Research On Failure Mode And Progressive Failure Mechanism Of Silt Retention Dam

As a new gully land management mode in the Loess Plateau area,land transformation has brought huge economic and ecological benefits with its construction and use.However,new problems such as slope collapse,collapsibility andsilt retention dam deformation and damage have also arisen,which seriously affect the benefits of gully land management project.This paper focuses on the long-term stability of thesilt retention dam in the site of Yan’an gully construction,takes the site of Yan’an Nangou gully construction as the research area,analyzes the influencing factors of thesilt retention dam deformation and failure through the field investigation,summarizes the dam break mode of Yan’an gully construction,and selects one of the progressive dam break modes for in-depth study Through the indoor test of plastic soil,the basic physical and mechanical properties,soil water characteristic properties,the relationship between water content and strength,and disintegration properties of loess in the study area are obtained;through the indoor physical model test,the gradual dam break process ofsilt retention dam and various sensor data are obtained,so as to analyze the stress change,crack development and soil displacement in the process of gradual dam break,and reveal the gradual dam break process ofsilt retention dam Mechanism of dam break.The main research results are as follows:(1)The deformation modes ofsilt retention dams in small watersheds are divided into four types: gully type on dam slope,sinkhole type behind dam,settlement type on dam crest and instability type on dam slope;the failure modes ofsilt retention dams in small watersheds can be divided into three types: progressive failure type,piping failure type and overtopping failure type according to failure causes;the main influencing factors ofsilt retention dam deformation and failure are analyzed as follows: loess nature and dam slope shape State,rainfall and storage water level.(2)A set of multi-functional visual large-scale physical model equipment integrating rainfall and water injection is developed.The water injection rate and rainfall intensity can be controlled by adjusting the valve.Combined with the layout of cameras,the whole process of model test in three directions of side,top and front can be obtained.(3)Based on the analysis of the experimental phenomena of the model test,the progressive failure mode of thesilt retention dam is divided into three stages,namely,the wetting stage of the dam body,the sliding stage of the dam slope and the overtopping erosion stage.During the wetting stage of the dam,the wetting line of thesilt retention dam will gradually move from the upstream slope to the downstream slope,and the shape of the wetting line will develop from a curve inclined to the upstream to a nearly vertical curve,and then to a curve inclined to the downstream,and there is no obvious deformation of thesilt retention dam;during the sliding stage of the dam slope,it can be seen that the downstream slope collapses four times in turn,coupled with the collapse of the upstream slope,resulting in warping The height of the dam crest is basically level with the upstream level;in the overtopping erosion stage,the water in the reservoir can be seen to flow downstream over the dam crest.Under the erosion effect of the water flow,the fine gullies first appear on the dam slope,and then develop into a number of stepped scarps,and finally the scarps merge into 1-2 large scarps.(4)Through the analysis of the sensor data in the process of model test,it is found that the water content data only increases in the wetting stage of the dam body,and the water content in the soil changes little in the later stage;the pore water pressure in the soil will be generated after the soil reaches saturation,and the generation of pore water pressure obviously lags behind;the change of horizontal stress in the soil is related to the pore water pressure in the soil The results show that there is a certain positive correlation between the stress changes,but the change range of the horizontal stress is obviously small,and the change of the horizontal stress in most of the soil is small during the model test;the pore water stress will rise and fall sharply with the internal deformation of the dam body,especially the large deformation of the landslide,and generally the pore water pressure will drop sharply near the sliding surface after the landslide,which is the reason of the water pressure In addition,the pore water pressure in the non sliding dam near the sliding surface will rise sharply,which is due to the blockage of some seepage channels after the collapse.(5)Combined with the phenomenon of model test and sensor data,the mechanism of gradual dam failure ofsilt retention dam is discussed: the wetting stage of dam body is the premise of gradual dam failure ofsilt retention dam,after this stage,the stability of dam body will decline,and the downstream slope will be more prone to collapse;the sliding stage of dam slope is an important part of gradual dam failure ofsilt retention dam,in this stage,due to the influence of seepage When a large amount of collapse deformation is superimposed,cracks will gradually appear on the surface of the dam body and expand,resulting in multiple landslides of the downstream slope.Coupled with the collapse of the upstream slope,the crest height of thesilt retention dam is basically level with the upstream level.Overtopping erosion stage is the final result of gradual dam failure of thesilt retention dam In the process of dam erosion,there will be scarps,and the scarps will gradually merge to form larger scarps.It is precisely because of the existence of scarps that the erosion damage of water flow to thesilt retention dam is intensified.