ABSTRACT: Soil near ground surface is usually unsaturated and the modulus value is affected by the seasonal variation of water content and the corresponding capillary pressure. The Stokoe-type torsional resonant column (RC) testing equipment was modified to control the pore air and pore water pressures separately and to measure the variation of water content due to capillary pressure. Subgrade soil was collected at a test road site where the crosshole and water content measurements were performed with depth. RC tests were performed at various water contents by controlling capillary pressures using a given specimen and by using specimens compacted at various water contents. As the capillary pressure increased, the shear modulus was increased at the whole strain range due to the increase of effective stress between soil particles. The relation between maximum shear modulus (G_max) and water content was much flatter and G_max, value is smaller at a given water content for specimens prepared by controlling compaction water contents compared with those tested by controlling capillary pressures. The normalized modulus (G/G_max) reduction curve was almost unique independent of compaction water content and density during sample preparation and variation of water content due to capillary pressure during testing. Field crosshole tests were also performed which showed that G_max values determined by crosshole tests matched well with those determined by controlling capillary pressures in the laboratory, but also showed substantial difference with results obtained by controlling compaction water contents.

Key words:  capillary pressure, crosshole test, maximum shear modulus, modulus reduction curve, resonant column test,     water content (IGC: D7)