Investigation On Rainfall-induced Colluvium Landslides Using Laboratory Model Tests

Colluvium landslide, which is a common type of landslides in china,is catastrophic and dangerous for infrastructures, human lives and property.Rainfall is the main triggering factors for colluvium landslides.Investigation of the mechanism of rainfall-induced colluvium landslidesand a reliable evaluation method for the colluvium slopes are essentiallyimportant for theoretical research and practical engineering. In this study,the seepage, deformation and particles migration of loose colluvium soilslopes under rainfall condition are investigated through laboratory modeltests. The effects of particle size distribution, slope angle and rainfallcharacteristics on infiltration and slope stability on performance ofcolluvium slopes are discussed. The main research findings aresummarized as follows.(1) A fully instrumented laboratory model test system forrainfall-induced landslide is developed. Seven model tests are conductedfor three types of reconstituted colluvium soils.(2) The laboratory model tests show that there is a remarkabledifference in the failure modes for soils with different particle sizedistributions. The failure modes for the three slopes, which are composedwith sandy soil S1, colluvium soil S2and colluvium soil S3, respectively,are multi-level retrogressive sliding failure, shallow sliding failure，massive sliding failure，correspondingly. The volumetric water content,pore water pressure and soil suction in the slope varied with rainfallinfiltration. When the wetting front reaches the corresponding measuringpoint, the volumetric water content and pore water pressure continue to increase while suction continues to decrease with time. After some time,the measured values become stable. Once the rain stops, the pore waterpressure and water content response immediately and gradually decreases,while soil suction in the slope gradually increases. The displacement of theslope is accelerated when a failure occurs. Fine particles in a colluviumsoil slope are transported to lower part of the slope under rainfallinfiltration. The amount of the particle transported is influenced by theinitial particle size distribution.(3) The impact of the particle size distribution, slope angle andrainfall characteristics on soil deformation, infiltration and slope instabilityis discussed through the comparison of the model tests. Increasing therainfall intensity may not change the failure mode of the slope. Under anintermittent rainfall, the peak value of pore water pressure is slightly largerthan the value under a continuous rainfall and the time of initial failure isearlier, as pore water pressure is cumulated in the slope during anintermittent rain. The slope angle significantly affects the variation of slopestability during rainfall. The time of initial failure for a gentle slope isearlier than that of a steep slope. The failure surface of a gentle slope undera less intense precipitation is deeper than that of a steep slope which isunder a more intense rain. The initial particle size distribution, e.g., thecontent of stone, has a significant impact on the failure modes. With asmaller stone content, the slip surface is deeper. The transport of fineparticles is more remarkable and the content of fine particles near the toeof a slope is greater when the stone content is increased.(4) A numerical simulation of seepage in saturated-unsaturated soilslope and a slope stability analysis are conducted. The results arecompared with the experimental data. The results indicate that there aresome differences in the distribution of pore water pressure during theinitial stage of tests while calculated simulations and measurements areconsistent after the soil slope is saturated in some parts. The results ofslope stability analysis show that the effect of slope angle on infiltration and slope stability is not significant for the given range of slope angle (37to45degrees). Stone content however significantly affect rainfallinfiltration and slope stability. With a greater amount of stone in a slope,the depth of wetting front is the smaller and the time of initial failure islonger and hence the safety factor of the slope is greater.