SWELLING CLAY MINERALS AND SLOPE CUT FAILURES IN THE GARINONO FORMATION ALONG JALAN SUNGAI HITAM, LIBARAN, SANDAKAN

Abstract

Jalan Sungai Hitam in the Sandakan Peninsula traverses a flat to gently undulating terrain of low hills and ridges surrounded by broad alluvial flats. Recent slope cuts expose bluish to dark grey mudstones of the Garinono Formation containing pebble to boulder sized, angular to rounded, blocks of sandstone and other rock types. Slump-mud flows have occurred at most of the slope cuts; the failed materials characterized by desiccation cracks and fissures. Soil index properties, including consistency limits and grain size analyses, indicate that the mudstones have a medium to high swell potential. X-ray diffraction analyses show the clay minerals present to be kaolinite, illite and randomly interstratified illite-montmorillonite (a swelling clay mineral). The mudstones are thus sensitive to atmospheric wet-dry cycles with repeated swelling and shrinkage giving rise to the desiccation cracks and fissures, The cracks and fissures reduce the shear strength of the mudstones and allow for the infiltration of rainwater which initiates the slump-mud flows. It is concluded that earthworks in areas of the Garinono Formation in eastern Sabah need to consider the presence of swelling clay minerals; an occurrence that can be inferred from evaluating soil index properties.