Failure And Motion Mechanisms Of A Rapid Loess Flowslide Triggered By Irrigation In Hei Fangtai Platform, Gansu Province

Irrigation-induced slide-flow is one of the most general geological disasters in loessirrigation areas and there is hardly any research of it. Based on the survey and research of thetwo platforms named HeiTai and FangTai, which are located to the north of the Yellow Riverand south of the MoShiGou, there are12quick collapse landslides,15slide-flows,10slowtransitional landslides and no rapid long run-out landslides. Slide-flow distributes the mostextensively and it always originates from the head of the gully which presents the concavetype. Meanwhile, the slope general stays stable under the natural state for its narrow slidingpath and the thick vegetation. During the investigation, this kind of landslide is easily ignoredfor the reason that it doesn’t have the typical physiognomy. At last, the landslide grows in thetop while the effected area in the bottom, when it turns to slide-flow, its destructive scope isexpanded significantly.Through the physical and mechanical test of the soil for every1.0meter taken fromsouthwest of HeiTai, we get the result of the moisture content, liquid limit, plastic limit,saturated moisture content, void ratio, plasticity index and particle composition. The resultshows the three typical stratums, namely24m Q3loess,20m Q2loess and8m alluvial siltyclay.Physical mechanical performance tests were taken on the soil samples which were takenfrom the slope with intervals of1m in the vertical direction. The slope is on the north of theYongjing PowerStation which is located in the southwest end of Heitai. Through the tests, Igot the data of water content, liquid limit, plastic limit, saturated water content, void ratio,plasticity index and grain composition at different height, and find that the upper formationmainly consists of three layers: Q3loess layer of24m, Q2loess layer of20m and alluvial siltyclay layer of8m. I find that the loess that can became mudflow has several characteristics:void ratio bigger than0.8, saturated water content bigger than liquid limit, silty claysubstratum relative impermeable. Thus, we find the ground water condition and physicalcondition of landslide-mud flow.Loess with different moisture content under different confining pressures in three axle shear test should be got three failure models of environment, namely the splitting failure,shear failure and destruction of bulging. The field investigation shows that the top of theslope loess mostly appears splitting failure with low moisture content and small confiningpressure. The loess in bottom of the slope more performance for the creep damage, both ofconfining pressure and moisture content are large. The soil in the middle of the slope oftenappears obvious failure surface of shearing, and value of the confining pressure and watercontent are always between the aforesaid two cases.Test result is in conformity with the actual, so the test results revealed the deformationand failure characteristics of unsaturated soil. The triaxial test shows that with the increasingof the pore pressure, the stress path is turning to the left that is the direction of damage. Wealso measured the effective c’ and and total residual c and of alluvial silty clay, Q2loess,and Q3loess. The result shows that the total stress value is lower than the effective one,suggesting the great possibility of the slide flow.Based on the saturated permeability tests, we used the tensiometer to measure soil-watercharacteristic curve and calculate the unsaturated permeability coefficient using the empiricalformula.The sample usually used in permeability test is unloaded, not in the real stressenvironment. The permeability coefficient we get is not what we want. In order to restore thereal stress environment of the sample, the instruments have been improved. The loess inHeifangtai platform from different depth has been choose as the object of study, and theconsolidation-permeability test under different stress environment have been done. Thecomparative analysis is made on the deformation and permeability of the loess under theeffect of different loads. It turned out that the permeability coefficient obtained from thepenetration test under consolidation is25-40times as high as the one obtained from theconventional penetration test.After that, we make the parameters into SEEP/W, SIG, MA/W, SLPOE/W of GEO-Studiosoftware, to analysis the seepage, stress and stability. The result shows that with the rising ofgroundwater levels, oil pore water pressure is changing, resulting in redistribution of stress.When the water level rising to a certain extent, the potential sliding surface soil is close tosaturation, the landslide occurred.Based on the theory of fluid mechanics and three-dimensional landslide motion model, K.Sassa developed LS-RAPID which is appropriate for the simulation of the landslide, rapidslide-flow and debris flow. By ring shear test on soil sample points, we get the residualfriction angle, saturated undrained shear strength τss, estimating degree of pore waterpressure Bss, then simulate the movement process. From the result, we can get theinstantaneous velocity and scope of accumulation that is close to the actual situation. So thesimulation provides a basis for the prediction of the damage ability and disaster scopes.