Interactions between soil stability and pore water pressure as a function of the loading time during compaction test

Soil tillage and the accompanying compaction, can dramatically alter soil physical properties. Quantifying soil compaction modifications of soil bulk density is often used while interactions between hydraulic and mechanical processes are seldom addressed. The pre-compression stress value is used to predict irreversible soil deformation. We analysed the effects of mechanical stress and the time required for changes in soil strength due to soil settlement and associated interactions between stress dependent changes in pore water pressure and pre- compression stress. Disturbed soil materials from a clayey silt (Ut3-German classification) with a defined bulk density of 1,4 g/cm 3 and water contents of 25g*100g -1 were used for these studies. Soil samples were compacted, using 8 different loading steps between 20 and 400 kPa and 4 different loading times. Changes in pore water pressures and the settlement rates were recorded. Increased loading times increased soil settlement as pre-compression stress values decreased. Parallel to these variations in settlement we also found changes in the pore water pressure values. These responses appeared to result from decreasing pore diameters with increasing water saturation increases. Thus, the proportion of neutral stresses on total stress increased. These results suggest that changes in water suction (= unsaturated) conditions up to even positive pore water pressure values (from less negative to positive pore water pressure values) directly influenced pore diameters. From our experiments we can conclude that the changes in pore water pressure values already occur at normal stress values smaller than the pre-compression stress and it underlines the increasing sensitivity of soil deformation processes close to the internal soil strength. The results support the idea, that in order to quantify the mechanical strength of structured unsaturated soils, changes in pore water pressure values cannot be ignored.