AbstractsEngineering

The cone penetration test (CPT) is widely used for in-situ characterization of saturated or dry soils for which interpretation methods are well established. The CPT is also performed in unsaturated soils yet very little is known about how to interpret the recovered results. The few published studies on the CPT in unsaturated soils have focused on either clean sands or a silt. Never before have the effects of hydraulic hysteresis and suction hardening, features known to influence the mechanical behavior of many unsaturated soils, on cone penetration resistance been investigated. This research aims to fill these knowledge gaps. The soil considered in the study is a silty sand, a soil for which hydraulic hysteresis and suction hardening are present. A theoretical component includes a new cavity expansion analysis. This is useful since the pressure required to expand a cavity has a relationship with the cone penetration resistance. The effects of where the initial hydraulic state is located on the soil-water characteristic curve is investigated and found to have a significant influence on cavity pressure. Also, the effects of three different drainage conditions on cavity pressure are studied. It is found that the condition in which the contribution of suction to the effective stress is constant offers a good approximation to the others. One experimental component includes soil element tests to obtain the constitutive properties of the silty sand. Test results were used to calibrate a bounding surface plasticity model and generate cavity expansion results. Another experimental component includes laboratory-controlled CPTs in silty sand samples. Suction is observed to have a pronounced affect on cone penetration resistance. A new fractal-based hydraulic conductivity model is developed to study the time needed to achieve suction equilibrium in the samples during set up. Based on the cavity expansion and experimental test results, a semi-theoretical expression is presented that links cone penetration resistances to initial relative density and mean effective stress along with a parameter to account for suction hardening. It is shown that failing to account for suction may result in significant overestimation and unsafe predictions of soil properties from measured cone penetration resistances.