Gas-cause of Devastating Hengtaiyu Landslide of December 20, 2015 at Gentle Filling Slope with Soils Excavated from Decomposed Granite Sites

Abstract

This paper investigates the cause of the devastating landslide in fill soil slope on December 20, 2015 at Hengtaiyu Industrial Park, Shenzhen, P. R. China. It is a catastrophic rapid long-runout and wide-spreading landslide in a very gentle and large fill slope. The total volume of the slipped soil fills was 2.7 million m3. The soil debris destroyed and buried 33 buildings on the vast flat ground at least 300 m to the failed slope toe, which resulted in 73 fatalities, 4 missing, and 17 injured. It occurred in the middle of the dry-season when the site almost had no rainfall for many days. The overall angle of the fill slope was between 9 and 15 and is less than the 15 minimum angle for registration as potentially dangerous fill slope in Hong Kong Slope Safety System. The fill soil slopes are common in Shenzhen and Hong Kong. The fill soil is classified as well graded clayey silty sand with gravels. The clay content is about 7% by weight. The soil permeability is very low. The soils were excavated recently from construction sites comprising completely decomposed granitic rock soils. The landslide was strongly lack of the noticeable presence of free water or substantial amount of liquefied mud soils. The conventional slope stability assessment shows that for the slip surface to have a unit (1.0) FOS, the groundwater table had to be very high and the soil internal frictional angle had to be less than 5, which is impossible. The paper presents a new underground compressed gas cause of such landslides. The observed phenomena are well consistent and describable with this gas cause. This finding can allow us to understand the new type of landslide cause and mechanisms, to better assess and design slopes, and to clarify responsibilities of different parties involving in landslide disasters.