EFFECTS OF PHYSICAL AND MECHANICAL PROPERTIES OF RESIDUAL SOIL ON SLIDING AREA AT BUNDU TUHAN, SABAH, MALAYSIA
Authors: Roslee Rodeano 
Published: November 2020

The study of the earth mass movement has long been regarded as one of the most important and interesting aspect of engineering geology and geotechnical engineering, which the designers and planners from the private and public sectors address when implementing the initial stage of urban and rural development projects. This involves highways and infrastructures construction and land use planning among the others. Failure to appreciate the problems relating to mass movements of earth material could lead to damage of man made structures and even the loss of lives. These studies focused on the mass movement in Bundu Tuhan to Kundasang highway area approximately 84 km to 96 km from Kota Kinabalu city, Sabah, one of the most vulnerable to mass movements occurrence in west coast of Sabah. It is bounded by longitude line E 116o 31.592’ to E 116o 36.183’ and latitude line N 06o 00.269’ to N 05o 57.610’. The main objectives of this study are; 1) to map and locate the landslides in the study area; and 2) to study the mechanism and the influence of geological factors causing the mass movement. Geology of the study area and its surrounding is hosted mainly by three sedimentary rock formations: Trusmadi Formation (Palaeocene to Eocene age), Crocker Formation (Late Eocene to Early Miocene age) and Pinousuk Gravel (Upper Pleistocene to Holocene age). These three geologic formations dissected by numerous geological lineaments structural produced by a complex tectonic history of multi phase folding and thrust, normal and reverse faulting. These tectonic setting reduce the physical and mechanical properties of the soil and produced intensive displacement in substrata resulting in intensive high degree of weathering processes. The weathered materials are weak and cause sinking, subsidence and sliding due to high pore pressure subjected by both shallow and deep groundwater. Evaluation 10 boreholes data in study area indicated that the groundwater table in study area is shallow and range 1.9 meter to about 11.3 meters. The groundwater in study area fluctuate drastically even within short period. Sand and gravel layer with variable thickness defined the major shallow aquifers within the top weathered materials while the highly fracture sedimentary rocks defined the major deep aquifers. Most of the aquifer within top unconsolidated weathered material is under unconfined condition. Most of significant aquifers within the sedimentary rocks are sandstones. The sandstones generally fracture and contain coarse sediments, which increase the permeability. Geologic and geotechnic evaluation of the study area indicates that the mass movement take place when slope materials are no longer able to resist the force of gravity. These decrease the shear resistance resulting mass movement, which is due to internal and external factors. Internal factors involve some change in either physical or chemical properties of the rock and soil. External factors involve increase of shear stress on slope, which usually involves a form of disturbance that is induced by man. The triggering mechanism in the study area most likely involves heavy rainfall causing water saturation of the slope material and loss of cohesion along rapture planes. The sheared shale, bedding and fault planes, and opening fractures are all structural weaknesses, which acting as pathways for water seepage, hastening the weakening and eventual mass movement in the study area. Development planning has to consider these hazards in order to counter their effect. An environmental management program should be implemented to prevent these losses. Geological and geotechnical studies will play a vital role in ground stability assessment that critical in public safety.