A semi-empirical approach for the interpretation of the bearing capacity of unsaturated soils

In many parts of the world, and particularly in semi-arid and arid regions, shallow foundations are located above the ground water table where the soil is typically in a state of unsaturated conditions. The capillary stresses (i.e., matric suction or negative pore water pressure) above the ground water table contribute significantly towards the bearing capacity of unsaturated soils. However, the bearing capacity contribution due to the capillary stresses is routinely ignored in the conventional engineering design of shallow foundations. The bearing capacity of unsaturated soil is also estimated using conventional approaches that are used for saturated soils ignoring the influence of matric suction. This is a conservative approach used in the design of shallow foundations for unsaturated soils. For this reason, it is important to understand the influence of matric suction on the bearing capacity of unsaturated soils. In addition, a framework to interpret the bearing capacity of unsaturated soils that takes account of the influence of matric suction will be useful. In the present research program, an extensive experimental study is undertaken to investigate the bearing capacity of a compacted coarse-grained soil under both saturated and unsaturated conditions. Two different series of tests were carried out in a bearing capacity tank using two different sizes of square shaped model footings (i.e. 100 mm x 100 mm and 150 mm x 150 mm). The bearing capacity tank of the size 900 mm x 900 mm and 750 mm height was specially designed and fabricated in the University of Ottawa, Faculty of Engineering Student Workshop to undertake the present research program. The first series of tests were conducted to determine the bearing capacity of the compacted sand in the tank under fully saturated conditions. The second series of tests were conducted to study the influence of matric suction on the bearing capacity of compacted sand under different unsaturated conditions. The ground water table level in the bearing capacity tank was varied using the drainage valves to achieve different values of matric suction below the model footings. The variation of the matric suction with respect to depth below the model footing was measured using four commercial Tensiometers. These Tensiometers were located at different depths in the proximity of the expected stress bulb (i.e., 1.5 times the width of the footing) below the loaded footing. In addition, piezometers were also placed in the tank to monitor the ground water table levels. The model footings were loaded using a loading frame until failure to measure the bearing capacity. The sand used in the bearing capacity tank was compacted to achieve a density index value of approximately equal to 65% such that the failure of the footing will be in accordance with the general shear failure criteria. The experimental results of the test program demonstrated that the bearing capacity of unsaturated soils is approximately 5 to 7 times higher than the bearing capacity of the same soil under saturated conditions. As expected, there was a non-linear increase in the bearing capacity of the compacted sand with an increase in the matric suction under the model footings. In addition, it was also observed that there is a relationship between the soil-water retention curve (SWRC) and the bearing capacity of unsaturated soils. Terzaghi's bearing capacity theory for saturated soils was extended to interpret the bearing capacity of unsaturated soils using the saturated shear strength parameters and taking account of the influence of the matric suction. In addition, a simple technique was proposed for predicting the variation of the bearing the capacity of unsaturated soils using the soil-water retention curve (SWRC) and the saturated shear strength parameters. The proposed prediction procedure was similar to the procedure used for predicting the shear strength of unsaturated soils by Vanapalli et al. (1996). The procedure proposed for predicting the bearing capacity of unsaturated soils was extended for another four soils, of which two of them were coarse-grained and the other two were fine-grained in nature. The data for the four soils was obtained from the literature. The results of the study suggest that the variation of the bearing capacity of unsaturated soils can be reasonably well predicted using the SWRC and the saturated shear strength parameters. The framework of the research program developed based on the studies undertaken on model footings shows considerable promise for extending it to field studies.

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