Experimental Studies on the Effect of Vegetation Density to Change Underground Seepage Rate and Stability of Slopes

Abstract

Vegetation plays in an important role in the amount of surface runoff or infiltration after a rainfall event, specifically on slopes. High infiltration or seepage reduce shearing resistance of soil through the reduction in matric suction of partially saturated slopes, ultimately causing slope instability. On the other hand, low infiltration increases surface runoff and velocity of surface flow, which ultimately causes soil erosion. In this study, three different slopes were prepared using fine sand, specifically at the same dry density and geometry, but with three different vegetation cover densities. The slopes were instrumented with tensiometers at different depths. The slopes were subjected to a rainfall event with an intensity of 30 mm/h and advancement of the wetting front of the seepage water and soil matric suction were recorded with time duration after the rainfall. The experimental result shows that up to certain threshold vegetation cover density, amount of surface flow increases causing soil erosion. For the vegetation density above that threshold, vegetation covers intercept the rainwater and the intercepted water is gradually passed to ground. Moreover, vegetation cover will ease the flow of surface runoff water. These actions, on the other hand, increase the seepage velocity. As such, less dense vegetation cover triggers more rainfall induced soil erosion and denser vegetation cover increases the instability associated with rotational or translational slides.