Mechanical behavior of a volcanic ash soil (Typic Hapludand) under static and dynamic loading

Abstract

Andisols are often subject to landsliding and coinciding liquifaction of the existing soil structure because they dominate at steeper slopes and show often an insufficient hydraulic conductivity. Thus, soil erosion at those sites is considered as one of the major risk. In order to quantify the proportion of the mechanically and of the pore water dependent soil strength, we evaluated their dependency during compression (divergent process) and shear tests as a function of the pore water pressure in a volcanic ash soil (Typic Hapludand). Undisturbed samples collected at two depths (0-15, 40-55 cm) were equilibrated at two pore water pressure values (-60 and -300 hPa). Thereafter they were compressed for either 10 or 30 min, respectively at each total stress applied. Shear tests with a shear speed of 0.2 mm min-1 were performed at the same two initial pore water pressure values, after the samples had been statically pre-stressed with identical loads all smaller than 400 kPa. The pre-compression stress was similar between time intervals when the samples were dried at -60 hPa, but on average for the two depths the pre-compression stress was 32% lower when 30 min steps were performed because of a more complete settlement due to the time dependency of water flow during soil deformation. There was an increase in cohesion of 30% in topsoil and 900% in subsoil when the pore water pressure changed from pF 1.8 to pF 2.5.

Irrespective of the loading time interval, pore water pressure increased to even positive pore water pressure values when mechanical stresses of > 300 kPa were applied, being more important in samples after pre-drying at -60 hPa than at -300 hPa. In the shear test the behavior was similar, but positive values of pore water pressure already occurred when the vertical stress exceeded 200 kPa. Thus, we can conclude, that hydraulic properties are specially important in soils showing thixotropic behavior and/or when shear stresses are applied resulting in a more pronounced weakening by increasing the destabilizing pore water pressure as a reason for an enhanced particle rearrangement and soil homogenization compared with the original soil structure.