Mechanics of sinkhole due to ground tremor / Mohd Firdaus Mohd Akhbar

The abundance of limestone formation outcrops mostly in west peninsular Malaysia such as Perak is vulnerable to the geological hazards such as sinkhole. The primary reason for the sinkhole incidences are due to the uneven and dissolution of limestone formation. A sinkhole is a phenomenon where a natural depression or hole in the earth's surface occurred due to several reasons. Before sinkhole incidences happen, there is an initial condition in the ground, which is underlain by soluble bedrock such as limestone formation and topped off by its residual soil. When the effective stress from the overlying residual soil increase, the cavity roof on the limestone formation may not be able to support and the residual soil may sink into the existing cavity. A study was carried out at the sinkhole prone area in Perak and Selangor. The physical characteristic of the limestone residual soil that vulnerable to sinkhole incidence that occupied these two sites was determined. The site investigation report consisted of Mackintosh Probe and Boreholes data from these two sites was analyzed in order to determine the characteristic of the limestone residual soil that vulnerable to sinkhole incidence. Result showed that the limestone residual soil above cavity is normally constituted of loose silty SAND and soft sandy SILT. The application of the non-intrusive method can be useful to detect the abnormalities and type of material in the ground based on the geoelectrical resistivity index. A non-intrusive method using geoelectrical resistivity survey was applied to determine the geoelectrical resistivity index of the limestone residual soil at these two sites. The finding from this method was used to determine the relationship of the geoelectrical resistivity index with the physical and engineering properties of the limestone residual soil. The earthquake event can pose a hazard known as ground tremor. The sinkhole incidence may also formed in response to dynamic loading from earthquake tremor. The energy from an earthquake tremor released a force which is a shear stress. The vibration from the ground tremor will start to accumulate the shear strain that occurred between the soils particles. Because of that, sinkhole incidences triggered as a result of gravity forces pulling the soil particles down when the soil particles experienced slippage or reorientation. This is happened when there is a significant of reduction in the shear modulus of the soil that caused the soil loose it strength below the amount needed for static stability. In order to simulate the sinkhole incidence during the ground tremor, the laboratory sinkhole model was setup using a glass tank with the remoulded limestone residual soil that was collected at Perak site. The laboratory sinkhole model was tested under vibration test using shaking table. The objective of the laboratory sinkhole model is to investigate the behavior and effects of percentage of fines material and presence of moisture content to the shear modulus of the remoulded limestone residual soil when subjected to ground tremor. The finding from the laboratory sinkhole model showed that the shear modulus of all remoulded limestone residual soil samples had significant reduction during the vibration and sinkhole incidences were governed by the effects of different percentage of fines material and with the presence of moisture content.