Dynamic deformational characteristics of soil, such as maximum shear modulus ($G_{max}$), normalized shear modulus ($G/G_{max}$) and damping ratio (D), are affected by many factors. Among those, the water content is important for the evaluation of dynamic deformational characteristics on compacted soils. Most compacted soils in field are under unsaturated conditions, and they are compacted at a given design water content and water content varies seasonally by precipitation and evaporation, freezing, thawing, etc.
The overall purpose of this study is to investigate the influences of water content and matric suction on dynamic deformational characteristics of subgrade soils in pavement, by using field cross hole and laboratory resonant column and torsional shear (RC/TS) tests. To reproduce the in-situ variation of water content, in the conventional laboratory test specimens are compacted at variable water contents but this method causes soils to have different fabric structure and stress history. In this study, specimens are compacted at a given water content and water contents vary with the control of matric suction in which the situation is more related with field condition. The RC/TS equipment was modified to control the matric suction pressure and to measure the water content change caused by the variation of matric suction. To compare test results obtained by suction control method with those by conventional method, specimens are also prepared at various molding water contents of same dry density, and RC/TS tests were performed. Field cross-hole tests were performed and test results were compared with laboratory test result to check up validity of each method
Shear modulus increases at whole strain ranges as increasing matric suction, and decreasing water content. Normalized shear modulus and damping ratio are not affected by matric suction and water content. The relation between maximun shear modulus and water content obtained by suction control is different from that by controlling molding water content, and matches well with field test results.
