| After the"5.12"Wenchuan Earthquake,under the scouring of the water flow in the rainy season,a large number of landslide debris accumulated in the Chutou Gully,which became an important material source for the formation of debris flow.At the same time,the earthquake also greatly reduces the critical rainfall inducing debris flow.In addition,the geomorphic characteristics of steep mountains and valleys in the study area,the frequency of debris flow disasters in hoe gully continues to increase.Before the Wenchuan earthquake,debris flow broke out for twice in Chutou Gully in 1976 and 1999,with a low frequency;after the Wenchuan earthquake,deadly debris flow erupted four times in 2013,2014,2019,and 2020,and the eruption frequency also changed from low-frequency to medium-high frequency;and at present,there are still abundant loose material in the gully,so it is judged that the debris flow in the basin is still in the development stage and there is still the possibility of inducing large-scale debris flow.Since the debris retaining dam has the function of intercepting mud and sand,reducing the undercutting of the channel and stabilizing the ditch banks to reduce the occurrence of collapse,the installation of multi-stage debris retaining dams in the formation and circulation areas has become the most effective and important engineering measures to control and prevent debris flow disasters.However,while the dam is functioning,a large number of debris and gully water sediments are also accumulated downstream,which is becoming the key source of the next debris flow.Therefore,taking the debris flow deposition formed under the influence of debris retaining dam as the research object,the analysis of the initiation and erosion characteristics of the deposition under the effect of rainfall and over-dam flow is of great significance to the prevention and prediction of gully debris flow.By investigating the regional engineering geological conditions of Chutou Gully basin,this paper understands the landform,loose material source and hydrodynamic conditions of the study area;Using the method of debris flow indoor physical modeling test,taking the debris flow deposition downstream of the secondary retaining dam in Chutou Gully as the research object,combined with rainfall,digital terrain data and in-situ debris flow deposition parameters,this paper simulates the erosion process of deposition under different intensity rainfall and over-dam flow conditions,and use sensors to deeply explore the variation features of the pore pressure and moisture content in the erosion process.Main researching results are as follows:(1)Comprehensively collect the geological environment data of Chutou Gully basin,especially the topographic and geomorphic characteristics,material source characteristics and meteorological and hydrological characteristics controlling the development of debris flow,and preliminarily analyze the causes of debris flow.Chutou Gully belongs to deep cutting structure and middle mountain erosion landform.The average slope on both sides of the ditch is about 34°.The erosion ditch is developed in the basin,the gradient of the main ditch is about 260‰,and the basin area reaches 21.7 km~2.The steep terrain and vast basin area provide favorable conditions for rainwater collection;Affected by earthquake,a large number of material sources have been added in the gully.After the"8.20"debris flow in2019,the total amount of material sources in the gully will reach 1426.10×10~4 m~3,of which the total amount of gully material source accounts for nearly 61%,which is the main material source type in Chutou Gully basin;The groundwater in the gully area is not rich,and the concentrated rainfall in the rainy season from May to September is the main water source for the start of debris flow.The steep terrain,abundant loose material sources,vast catchment area and concentrated rainfall have laid the foundation for the high-frequency and large-scale outbreak of Chutou Gully debris flow.(2)In order to obtain the basic physical parameters of source soil,a series of experiments are carried out in this paper,including particle test,density test,moisture content test and permeability test.Through the combination of in-situ test and laboratory test,the basic physical properties of soil samples in Chutou Gully are studied,and it is concluded that the source soil belongs to coarse-grained soil;the analysis of particle size curve shows that the particle size distribution is uneven and the particle grading is good;the permeability coefficient is within the range of coarse sand permeability coefficient,which has good permeability and is conducive to the rapid infiltration of rainwater.These characteristics are conducive to the rapid saturation of source soil under the action of rainfall and over-dam flow,which creates favorable conditions for the rapid initiation of debris flow.(3)Using the method of large direct shear test to obtain the relationship between water content and shear strength,and the internal cause of debris flow starting is analyzed.When the normal pressure is constant,the higher the water content is,the weaker the shear strength is.That is,during the continuous rainfall,the infiltration of rainwater causes the increase of water content,resulting in the reduction of the ability of soil to resist shear failure.When the shear strength decreases to a certain value,the deposition will be damaged,and it will lead to the start of debris flow.(4)Using the method of physical modeling test research,taking the debris downstream of Chutou Gully secondary debris retaining dam as the research object,the initiation and erosion characteristics of debris flow accumulation are studied.In this test,the indoor remolded soil test method is adopted,and the indoor small-scale model is established in the ratio of 1:100.According to the rainfall probability of 10%,5%,2%and 1%in the study area,four rainfall intensities of 29.4 mm/h,33.4 mm/h,38.4 mm/h and 42.2 mm/h are set respectively,and the overflow flow corresponding to each rainfall intensity is determined in combination with the terrain characteristics of the study area.According to the three precipitation forms of separate rainfall,separate over-dam flow and their joint action,a total of 12 groups of tests are completed.It is analyzed and discussed from three aspects:image recording,water content monitoring curve and pore pressure monitoring curve.(5)According to the test image,rainfall mainly produces seepage erosion on the deposition,and rainwater infiltration leads to slope instability,which leads to traction landslide damage;Under the action of over-dam flow,the initiation of debris can be summarized into five stages:surface erosion,V-groove erosion,traceability erosion,lateral erosion and equilibrium stage;When rainfall and overtopping flow act together,the starting process of debris flow is also divided into five stages:splash erosion,surface erosion,V-groove erosion,traceability erosion and lateral erosion.(6)The starting mechanism of debris flow is determined according to the sensor data.Under the infiltration of rainfall,the variation of moisture content curve is relatively slow.When the deposition is damaged,the water content at each position reaches the saturation value of about 23%.However,under the action of overt-dam flow,the erosion rate is fast,and the water content curve shows a steep rise.When the soil body is damaged,the water content exceeds the supersaturation value,that is,the sensor may be in direct contact with water.The variation characteristics of pore pressure under different precipitation forms are similar to those of moisture content curve under the same condition.Therefore,the failure mechanism of the accumulation body can be summarized as follows:with the increase of moisture content in the process of rainfall,the pore pressure in the soil increases,resulting in the reduction of effective stress and the weakening of soil shear strength,which leads to the destruction of the accumulation body.The over-dam flow further induces the initiation of debris flow,and the rainfall mainly accelerates the collapse of the gully bank in the lateral erosion stage.Over-dam flow is the main factor inducing debris flow. |
PDF Full Text Request