Determination of hydro-mechanical properties of unsaturated soil and its application for slope stability analysis

HANDOKO, Luky, Dr. Ir. Ahmad Rifa'i, M.T

2010 | Tesis | S2 Teknik Sipil

Rainfall may occur seasonally, and the rain infiltration will change the initially unsaturated soil become more saturated or fully saturated. This condition may lead the reduction of shear strength of soil due to loss of suction. Conventional methods of slope stability analysis often ignore the negative pore water pressure above the ground water level. Recently, many literatures discuss about the behavior of unsaturated soil, thus, simplification with ignoring the negative pore water pressure is not needed. SWCC can be approached from several methods. SOILVISION is needed to provide hydro-mechanical properties of unsaturated soil. Rainfall models to analyze the seepage at the slope are parted into 3 types: a model represents no rainfall occurrence, a model of hourly rainfall data refers to intensity of prescribed return periods, and several models represent the frequently occurrence of rainfall at study area. Slope stability analysis integrated with seepage simulation is conducted to calculate safety factor and to determine the slip surface location. A countermeasure work with combination methods has been proposed to stabilize the slope through reducing the increase of pore water pressure by installing subsurface drain and providing a weight at the toe of the slope in order to increase the shear strength of the toe. An observation at Section A-A’ shows that Model VI has the greatest increment of pore water pressure at the end of simulation. From -42.16kPa of the minimum pore water pressure at initial condition, it increases become -11.965kP at Model IV, followed by Model V which is about -14.668kPa, Model III is about -17.099kPa, Model VI is about -21.169kPa and the smallest is in Model II which is about -32.382kPa. These results show the same sequence that occurs in increasing of pore water pressure at base of slices and decreasing of safety factor. This result indicates that there is a strong relationship between pore water pressure distribution and safety factor. Safety factor of initial condition (no rainfall occurs) is about 1.186. Those, the slope can be considered to be in unsafe condition at initial condition. With involving rainfall in simulation, the safety factor is more decrease due to increase in pore water pressure. The greatest reduction in safety factor occurred in Model IV which is about 1.085. The next sequence is held by Model V (SF=1.101), Model III (SF=1.115), Model VI (SF=1.134) and Model II (1.167). Applied countermeasure works well in increasing average base of slices and increase the safety factor. At initial condition, the safety factor is about 1.491 and at the end of simulation is become 1.417, higher than 1.3, meaning the slope is in safe condition.

Kata Kunci : Unsaturated soil,Hydro,mechanical properties,Infiltration,Seepage analysis,Slope stability analysis