Moisture retention characteristics of weathered graphitic-quartz-muscovite schist

Slope cuts in weathered graphitic-quartz-muscovite schist expose an upper pedological soil (3-4 m thick) of silty to sandy clays with lateritic concretions, and a lower saprock (>10 m thick) of alternating bands of variously colored, firm to stiff and hard, clayey silts and silts with indistinct to distinct relict bedrock minerals, textures and structures. Moderately sloping (<60°), low cuts (<15 m high) in unsaturated, weathered schist are unaffected by failures, though small slips and falls have sometimes occurred at steeper cuts during rainfall events. To validate the role of negative pore water pressures in influencing the stability of slope cuts, laboratory pressure plate tests were carried out on weathered graphitic-quartz-muscovite schist collected at depths of 5.83 m (A), 6.71 m (B) and 8.95 m (C) at a weathering profile in the Kajang Schist. Samples A, B, and C, with dry unit weights of 12.76, 14.73, and 13.23, kN/m3, and porosities of 51%, 44%, and 49%, are overwhelmingly fine gained with silt contents of 53.6%, 73.7%, and 76.6%, and clay contents of 35.0%, 14.0%, and 23.0%, respectively. Increasing suctions from 0 kPa through 0.98 kPa and 9.8 kPa to 33 kPa and 1500 kPa resulted in volumetric moisture retentions of 49.9% through 48.9% and 36.7% to 33.0% and 7.2% in sample A, from 51.8% through 48.6% and 41.1% to 34.3% and 4.5% in sample B, and from 55.5% through 53.2% and 47.6% to 30.2% and 8.6% in sample C. Best fit lines drawn with the van Genuchten (1980) parametric model indicate rapidly decreasing moisture contents with increasing suction due to large porosities. It is concluded that negative pore water pressures in unsaturated, weathered graphitic-quartz-muscovite schist give rise to an enhanced shear strength and stability of slope cuts, though rapid infiltration of rainwater results in saturation and failure.