STABILITY OF EXCAVATED SLOPES DUE TO GENERATION OF NEGATIVE EXCESS POREWATER PRESSURE

Abstract

Negative excess pore water pressure or depressed pore water pressure or pore water tensions are formed after a saturated soil in undrained condition is excavated. Because of this short and long term instabilities could arise following the dissipation of this depressed pore water pressure. Just after excavation this depressed pore water pressure starts decreasing slowly depending upon the drainage conditions prevailing in the soil and reach a certain equilibrate or steady state seepage values after a long period of time. Because of this, there are variations in average effective stress values in the zones below and adjacent to this excavated slope, which in turn pose serious threat to the stabilities of such slopes in the long run. In this paper a benchmark geotechnical excavation was investigated in a finite element software MIDAS GTS NX to determine the generation and dissipation of this depressed pore water pressure. Also, the effect of different in-situ stress values on the stability of slopes was studied. A fully-coupled stress seepage analysis was undertaken to simulate the real time dependent analysis. Results show that with higher removal of stresses or higher unloading, higher negative excess pore water pressures were generated which in the short term increased both the effective stress and factor of safety but with gradual dissipation of the same might lead to delayed failure of the slopes.