Numerical study of soil conditions under which matric suction can be maintained

Abstract

The effect of negative pore-water pressure is often ignored in slope stability studies. There is a perception among geotechnical engineers that negative pore-water pressures will dissipate with rainfall infiltration and cannot be relied upon in design considerations. The objective of this paper is to illustrate that under certain conditions soil suction can be maintained. Based on the theory of infiltration and seepage through a saturated–unsaturated soil system, steady state and transient finite element seepage analyses were conducted using Seep/W on a 20 m high slope inclined at 30°. The results of the analysis showed that under steady state conditions, the most important factor influencing the permanency of matric suction in the soil is the magnitude of rainfall flux expressed as a percentage of the saturated coefficient of permeability of soil. For the analysis under transient seepage conditions, the results showed that the pore-water pressure profile depends on the magnitude of the rainfall flux, the saturated coefficient of permeability, the soil-water characteristic curve, and the water storage function. For a soil with a low coefficient of permeability and a large water storage capacity, the matric suction needs a substantial amount of time to dissipate and thus may be maintained over a longer time period than the rain is likely to fall, even if the ground surface flux is equal to or greater than the saturated coefficient of permeability. Engineers should address more appropriate engineering design assumptions that can be related to the permanence of matric suction in soil slopes based on the numerical analysis. Measures such as slope cover or surface recompaction can be taken into consideration to minimize the rainfall infiltration and thus maintain active matric suction in slopes.Key words: unsaturated soils, slope, rainfall infiltration, matric suction, permeability.