Study On Formation Mechanism And Stability Of Chengzhai Landslide In Xishui County Of Guizhou Province

On June4,2012, a landslide suddenly occured in chengzhai village of xishuicounty in zunyi city of guizhou province,which caused about$30million of financialloss directly,including more than10houses collapsed, in nearly30buildings badlydeformed, more than520people’s life and propery’s safety of136families are alsoin dangered, and so dose the important traffic channel,which from xishui tochengzhai are interrupted.Relying on the scientific research project "guizhou province geological disastermonitoring and early warning and decision support platform for the study, This articlehas set up the" conceptual model " of landslide formation mechanismby analyzing thefield investigation and collect related data of landslide area,we also research themechanical properties of slope rock mass based on rock mass physical andmechanical test;and then I analyse the influence of excavation and rainfall onlandslide by numerical simulation according to the physical and mechanicalparameters obtained; Finally, evaluate the stability of the landslide and make aprediction. The main research contents and results show that:(1)We confirm the chengzhai landslide is composed of ancient landslide deposit.Which based on the survey of the topography and landforms and the structurecharacteristic of slope bodys, and also combining with borehole survey data.Becauseof the slope creeping at weak argillite and unloading effect of river, the bed rocklose it’s balance at slip-bending mode and come into deposit at last, and the debrispiled up at the front of the bending section, the difference weathering leads to thedouble layer structure which contains silty clay soil at front and stone at hindside.(2) We have made particular investigation and analysis on thedeformation-falilurefeature,then we have divided this landslide into the following three zones: theoverall-slip zone in the central of the landslide(A1), the resistant zone in the front ofthe landsilde (A2) and the damage caused by traction zone at the back of thelandslide(A3). According to the different deformation and failure characteristics, wehave defined four secondary zones for the resistant zone:the bulged zone at the mainsilde direction (A2-1),the pushed and slipped on the right side zone (A2-2),the pushedand bulged zone (A2-3),the extrudedand slided zone on the left (A2-4); and we dividedthe traction zone into strong traction’s damage zone (A3-1) and a weak drawingdamage zone (A3-2). (3) Accroading to the investigated datas of the deformation failure in the field anddrilling datas, the morphology of the sliding surface in the front of landslide forms adeep groove in the left side, and the right side becomes shallow because of lifting up.Sliding zone is constituted by clay powder, which has the water swelling andsoftening effect.(4) based on the field investigation,we consider that the sustained rainfall and theexcavation on the left front side of landslide is the main factors of the formation.Socombined with the detailed analysis of the landslide deformation failure,we hadestablished the concept model for the landslide formation:the excavation at the frontalpart reduced the stability of the ancient landslide deposit, continuous rainfall causedthe central part sliding,and pushed the bottom part sliding, and leads to the upper parttowed sliding.(5) Accroding to the expreriment, the sliding zone contains a lot of clay mineral,which not only make the sliding zone soil strong hydrophilic,and butalsowater-swelling obviously,they mainly manifest inreducing cohesive force andinternal friction Angle, especially the cohesive force which is the important reasonthat the landslide happened after the continuous rain has dropped by eight pecent.(6) The three-dimensioned mumerical analysis results can be concluded that theexcavation effect changed the slope stress state, the excavation of the resistantzonemake the front of deposit uplift about20.5mm, which contributed to the overall creepdeformation of the slope, and provides the international airport in surface; Rainstriggered the slide instability, it result in thecentral of the deposit moving first, andsliding out along the excavation face. The maximum displacement of the centralparty is35cm.The results of Numerical simulation show that the deposit mechanismfor resurrection is "start pushing central upper-lower resistance slide uplift-step bystep drawing", which consistent with the survey results.(7) we have analysed the seepage of the landslide under the condition of5daysrainfall by2d seepage numerical simulation,it showed that the rainfall loaded theslip-mass by vertical infiltration at the first2days of the rainfall, then by osmosisand buoyancy at the later3days.(8) Field monitoring results and stability calculation results show that chengzhailandslide is in basically stable state under natural condition,but it will in unstablestate if it is under the rain condition.And it is proposed the prevention advice of‘cuttingcentralslope,pressing the slope foot and strengthen drainage’.