Characterizing a weathering profile over serpentinite in the Senaling area, Negeri Sembilan Darul Khusus

Abstract

Three broad morphological zones can be differentiated; the top, 2.5 m thick, pedological soil comprising IA and IB sub-zones of brown, soft clays, and IC1 and IC2 sub-zones of reddish brown, stiff silty clays with lateritic concretions and lateritized core stones. The intermediate, 1.5 m thick, saprock zone consists of brown, stiff, silty clays with indistinct relict fault planes and core-stones, whilst the bottom bedrock zone consists of serpentinite with effects of weathering along discontinuity planes. Constant volume samples show the earth materials to be rather loose with dry unit weights of 10.32 to 16.28 kN/m3 and porosities of 39 to 60%. Particle size distributions are variable with depth; silt contents decreasing from 38 to 58% in saprock to 28 to 44% in the pedological soil zone. Increasing fine clay (<2 μm) contents up the profile from 34 to 43% in saprock to 40 to 51% in sub-zones IA and IB indicate that pedological processes result in continued disintegration of weathered serpentinite. Plastic and liquid limits have limited variation ranging from 24 to 33%, and from 36 to 50%, respectively. Weathered rims around core-stones show alteration of serpentinite to start with the opening-up of grain boundaries and formation of micro-cracks (Stage 1) followed by staining along chrysotile veinlets (Stage 2). More extensive staining then occurs with decomposition (to clay minerals) of most antigorite and chrysotile grains (Stage 3) and ending with formation of a brown, stiff silty clay (Stage 4). Increasing stages of weathering are marked by decreasing dry unit weights and uniaxial compressive strengths, but increasing apparent porosities. It is concluded that in situ alteration of serpentinite through lowering of an unconfined groundwater table has led to development of the weathering profile.