible, not only in presence of expansive clays but also for drainage purposes. The study suggests that the inclusion of such layers decreases the moisture induced stresses/deformation of a wall constructed in unsaturated expansive clay. The parametric study revealed that more drastic changes in soil suction profiles between construction and in-service conditions may generate swelling stresses that result in excessive wall deflections. In conclusion, this paper suggests the need for a detailed analysis of clay swelling pressures in designing walls in expansive soils.

6.

REFERENCES 1.

P. F. Hudak, Geologic controls on foundation damage in north central Texas. Geojournal 45(1998), 159-164.

2.

R. L. Lytton, Prediction of Movement in Expansive Clay.” Geotechnical Special Publication, No. 40(1994), ASCE, NY, 2, 1827-1845.

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3.

S. Bin-Shafique, I. Azfar, and K. Rahman, The Effect of Freezing-Thawing Cycles on Performance of Fly Ash Stabilized Expansive Soil Subbases,” Proceedings of Geo Frontiers 2011, Advances in Geotechnical Engineering, Mar. 13-16, 2011, Dallas.

4.

TxDOT, Geotechnical Manual, Texas Department of Transportation, Austin, Texas, August 2006.

5.

Fredlund, D.G. and Rahardjo, H. (1993). Soil Mechanics for Unsaturated Soils, John Wiley and Sons, Inc., New York.

6.

ASTM D5298 - 10 Standard Test Method for Measurement of Soil Potential (Suction) Using Filter Paper.

7.

ASTM C1699 - 09 Standard Test Method for Water Retention Curves of Porous Building Materials Using Pressure Plates.

8.

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