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Engineering Geological Investigation on Slope Stability in The Sandakan Town Area, Sabah, Malaysia

Writers : Rodeano Roslee, Sanudin Tahir & S. Abd Kadir S. Omang

Date : 20-23 Nov 2007

Publisher : 2nd Malaysia-Japan Symposium On Geohazards And Geoenvironmental Engineering

Location : Langkawi

Abstract :

The geology of the Sandakan town area provides a favourable setting for engineering geological instability. The exposed rocks in the study area and its surrounding vary in types and ages, from Late Eocene-Early Miocene Neogene’s clastic sediment of the Garinono Formation, the Sandakan Formation and Volcanic Facies to vary recent Quaternary alluvial materials which are still being deposited. These rock units show numerous lineaments with complex structural styles developed during several regional Tertiary tectonic activities. The tectonic complexities influenced the physical and mechanical properties of the rocks, resulting in a high degree of weathering and instability. The weathered materials are unstable and may experience sliding due to by high pore pressure and intensively of geomorphological processes. In this study, a total of 25 selected critical slope failures were studied and classified into two main groups: rock slope and soil slope. Failures in soil slopes (including embankments) are 18 (72 %) whereas 7 of all failures (28 %) of rock slope. Soil slope failures normally involved large volumes of failed material as compared much rock slopes, where the failures are mostly small. Of the 18 failures in soil slopes, 5 (28 %) are embankment failures making them 20 % of all types of failures. Kinematics rock slope analyses indicates that the variable potential of circular, planar, wedges and toppling failures modes as well as the combination of more than one mode of aforementioned failure. Rock and soil slopes stability analysis indicates that the factor of safety value as unsafe (0.56 to 0.98). Engineering geologic evaluation of the study area indicates that the slope failures took place when rock and soil materials were no longer able to resist the attraction of gravity due to a decrease in shear strength and increase in the shear stresses due to internal and external factors. Internal factors involve some factors change in either physical or chemical properties of the rock or soil such as topographic setting, climate, geologic setting and processes, groundwater condition and engineering characteristics. External factors involve increase of shear stress on slope, which usually involves a form of disturbance that is induced by man includes removal of vegetation cover, induced by vehicles loading and artificial changes or natural phenomenon such as tremors. Development planning has to consider the hazard and environmental management program. This engineering geological study may play a vital role in slope stability assessment to ensure the public safety.