Experimental tests of slope failure due to rainfalls using 1g physical slope models

In order to mitigate the damage due to sediment disasters, knowledge about how slopes fail due to rainfall is indispensable. The main objectives of this paper were to investigate experimentally the effects of surface sand layer density and rainfall intensity on the slop failures due to rainfalls. We conducted a series of experimental tests using 1g physical slope models constructed of Kasumigaura sand and a silt soil named DL clay for the permeable residual surface layer and the firm rock foundation, respectively. A total of nine cases with different combinations of surface sand layer densities and rainfall intensities was tested. Two types of failure: surface slide failure and retrogressive failure, were observed depending on the rainfall intensity and the surface sand layer density. The following mechanism of failure was accounted. At first some sands, which contained a lot of accumulated rainwater, flowed out (flowslide) at the slope toes. The flow slides may be due to the reductions of effective stresses as a result. When a surface slide failure occurred, most of the PWP (pore water pressure) values were still negative but the whole sand layers were almost at the saturation condition. In the case of retrogressive failures, seepage surfaces rose up to higher positions and excess PWPs appeared under the seepage surfaces. This difference of generation mechanism of PWP values may be the deciding factor in the difference in the type of failure.     

Effectiveness of filter gabions against slope failure due to heavy rainfall

Slope failures due to heavy rainfall events are phenomena that can cause serious damage to social infrastructures and the loss of lives. Based on previous studies, natural slope failures are generally shallow and originate at the slope toe where infiltrated rainwater has accumulated and saturated it. Hence, it is extremely important to prevent these initial failures from inducing entire slope failures. In the present study, firstly, 1 g model tests, called G series tests, were conducted. In the tests, a gabion filled with filter materials was placed at the slope toe of each model for reinforcement and to drain the accumulated rainwater from the slope toe. Filter gabions have been found to shrink the failure regions and to significantly extend the time until slope failures occur. The failure mechanism in the G series tests was almost similar to that in cases without filter gabions if focus was placed on the slope above the filter gabions. However, the drainage effect was small. Secondly, P series tests, in which a filter gabion with a pipe was introduced for each model, were conducted. The results of these tests indicated that the displacements significantly decreased as the diameter of the pipe and the depth of the pipe’s insertion to the surface layer increased. Water did not discharge through the pipe until the pore water pressure around the pipe reached positive values. The failures always started when a phreatic surface appeared on the slope surface. Thus, it is very important to prevent a phreatic surface from forming on the slope surface. The adequate arrangement of a filter gabion with a drainage pipe may increase the potential for slope stability.