Ring shear test for transform mechanism of slide–debris flow

Abstract

The paper investigates the transform mechanism of slide–debris flow. The investigation is based on a case study of an actual transformation from landslide to debris flow event. The event occurred on July 7, 2007 in Da County, Sichuan Province, China. The landslide was induced by continuous rainfall from June 16 to July 7, 2007. The total rainfall was 521.6 mm. A part of the landslide debris transformed into debris flow which destroyed houses for about 200 people. The field investigation showed that the ground surface below and in front of the slickensided slip bed was obviously delaminated. A layer of silt about 20 cm was found on the surface and grains under the silt layer became coarser and coarser as the depth increased. Samples from the slidebody near the slip surface were collected and used in the ring shear test with in-situ stress level. The test was to simulate the long sliding and shearing course of the slidebody on the slip bed and the ground after its failure and to analyze what happened to the slidebody. The test results showed that in addition to the excessive pore water pressure due to shearing shrinkage, there was another mechanism facilitating the mobilization from landslide debris into debris flow. A thick liquefied layer of silt forms at the bottom of the slidebody, which could be due to grains crushing and delamination. The thickness of the silt layer increases as the shear displacement increases. The liquefied layer carries landslide debris moving fast and reduces its whole integrity greatly, making landslide debris easily break down into debris flow. This mechanism is called the soft base effect in this paper.