The research into small strain characteristics of sedimentary soils has advanced to the stage where it has been widely applied by the profession in advanced countries such as Japan and U.K.. In spite of the progress, the knowledge of small strain behavior of geomaterials is relatively poor in the developing countries. Systematic study of the stiffness characteristics of granitic saprolite (Completely Decomposed Granite, CDG, and Highly Decomposed Granite, HDG) at small strains (0.001% to 1%) has attracted less attention....[ Read more ]

The research into small strain characteristics of sedimentary soils has advanced to the stage where it has been widely applied by the profession in advanced countries such as Japan and U.K.. In spite of the progress, the knowledge of small strain behavior of geomaterials is relatively poor in the developing countries. Systematic study of the stiffness characteristics of granitic saprolite (Completely Decomposed Granite, CDG, and Highly Decomposed Granite, HDG) at small strains (0.001% to 1%) has attracted less attention.

This research project studies the stiffness characteristics of granitic saprolites at small strains systematically. It consists of three parts: laboratory tests, field tests and numerical analysis. In the laboratory tests, both bender elements and local transducers (Hall Effect transducer) were used to measure the small strain characteristics of CDG subjected to various triaxial stress paths including compression and extension conditions. Both intact and recompacted samples were tested in laboratory. While in the field tests, both cross-hole seismic techniqu, self-boring pressuremeter and high pressure dilatometer were applied. Based on the test results, the numerical analysis was carried out to model two deep excavations in Hong Kong and comparisons between the predictions and the observations were conducted. The numerical analysis was performed with the finite element program called "SAFE", in which the non-linear and path-dependent behavior of geomaterial can be taken into account with the "Brick" model.

It was found that the shear stiffness of granitic saprolite at small strains is highly non-linear. Shear modulus and bulk modulus decreases as shear strain and volumetric strain increases, respectively. Measured average shear stiffness obtained from extension tests on intact samples is higher than that from compression tests by up to about 60% at 0.01% shear strain. But the difference becomes smaller as strain increases and it vanishes at about 1%. Recent stress history also leads to higher non-linearity stiffness. Tests with 90° rotation of stress path have about 50% greater stiffness at 0.01% than that of tests without stress path rotation. The structure and bonding inherent from parent rock is one of the likely reasons to explain the higher stiffness obtained from cross-hole seismic tests than that from bender element tests in laboratory. The elastic shear modulus obtained from field cross-hole tests is about 30-100% higher than that from bender element tests on intact samples in the laboratory. Much higher shear modulus is obtained for HDG than that for CDG in the field. From the numerical analysis, the simulation with the "Brick" model matches the observed data better than those with Mohr-Coulomb model. The computations with Mohr-Coulomb in general under-predict the deformation of ground. The effect of extension stiffness on ground deformation was also investigated. It was found that when the higher stiffness from extension tests is adopted for soil elements under the bottom of excavation, the computed results match the observed data more properly as compared with those neglect the effect of extension stiffness. It was also found that all computations over-predict the ground settlement behind the wall and some potential reasons such as dilatancy and anisotropic stiffness were investigated. But they can't give a complete explanation on the poor computations of ground settlement.