The Mechanism Of Irrigation-induced Flow-slide And Its Motion Simulation

Since the1970s, irrigation-induced loess landslides have been increasingly widespread.The loess flow-slide, as one type of the irrigation-induced loess landslides, is drawingincreasing attention, because of its short duration and wide threat range. Based on the fieldinvestigation, site tests, laboratory experiments and modeling analysis, this papersystematically summarized the landform, groundwater conditions and physical conditions ofthis type of landslide, and studied the whole process from gestating, developing to occurring.The research work and conclusions are listed as follows.The results of field investigation on irrigation-induded loess flow-slide show that theformation of this type of landslide needs specific landform, groundwater, and physicalconditions. Generating area is usually located on the gully head, so the landslide has a highpotential energy and circulation channel. There is a relative impermeable layer in the upsidepart of the slope or loess tableland. Thus higher perched water can be formed under irrigation.Besides, the structure of slip mass should be loose with the saturated water content lager thanliquid limit. Once the slip mass is disturbed, it will change from the solid to flow state rapidly,and a mudflow will occur.The saturated permeability parameters were obtained from the double-rings infiltrationtest of Gaolou village landslide in Huaxian County. Combined with the soil-watercharacteristics curve of corresponding layer measured by tensiometer, unsaturatedpermeability function can be obtained. Then, the seepage field inside the slope under seepageof water in the channel was simulated. According to the results of simulation, the stabilityfactors of slope at different times were calculated. Results indicate that the paleosoil isrelatively impermeable layer. Therefore, although the upper slope was in unsaturated stateunder channel leakage, saturated zone of perched water can be formed inside the relativelyimpermeable layer. As to Gaolou village landslide, saturated zone is firstly formed in the layerS3under irrigation. Water level rises continuously and eventually leads to landslides.The undrained triaxial consolidation tests on soil samples with different moisture contentwhich is taken from the layer above the groundwater table of Gaoloucun landslide in Huaxian county were conducted, the deformation failure modes of loess with different moisturecontent and stress environment were analyzed. The results show that the failure modes ofloess in triaxial stress environment can be divided as crack, shear and tympanites. Crackoccurs in the presence of low confining pressure and moisture content, the orientation ofrupture plane is consistent with that of the maximal principal stress, the stress-strain curvebelongs to typical strain softening, obvious stress drop appears after the presence of peakvalue. Shear has obvious shear plane occurring in the presence of moderate confining pressureand moisture content, the orientation of shear plane is skewed to the maximal principal stress,the stress-strain curve also belongs to strain softening. Tympanites belongs to typical plasticdeformation, which occurs in the presence of high confining pressure and moisture content,the sample bulges and axially shortens without obvious shear plane after tests. Failure modesof the layers above the groundwater table are mainly shear and crack. Thereby, greateracceleration will be generated at the instant when the unsaturated part of landslide mass slips.Triaxial anisotropic consolidation on saturated samples was conducted. In order tosimulate the rise process of groundwater, the total stress was set constant and the backpressure increased gradually until the sample failed. The results show that the cohesion of thesamples which is in this stress path is almost zero. Visibly, the structure of saturated soil massis disintegrating gradually with continually rising of the water table. And the sample isbasically in flow sate when unloading confining pressure. CU tests on these saturated sampleswere also conducted because the deformation of unsaturated layer will transfer part of load tosaturated layer. The results show that greater excess pore pressure will be generated under thecondition of very low strain, which illustrates that greater pore water pressure will begenerated due to the slope deformation, which results in the loss of strength, and then highspeed sliding occurs.The ring shear test on disturbed soil taken from the slide path was conducted. The resultshows that the residual effective friction angle of soil on the slide path is21°, stable residualstrength is5kPa and the cohesive strength is almost zero. When sliding body begins moving,shear strength is independent from normal stress, friction angle is nearly zero, and saturatedsliding zone is in flow state. Because of the uplift force of saturated sliding zone onunsaturated layers, slip mass will flow at high speed in circulation area. Combining the strength parameters of ring shear test with field survey data of this flow-slide, kinematicssimulation was carried out, then movement distance and its coverage were obtained. Theresults basically coincide with the actual accumulation scope, thus it provides a feasiblemethod for the prediction of hazard range for potential landslide.Combined with the results of the experiments and simulations, the formation offlow-slide can be divided into two parts: the formation of landslide and the formation of mudflow. The forming process of the landslide is divided into five stages, namely, the formation ofthe saturated zone, the deformation of saturated zone, the deformation of slope body, theliquefaction of saturation zone and the change from landslide to mudflow. The formation ofthe saturated zone is mainly due to the differences in permeability. Saturated zone can beeasily formed inside the layer L3,which lies on the relative impermeable paleosol layer S3.With the continually rising of underground water level, deformation of saturated zone willeasily occur under the external force, since the saturated water content is greater than plasticlimit. Then, it will result in stress redistribution inside the upper strata and formation ofcracking and shear plane. The hydrostatic pressure caused by the rise of underground waterlevel and quick load from the upper strata will produce an excess static pore water pressure insaturated zone and liquefaction happens. Collapse of the structure and release of stress lead tothe liquefaction of saturated zone and high-speed mudflow runs along the gully.