Geotechnical and Geological Characterisation of Decomposed Volcanic Rocks from Hong Kong

香港地區風化火山岩岩土工程及地質特徵研究

Student thesis: Doctoral Thesis

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Award date14 Aug 2017

Abstract

There are many extensive and detailed investigations into the effects of weathering on the mechanical behaviour of sedimentary soils in temperate regions of the world, generally testing the soil both in the reconstituted and the intact state within the critical state framework. However, research on the behaviour of geomaterials resulting from chemical decomposition of rocks (saprolites) which are abundant in tropical and subtropical areas of the world, applying critical state framework, are much less common. In addition, the little research available has generally discussed the weathering along a vertical profile and/or has investigated only a few weathering degrees. This research presents a comprehensive laboratory investigation into the effects of weathering on the nature and the mechanics of decomposed volcanic rocks from Hong Kong. The evolution of the physical properties and mechanical behaviour of these materials are studied covering a wide range of weathering degrees ranging from Highly Decomposed Volcanic rocks (HDV) to Completely Decomposed Volcanic rocks (CDV), from different locations, depths and formations which are both fine and coarse grained. A critical state framework that accounts for the effects of structure has been used and also the spatial variations of the properties have been investigated. In addition to index tests, optical microscope, Scanning Electron Microscope, X-Ray Diffraction and X-Ray Fluorescence, the behaviour at large strains was studied with an extensive series of oedometer and triaxial tests at low, medium and high pressures, while the small strain behaviour was investigated using multi-directional bender elements and high resolution local strain gauges, comparing the behaviour of samples from different locations and depths in the intact and the reconstituted states. This made it possible to trace the changes in the intrinsic behaviour throughout different degrees of weathering with the changing effects of structure in the intact samples. This research also showed that a critical state framework that accounts for the effects of structure can be applied to this type of geomaterial. The changes in the intrinsic properties are generally consistent with the increase in fines content and plasticity as a result of weathering. The sampling depth and chemical weathering indices are the most appropriate factors to predict the compressibility of the decomposed volcanic rocks. The strength will be satisfactorily predicted by fines content, clay minerals and chemical weathering indices while the depth and chemical weathering indices are most appropriate for predicting the in-situ specific volume. The chemical weathering indices alone are the most suitable for predicting the grading of the decomposed volcanic rocks. These approaches (depth, fines, mineralogy, and chemical weathering indices) are suitable for different parameters and are therefore recommended for the practising engineers working on these geomaterials depending on which properties are needed. The spatial variability of the mechanical parameters is relatively moderate but the variability in the critical state line gradient M is small compared to other mechanical parameters. The effects of structure are small to medium in compression but are less evident at small strains and at failure for shearing and generally they increase with depth and reduce with weathering. The degrees of anisotropy for different weathering degrees in the intact and reconstituted states are quite small.