Physical characterization of the weathering profile over a sheared, biotite-muscovite granite in Peninsular Malaysia

Abstract

The weathering profile can be separated into an upper, 11.8 m thick pedological soil (zone I) comprising gravelly clayey sands and a lower, >31.9 m thick saprock (zone II) consisting of gravelly silty sands with distinct preservation of the minerals, textures and structures of the original granitic bedrock material and mass. Zone I can be separated into A, B and C soil horizons, whilst zone II can be differentiated into sub-zones IIA, IIB, IIC and IID based on differences in preservation of relict structures and content of core-boulders. The earth materials of zone I represent rock mass weathering grade VI, whilst those of sub-zones IIA and IIB represent grade V, and those of sub-zones IIC and IID represent grades IV and III respectively. Constant volume samples show the earth materials to have variable dry unit weights (11.98 to 17.66 kN/m3), but a limited range in specific gravity (2.62 to 2.70) due to similar primary and secondary minerals. The zone I earth materials have relatively large clay contents (>19%) and are more porous (33% to 55%) than those of zone II (36% to 44%) which have large silt contents (>23%). Sand contents are more variable (23% to 44%) though relatively large total sand and gravel contents (37% to 68%) point to the original, coarse grained granitic bedrock. Increasing clay contents (of kaolinite and illite) up the profile, and a corresponding decrease in silt contents (of mainly sericite), reflect increasing alteration of the bedrock; a feature also shown by increasing values of the textural weathering index (Iw). Distinct preservation of granitic textures and structures in saprock (zone II) indicate in situ alteration of bedrock; weathering resulting from gradual lowering of an unconfined groundwater table.