Properties of High-Plasticity Clays (0-2100-2)

The process of slope failures in high-plasticity clays involves formation of surface cracks, moisture infiltration through the cracks into the soil mass, a reduction in suction and hence shearing resistance of the soil, and ultimately slope failure when the driving stresses exceed the shearing resistance of the soil. Similar processes can impact other earth structures such as retaining walls and pavements. Two issues addressed in this report are the rate of moisture diffusion into the soil mass and the practical limit to which suction and soil shearing resistance degrade. The rate at which moisture diffusion infiltrates into the soil is a key factor in assessing the depth to which strength degradation due to seasonal fluctuations in moisture is likely to occur. This project adopts an approach originally proposed by Australian researchers for analyzing moisture infiltration through partly saturated soils. This approach utilizes a linear diffusion equation for characterizing moisture infiltration. This approach requires a single material parameter: a-coefficient governing the rate of moisture infiltration. Researchers on this project developed a simple laboratory test for evaluating this parameter by measuring the changes in suction that occur in an undisturbed soil sample when one end of the sample is dried. Preliminary results are very encouraging, although improved methods of measuring soil suction would greatly improve the test. Researchers on this project also investigated the lower limit of suction that will occur in the field due to wetting of soils. These studies were largely based on back-analysis of apparent soil suction from documented slope failures. These studies showed that the matric suction at failure generally is in the range pF=1 .6 to 1 .8.