Completely decomposed granite (CDG) is a common soil, and hence a common geomaterial, in Hong Kong and many others countries around the world. In the light of the fact that the literature on its dynamic properties are quite limited, this research aims to study the characteristics of shear modulus and damping of a completely decomposed granite in Hong Kong and the influence of wetting on its shear modulus and damping ratio at small-strain level....[ Read more ]

Completely decomposed granite (CDG) is a common soil, and hence a common geomaterial, in Hong Kong and many others countries around the world. In the light of the fact that the literature on its dynamic properties are quite limited, this research aims to study the characteristics of shear modulus and damping of a completely decomposed granite in Hong Kong and the influence of wetting on its shear modulus and damping ratio at small-strain level.

The completely decomposed granite exhibits practically linearly elastic behavior at small-strain level up to a threshold at 0.001 %; thereafter, it becomes nonlinear and shear modulus degrades and damping ratio increases significantly. The small-strain shear modulus of the soil varies with effective mean normal stress to an exponent of 0.64 and decreases moderately with increasing void ratio. An empirical equation is suggested to relate the small-strain shear modulus to effective mean normal stress and void ratio in this research.

To study the influence of wetting on the small-strain shear modulus and damping ratio of the soil, a dry specimen in the resonant column apparatus was wetted to different degrees of saturation by the infiltration of water, through the filter paper side drains placed on its circumferential surface, under constant external water confining stress. In the wetting sequence, each degree of saturation was cured for 72 hours for homogenization of water. Pore air pressure was always kept at atmospheric. It was demonstrated that the wetting was uniform and water phase was continuous for each degree of saturation greater than 32 %. The monitor of small-strain shear modulus, small-strain damping ratio and negative pore water pressure with time indicated that they became essentially constant after 72-hour curing period when the water phase was continuous .

The small-strain shear modulus of the completely decomposed granite increases noticeably as degree of saturation increases up to around 25 %, after which it decreases as degree of saturation increases further. The effect of degree of saturation on small- strain shear modulus decreases when external confining stress is increased, and increases when void ratio is decreased.

When a dry CDG specimen is wetted, the small-strain damping ratio increases noticeably. Since the test data is scattering, more test results are needed to draw a clear conclusion for the variation of β_min with degree of saturation. When the external confining stress is increased, the small-strain damping ratio becomes smaller in the whole range of degree of saturation.