| As one of the most serious forms of soil erosion in southern China,collapsing gully phenomenon is a kind of comprehensive erosion characterized by both hydraulic erosion and gravity erosion,it is mainly caused by gravity and hydraulic force is an important factor to promote the occurrence of collapsing mound.Once the Benggang erosion occurs,it may bringing about severe damaging and threats and causing numerous holes in the surface soil,the hazards not only will threaten people's lives and property,but also causing heavy pressure on flood control safety?food security?ecological balance.In addition,collapsing gullies are obstacles to the development and prosperity of national economy in granite red soil hilly areas.Thus,the process of granite collapsing gully erosion and its collapsing mechanism have long been concerned by the whole society and government.Collapsing wall is the main body of collapsing hill landforms,while the collapse of collapsing gully is mainly caused by the decrease of the stability of the rock and soil layers.The soil moisture content and the gradient are the significant parameters controlling the present stability(equivalent to gravity erosion)of the soil layer of the collapsing wall section.Collapsing gullies erosion is inextricably related to rainfall infiltration,the hydraulic factors such as rainfall and runoff provide the main erosion power for the the development of geomorphological,such as concave cavity,which is the prophase process to the formation of the collapsing gully.However,due to the deep weathering crust of granite rock and the significant difference of weathering in the collapse section,the erosion resistance,collapse resistance and scouring resistance of different soil layers of the collapsing wall are quite different,which makes the sand layer with very low water erosion resistance in the collapse section structure easy to be hollowed out in the lower layer(named concave cavity)during the continuous rainfall period.With increase of weathering and spalling degree of sand layer,the depth of concave hole increases,which results in the decrease of stability of the red clay layer overlying the hollow cavity.Therefore,there must be a critical value D0 of the splash erosion pit depth required for triggering the obvious collapse of the collapsing wall precisely.From another point of view,if the splash erosion pit depth is less than the D0,but the water content of the overlying soil layer is higher than the natural water content,the collapsing wall is still in a stable state.If the water content of the overlying soil layer of the concave cavity continues to increase(for example,when the rainstorm comes),because the strength of the overlying soil layer of the hollow cavity is weakened by water flow,there will also be a water content value(named as the critical moisture content of the splash erosion pit)which causes the overlying soil layer of the hollow cavity to fall precisely.Nevertheless,up to present,studies on the collapsing gully wall(also can be called"headwall")have been mostly carried out qualitatively,and reports on quantitatively assessing and predicting the probability of occurrence of collapse are quite limited.The distribution of granite collapsing hills in Tongcheng area accounts for 47%of the total number of collapsing gullies in Hubei Province.It is a key county for preventing collapsing mound in China,which is typical and representative and demonstrative.Therefore,in this study,a typical collapsing hill erosion area in Tongcheng County,Southeastern Hubei Province,was selected as the object of research.Soil samples were collected from various soil layers(surface soil?red soil layer?sand layer?clastic layer)of the collapsing wall in the field,and the basic physical parameters of each soil layer were calibrated by geotechnical tests.On the basis of previous studies and the theory of multiple interdisciplinary disciplines,this paper mainly discusses the stability of collapsing wall and the process of collapsing hill erosion from the perspective of slope gradient,water content and the evolution of concave cavity with the help of ABAQUS software,the main analysis contents of this paper and the results of calculation or the achievements of cloud images showed that:1)The alternation of wetting and drying caused by rainfall and surface evaporation will cause the deterioration of soil strength,especially the stability of longitudinal profile of collapsing wall which is susceptible to water content,While the outward expansion of collapsing gully wall is more sensitive to slope gradient.For this reason,eight kinds of slope gradient(30°?35°?40°?50°?60°?70°?80°?90°)of collapsing gully wall are selected according to actual conditions,and six kinds of dry and wet levels are designed by air-drying or immersion tests.The corresponding relationship between shear strength and water content of soil layer is obtained by rapid shear tests.The stabilities of collapsing wall under 48 working conditions are calculated and analyzed by using finite element strength reduction method,and the empirical formula for predicting the safety factor Fs of collapsing wall is derived through a series of data fitting.The results show that with the increase of slope gradient,the influence degree of slope gradient on the position of critical sliding surface decreases,and the safety factor decreases logarithmically;with the increase of water content,the safety factor increases first and then decreases,and the influence degree of wetting-drying level on sliding surface decreases first,then increases and then decreases,and the influence degree is greater than that of slope gradient.The slope gradient and water content of the collapsing mound can be brought into the binary empirical formula to predict the stability of the collapsing wall.2)Referring to the rainfall data in Tongcheng County,three kinds of rainfall conditions(working condition 1:total rainfall time is 164.00 hours and rainfall intensity is 6mm/h;working condition 2:rainfall duration is 22.88 hours and rainfall intensity is 43mm/h;working condition 3:torrential rain occurring once in two hundred years on June 10,2011)were formulated.We harbour the ongoing effort towards considering the coupling effect of seepage domain and stress field of collapsing wall,the mechanism of hydraulic factors contributing to collapse hills erosion during rainfall duration had been emphatically discussed through performing numerical experiments.The FEM results indicate that:under the condition of continuous low-intensity rainfall,the middle and upper part of sandy soil layer with unusual weak erosion resistance is emptied by water flow to form a deep-cut hole which also known as concave cavity,and subsequently the soil body above the concave cavity will occur deep integral slip failure;while in short-term heavy rainfall environment,the failure mode of collapsed wall appears as shallow layer failure(soil layered flowing).However,no matter what type of rainfall it belongs to,there must be an early stage(lasting for a long time)of rainfall process that facilitating the appearance of the splash erosion pit,which is precisely the prerequisites for the collapse of headwall.The seepage areas generated by rainfall infiltration mainly occurs in the shallow soil layers of the caved wall,as a result,the shear strength properties of shallow soil continuously weakened and the maximum shear stress increased obviously.With the prolongation of rainfall duration,transient saturated zone appears on the surface soil of catchment slope and the wetting peak gradually moves towards the deep stratal of the collapsing gully wall,the groundwater diving line gradually rises and thus causes pipe erosion to the bottom of the sandy soil layer in the way of downward springs.3)To investigate the relationships between the dimensions of concave cavity and the development of collapse hills and Benggang erosion,in this paper,the quantitative process of the failure of caved wall owing to the expansion of splash erosion pit or the increase of soil water contents in red clay layers were clarified.A two-dimensional numerical simplified model of the caved wall-concave cavity is established based on the Saint Venant principle,and a "digging simulation algorithm" for simulating the depths of concave cavity and a "dichotomy" for improving the calculation accuracy are proposed.Quantitatively analyze the D0 value needed to trigger the obvious collapse of collapsing wall in natural state,and at the same time,the sensitivity of the main factors that leading to the collapse of the cavedwall is evaluated by means of the orthogonal experimental,at last,this paper chose a series of the depths value(less than D0)of concave cavity,the quantitative relation between these depths of concave cavity and the saturation threshold value that causing the collapse of overlying red clay layer covering with cavity was explored.The results showed that:there is a significant linear negative correlation between the depths of concave cavity and the safety factor of collapsing wall;the water content of the red clay layer overlying the hollow cavity is the most vital factor that bringing about the collapse of headwall,in addition,the gradient of the collapsing wall has a distinct impact on the stability of the headwall,and the proportion of the height of concave cavity to the thickness of the sandy layer has little effect on the stability of collapsing gully wall,the negative correlation between depth of concave cavity and critical moisture content of red clay can be expressed by quadratic polynomial equation.Combined with the numerical results,the prediction formula for collapse of caved wall is established by least squares method,which is?Sr0*?16.577-0.9572d-0.8469d2.This formula has strong prediction ability and adaptability,and the results have some certain reference significance.The rationality and applicability of numerical results are verified by consulting literature and comparing with previous research results.The results can provide a clear direction for the prevention and control system of collapsing gullies. |
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