A long natural slope, extending over a large area, is inclined at 14 degrees to the horizontal. Observations over an extended period of time suggest that the slope reaches a state of imminent collapse following periods of very wet weather. A site investigation has revealed that 4m of fine sandy silt overlies a lightly cemented glacial till on the slope. Porewater pressure measurements in the silt indicate that, during adverse weather conditions, the groundwater flows parallel to the surface of the slope, which is itself a seepage surface. The bulk unit weight of the silt is estimated to be 18 kN/m³.
(a) Assuming a plane failure surface parallel to the surface of the slope, draw a schematic to illustrate the forces on the plane failure surface.
(b) Estimate (phi)' for the silt, assuming c'= 0.
It is proposed to stabilize the lower reaches of the slope by installing a system of drains to lower the level of the water table within the slope, assuming the bulk unit weight of the silt is always 18 kN/m³ regardless of the water table level.
(c) Calculate the amount by which the water table must be lowered to provide a factor of safety of 1.5 against collapse during adverse weather conditions.
(d)What would be the critical angle c of the long natural slope if the fine sandy silt on the slope is fully dry without seepage?