Completely decomposed granite (CDG) is abundant in Hong Kong and it is used extensively as a fill material. However, systematic research on this material is relatively less as compared to sedimentary type of soils like sand and clay. In this study, a series of comprehensive laboratory testing is performed, including triaxial compression and extension in both drained and undrained conditions, to study the stress-strain response of re-compacted CDG. A preliminary constitutive model is proposed based on the laboratory findings. The aim of this investigation is to improve the understanding of this material....[ Read more ]

Completely decomposed granite (CDG) is abundant in Hong Kong and it is used extensively as a fill material. However, systematic research on this material is relatively less as compared to sedimentary type of soils like sand and clay. In this study, a series of comprehensive laboratory testing is performed, including triaxial compression and extension in both drained and undrained conditions, to study the stress-strain response of re-compacted CDG. A preliminary constitutive model is proposed based on the laboratory findings. The aim of this investigation is to improve the understanding of this material.

Laboratory testing of the material is performed in triaxial stress space. The compaction process during sample preparation stage produces stress-induced anisotropy to the soil. As confining pressure increases, the degree of such anisotropy decreases. Isotropic compression shows that the material is highly compressible. However, volume change during unloading is found to be insignificant. Normal consolidation lines at constant stress-ratio loading are found to be unique in the e-p' plane and parallel to each other. Normally consolidated or lightly over-consolidated soil shows overall contractive behaviour whereas heavily over-consolidated soil exhibits overall dilation. During undrained constant cell pressure shearing, over-consolidated soil shows dilative response in a low stress ratio range. The soil then exhibits contraction at higher stress ratio. Therefore, a phase-transformation state, in which the dilatancy equals to zero, is identified. For heavily over-consolidated soil, one more dilation phase, right after the contraction, is noted prior reaching the critical state. Therefore, two phase-transformation states are identified in such case. A unique critical state line in e-p'-q space is found for soil under compressive shearing, regardless the initial states and drainage conditions. The critical stress ratio (compression) M_c of the soil is found to be about 1.40.

Necking occurs in triaxial extension tests at around 8% axial strain. Critical state of the soil is estimated from the ultimate states (before necking) of the tests. The critical stress ratio for extension M_e is found to be about 0.94. In e-p' plane, critical state obtained from extension tests lies very close to the CSL obtained from compression test. It may because the inherent anisotropy of the soil is not substantial.

A preliminary critical state bounding surface model in triaxial compression space is proposed. State-dependent dilatancy is incorporated in the model. The model works for normally consolidated and over-consolidated CDG, under either drained or undrained conditions. Also, it is capable of simulating both contractive and dilative behaviours of the soil with only one single set of model parameters. The simulations show good agreement with laboratory results.