Sai K. Vanapalli

Evaluation of Empirical Procedures for Predicting the Shear Strength of Unsaturated Soils

This paper provides a summary of nineteen empirical procedures or techniques that are available for predicting or estimating the shear strength of unsaturated soils. While six of these procedures use the soil-water retention curve (SWRC) as a tool; the remainder thirteen procedures are mathematical formulations for the prediction or estimation of the shear strength of an unsaturated soil. A comprehensive analysis is undertaken to predict the variation of shear strength with respect to matric suction of twenty soils using the six prediction equations that use SWRC. The studies presented in this paper highlight the suitability of these equations for successful predictions of the shear strength of the unsaturated soils.

The Relationship Between the Soil-Water Characteristic Curve and the Unsaturated Shear Strength of a Compacted Glacial Till

Soils compacted at various “initial” water contents and to various densities should be considered as “different” soils from a soil mechanics behavioral standpoint even though their mineralogy, plasticity, and texture are the same. The engineering behavioral change from one specimen to another will vary due to differences in soil structure or aggregation. The shear strength of an unsaturated soil and the soil-water characteristic curve are dependent on soil structure or the aggregation, which in turn is dependent on the “initial” water content and the method of compaction. The laboratory preparation of specimens must, therefore, closely represent the physical conditions and the stress state conditions likely to occur in the field if a proper assessment of the shear strength parameters is to be achieved. This paper is primarily concerned with the study of the relationship between the shear strength of an unsaturated soil and its soil-water characteristic curve. Consolidated drained direct shear tests were conducted on statically compacted glacial till specimens, both under saturated and unsaturated conditions, representing three “initial” water contents and densities. The “initial” water contents and densities of the specimens were selected to represent the dry, optimum, and wet of optimum water content conditions with reference to the compaction curve. Multistage, unsaturated, direct shear tests were conducted under three different net normal stresses with varying matric suction values for each case. The soil-water characteristic curves were also developed on specimens with “initial” conditions similar to those used for the unsaturated shear strength tests. The shear strength variation with respect to matric suction was found to be nonlinear for all the tests. The rate of increase in the shear strength contribution due to matric suction, however, was found to be related to the rate of desaturation of the soil. The desaturation characteristics are a function of the “initial” water content of the compacted specimens. For any particular net normal stress and matric suction, specimens compacted wet of optimum water content offered more resistance to desaturation and exhibited a higher shear strength when compared to specimens compacted at dry of optimum or at optimum water content conditions. Under similar “initial” conditions, the soil-water characteristic curve bears a close relationship to the unsaturated shear strength behavior of the soil.

Semi-empirical model for the prediction of modulus of elasticity for unsaturated soils

A semi-empirical model is proposed in this paper to predict the variation of modulus of elasticity with respect to matric suction for unsaturated sandy soils using the soil-water characteristic curve (SWCC) and the modulus of elasticity under saturated conditions. Using this model, comparisons are provided between the predicted and measured moduli of elasticity and elastic settlements from model footing test results on three different sandy soils. The results of this study are encouraging as there is good agreement between the predicted and measured moduli of elasticity and settlements.

Bearing Capacity of Model Footings in Unsaturated Soils

A simple technique is proposed to predict the variation of the bearing capacity of an unsaturated soil with respect to matric suction. This technique is based on extending conventional bearing capacity theory proposed by Terzaghi. The proposed equation in this paper is presented as a functional relationship such that the variation of the bearing capacity of an unsaturated soil with respect to matric suction can be predicted. This technique is developed extending the concepts for predicting the shear strength of unsaturated soils proposed by Vanapalli et al. (1996). Using the approach presented in this paper, the bearing capacity of an unsaturated soil can be predicted using the saturated shear strength parameters, c’ and π’ and the soil-water retention curve (SWRC).

A state-of-the art review of 1-D heave prediction methods for expansive soils

Abstract Expansive soils are typically in a state of unsaturated condition and are widely found in semi-arid and arid regions of the world. The engineering properties of these soils are highly sensitive to changes in water content and suction in the active zone depth. One of the key problems associated with expansive soils is their swelling characteristics due to an increase in their natural water content. Practicing geotechnical engineers consider reliable prediction of the heave behavior of expansive soils and understanding its impact on the structures as one of their greatest challenges. Several researchers and practitioners from all around the world have made significant contributions over the past 60 years to better understand these problematic soils. This paper synthesizes information of various techniques available in literature for estimating the swelling pressure and the 1-D heave behavior of expansive soils. In addition, the limitations of using them in geotechnical engineering practice are discussed.

Modelling the applied vertical stress and settlement relationship of shallow foundations in saturated and unsaturated sands

The bearing capacity and settlement of foundations are determined experimentally or modelled numerically based on conventional soil mechanics for saturated soils. In both methods, bearing capacity and settlement are estimated based on the applied vertical stress versus surface settlement relationship. These methods are also conventionally used for soils that are in an unsaturated condition, ignoring the contribution of matric suction. In this study, a methodology is proposed to estimate the bearing capacity and settlement of shallow foundations in unsaturated sands by predicting the applied vertical stress versus surface settlement relationship. The proposed method requires soil parameters obtained under only saturated conditions (i.e., effective cohesion, effective internal friction angle, and modulus of subgrade reaction from model footing test) along with the soil-water characteristic curve (SWCC). In addition, finite element analyses are undertaken to simulate the applied vertical stress versus surfac...

A model for predicting the modulus of elasticity of unsaturated soils using the soil-water characteristic curve

Abstract A simple model was proposed by Oh et al. (2009) to predict the variation of modulus of elasticity with respect to matric suction for unsaturated coarse-grained soils (i.e., sandy soils with plasticity index, Ip = 0). This model requires the modulus of elasticity under saturated condition (Esat) and the Soil-Water Characteristic Curve (SWCC) along with two fitting parameters (α and β). In the present study, this model has been extended and a general model has been proposed such that the modulus of elasticity can be estimated for all soils (i.e., both coarse- and fine-grained soils). The proposed model is developed using model footing and in-situ plate load test results of six different unsaturated soils. The study shows that there is a good comparison between the measured and the predicted modulus of elasticity with respect to matric suction for all the six soils whose plasticity index, Ip values are in the range of 0 to 16%.

Interpretation of the Bearing Capacity of Unsaturated Fine-Grained Soil Using the Modified Effective and the Modified Total Stress Approaches

AbstractThe bearing capacity of unsaturated soils is commonly estimated using the conventional shear-strength parameters determined from drained loading conditions regardless of soil type and drainage condition. However, the use of drained shear-strength parameters to interpret the bearing capacity of unsaturated fine-grained soils is not reliable because there are uncertainties with respect to the drainage conditions of both the pore-air and pore-water pressures. In the current study, a series of model footing tests is conducted in statically compacted unsaturated fine-grained soils. The model footing test results are interpreted using the modified effective-stress and total stress approaches considering the influence of matric suction. In addition, the validity of the modified total stress approach is tested using in situ plate-load test results in unsaturated fine-grained soils. The advantages, disadvantages, and limitations of using the modified effective-stress and modified total-stress approaches fo...

Modified Ring Shear Apparatus for Unsaturated Soils Testing

This paper presents the design details of an automated modified ring shear apparatus for the testing of unsaturated soils using the axis-translation technique. The components of this automated system are described, as well as the types of tests that it can be used for. The key cell components include a flushing system and a precise volume gage system. The paper presents typical test results including the continuous monitoring of water as the test proceeds. A methodology to correct the water content measurements for the effect of air infiltration and evaporation or condensation of water is included.

A Revised Contact Filter Paper Method

The filter paper method, ASTM Standard D5298-03 is generally accepted to be an inexpensive, technically simple, and reasonably accurate method that could be used to measure a wide range of soil suction. The method, however, is dependent upon the accuracy of the calibration curve that relates filter paper water content to soil suction. Additionally, applying a contact stress to the filter papers significantly influences this calibration curve. The effect of five separate contact stresses, namely, 0, 0.4, 1, 2, and 4 kPa on the calibration curve of Whatman 42 filter paper were examined. The study demonstrates that a contact stress of 1 kPa will ensure direct contact between the filter papers and the pore-water of the test specimens without significantly altering the ASTM calibration curve. The contact filter paper method was used to measure the matric suction of compacted Indian Head till specimens over a range of 20 to 300 kPa. This range was chosen to understand the performance and sensitivity of the filter paper method in the low suction range. A comparison of published calibration equations was then undertaken to show the drastically different matric suction estimates that can be obtained by the improper selection of published calibration equations.