Pore-pressure generation and movement of rainfall-induced landslides: effects of grain size and fine-particle content

Using a small flume, a series of tests was conducted to trigger rainfall-induced landslides. By performing tests on silica sand no. 7 (D50=0.13 mm), no. 8 (D50=0.05 mm) at different initial dry densities, the effects of grain size on pore-pressure generation and failure behavior of a landslide mass were analyzed. Results from tests of different initial densities showed that the optimal density index, at which the pore-pressure build-up after failure reached its maximum value, differed for samples of different grain size. Moreover, observed failure phenomena showed that the failure mode also depended greatly on the grain size. In fact, flowslides were initiated in the tests on finer silica sand (no. 8), whereas retrogressive sliding occurred in the tests on silica sand no. 7. Results of tests on mixtures of silica sand no. 8 with different contents of loess by weight showed that the existence of fine-particle soil (loess) could significantly change the flow behavior of a landslide mass during motion. The flow behavior of soils with 20% and 30% loess was different from these two silica sands and the mixture with 10% loess, showing greater velocity without deceleration. This suggests the existence of a mechanism that maintains high pore pressures during motion for these soils. In addition, by rotating saturated samples in a double-cylinder apparatus, a mechanism was examined in which pore pressure in saturated soils during motion was maintained. The results showed that the pore pressure of the saturated mixture increased with velocity because of the “floating” of sand grains that accompanied the movement for each test. In addition, the sample with finer grain sizes or greater fine-particle (loess) contents floated more easily, and high pore pressure could be maintained during motion. The floating ratios of grains reached a high value (>0.85) at a very slow velocity for samples with 20% and 30% loess. Based on these test results, it is concluded that grain size and fine-particle contents can have a significant impact on the mobility of rainfall-induced landslides.