The water content of soil in the field varies by precipitation and evaporation, freezing, thawing, etc. This seasonal variation of water content induces capillary pressure variation of soil. In general, dynamic deformational characteristics of soil, which are expressed in terms of maximum shear modulus $(G_{max})$, normalized shear modulus $(G/G_{max})$ and damping ratio (D) are mainly affected by variations of both water content and capillary pressure(i.e. matric suction). Therefore, it is important to determine the influences of water content and capillary pressure on dynamic deformational characteristics of soils for the proper design of geotechnical structures.
The overall purpose of this study is to investigate the influences of water content and capillary pressure on dynamic deformational characteristics of soils, particularly subgrade soils in pavement, by resonant column and torsional shear (RC/TS) tests. The RC/TS equipment was modified to control the capillary pressure and measure the water content change caused by the variation of capillary pressure. Specimens were prepared at various conditions of molding water contents and dry densities. RC/TS tests were performed at some given capillary pressure levels for each specimen. The variations of maximum shear modulus, normalized shear modulus reduction curves, loading frequency effect and damping ratio of tested soils, which were induced by changes of capillary pressure and water content, were investigated. It was observed that the shear modulus of soils increased obviously with the increase of capillary pressure and the decrease of water content at small-to-medium shearing strains. It was revealed, however, that the normalized shear modulus and the damping ratio remained constant. Finally the influences of specimen preparation conditions on relationships between capillary pressure and dynamic deformational characteristics of soil were evaluated.
