A systematic research has been carried out on the effects of weathering on the compression behaviour of decomposed volcanic rocks in the intact and reconstituted states. Mazier samples taken from different locations, depths and formations that are geologically different were studied in one-dimensional compression by carrying out several oedometer tests. The influence of structure was studied by applying different normalising parameters as well as quantitative stress sensitivity analysis. Weathering affects both intrinsic and intact behaviour of the samples. The compression paths of the reconstituted samples converge to unique intrinsic normal compression lines (NCL) and the NCLs move downward as the degree of weathering reduces. The intrinsic compressibility is highest in the sample that is most affected by weathering. The least weathered sample has lowest in-situ specific volume, that is, lowest porosity and it is less compressible. The influence of structure is most obvious in the most weathered sample and the degree of enhanced resistance of the least weathered intact material in compression is not clear because a very high stress is required to reach and cross the intrinsic normal compression line. By applying stress sensitivity analysis, the effects of structure are seen to reduce with the degree of weathering.


A more extensive study has been carried out on the effects of weathering on the mechanical behaviour of sedimentary soils (e.g. Cafaro & Cotecchia 2001) compared to weathered geomaterials. Weathered rocks are abundant in tropical (e.g. west Africa) and subtropical (e.g. south east Asia) areas of the world and the knowledge about the mechanics of their behaviour is small. In Hong Kong particularly, the geology is dominated by intrusive and extrusive volcanic and granitic rocks. The climate plays an important role in the decomposition of the rocks.

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