INVESTIGATION OF A HIGH SPEED AND LONG RUNOUT ROCKSLIDEDEBRIS FLOW AT DAZHAI IN GUANLING OF GUIZHOU PROVINCE

On June 28， 2010，due to sudden heavy rainfall，a high speed and long runout rockslidedebris flow occurred at Dazhai in Guanling of Guizhou Province. Its runout distance was about 1.5km.Its rock debris volume was about 174.9 M m3.Its destroyed two villages and resulted 99 fatalities.The landslide was located at the coalrelated sedimentary rock strata area.This rock system commonly presents in southwestern China.The upper strata are limestone and dolomite.The middle strata are sandstone with gentle dipangles.The lower strata are mainly shale and mudstone and have coal seams in local regions.The strata forming the mountains have a special geological structure with hard rocks at upper portion and soft rocks at lower portion and a hydrogeological structure with upper rich groundwater aquifer and lower impermeable beds，which can easily form landslide hazards.From the site topography，the hillside slopes are steeper at the upper portion and gentler in the lower portion.The topography looks like a shoeshaped geometry. The upper steep slopes can easily occur slope instability and the middle and lower gentle and wide slope valleys offer the kinematic conditions for landslide debris to flow long distance. Transformation of the larger potential energy into kinetic energy can easily form rockslidedebris flows with high speed and long runout distance. The heavy rainfall on June 27and 28 were the main factor triggering the major disaster.The rainfall was up to 310mm and exceeded the records over the last 60 years.Analysis indicates that the runoff in valley streams were two times more than those during usual rainfall at the region.The large runoff water formed static groundwater pressure and seepage pressure in joints and fossils of the sandstones at the landslide source zone，which led the landslide to occur.Secondly，the runoff was formed in stream valleys，which became a layer of saturated water bed for the rock debris to flow long distance.As a result，the runout distance and speed of the rock debris flow was evidently increased. The findings of this investigation are useful to reduce and mitigate the disasters of such high speed and long runout rockslides and debris flows，the occurrences of which seem to have been increasing in recent years in China due to extremely heavy rainfall events.