Residual shear strength of a weathered graphitic-quartz-mica schist in humid tropical Peninsular Malaysia


Author : J. K. RajPublication : Bulletin of the Geological Society of MalaysiaNumber : 68Page : 117-125Year : 2019DOI : https://doi.org/10.7186/bgsm68201911


Description

Bulletin of the Geological Society of Malaysia, Volume 68, December 2019, pp. 117 – 125

Residual shear strength of a weathered graphitic-quartz-mica schist in humid tropical Peninsular Malaysia

J. K. Raj
Engineering Geology Consultant
No.83, Jln. Burhanuddin Helmi 2, Tmn. Tun Dr. Ismail, 60000 Kuala Lumpur, Malaysia
Email address: jkr.ttdi.tmc@gmail.com

Abstract: In situ weathered bands of graphitic-quartz-mica schist some 0.5 to 1.0 m thick intercalated with 0.3 m thick layers and lenses of quartz-mica schist are found at the site of a compound slide at Km 25.4 of the Kuala Lumpur - Seremban Highway. These schists, mapped as the Kajang Schist of Silurian age, are seen in thin-section to consist of thin (<0.5 mm) layers of fine grained quartz in parallel alignment with thicker layers (up to 5 mm) of aligned sericite, muscovite and clay minerals; the graphitic variety also containing aligned graphite flakes. The weathered graphitic-quartz-mica schist shows variability in index properties with mean dry, and saturated, densities of 1,355 kg/m3, and 1,845 kg/m3, respectively, and a porosity of 49.5%. The weathered schist comprises on average 12.4% sand-sized, 62.6% silt-sized, and 25.0% clay-sized, particles. Three samples of the mortar ground weathered schist were tested with the Bromhead ring shear apparatus to determine residual strength using the pre-shearing procedure with multi-stage loading. Sample A comprised particles <0.18 mm in size, Sample B particles <1.00 mm in size, and Sample C was the original unsieved sample. Plots of effective shear stress at maximum displacement against effective normal stress yield “complete failure envelopes” that can be differentiated into two distinct linear segments. Linear segments for effective normal stresses below 150 kPa yield residual friction angles (Φr) of 24.7o, 25.5o, and 25.8o, for Samples A, B, and C, respectively, or an average residual friction angle of 25.3o. These residual friction angles are comparable with the residual friction angle of 26.5o determined in soil shear box tests along a pre-cut surface in similar weathered schist but under low normal stresses (<10 kPa) and limited displacements. Linear segments for effective normal stresses exceeding 150 kPa yield unwarranted cohesion intercepts due to increased friction as a result of sample extrusion and settlement of the top platen into the specimen container. It is concluded that the pre-shearing test procedure with multi-stage loading is only applicable to determining the residual strength of weathered graphitic-quartz-mica schist under effective normal stresses below 150 kPa.

Keywords: weathered graphitic-quartz-mica schist, residual friction angle

https://doi.org/10.7186/bgsm68201911