| Shimian County,with steep terrain and complex geological conditions,is located on both sides of the Dadu River,where river erosion and cutting are strong and the debris flow disaster within the territory is more developed.The disaster is more frequent there.The bank slope of Jiefanggou valley is high and steep,and the weathering of rock and soil mass is strong,which leads to the serious fracture of rock mass,the development of joint fissure as well as the reduction of bearing capacity.Under the influence of the interior vibration,the soil structure of the inner bank slope is more loose and broken,and the slope stability is poor.Affected by the adverse factors such as earthquake and continuous rainfall,the slope in the area becomes unstable and much sediment accumulates along the slope or gully,forming debris flow and landslide material sources.In this paper,the debris flow in Jiefanggou,Shimian County will be taken as an example.Through field investigation and theoretical analysis,numerical simulation was carried out based on PFC-2D software combined with the geological environment and rainfall characteristics of Jiefanggou.Finally,the prevention and control engineering was designed for the debris flow trench.The main research contents and achievements of this paper are as follows:1.The geological and geographical conditions of the study area were investigated.As heavy rain-low frequency-large-dilute debris flow,Jiefanggou debris flow is characterized by low frequency of activity and large scale,which occurs once every 50-100 years.The debris flow is relatively small,while the hydrodynamic force is strong,with the main characteristic of scour.The ditch bank slope erosion and the ditch bottom erosion are serious.The flow intensity of Jiefanggou debris flow is about 1.713t/m3.The average flow velocity is 3.5m/s～8.2m/s;The scouring depth of debris flow is 0.6～0.97m;The impact force of debris flow reaches the maximum of 14.7KN at the 24-24’ section downstream of the trench.2.The formation mechanism of debris flow is studied.Under the influence of rainfall,the debris flow in Jiefang Gully decreases the shear strength and stability of the slope.The surface runoff erodes and transports slope rock,soil mass and loose provenance,then the involved provenance continues to erode along the gully,at the same time produces lateral erosion on both sides of the gully.In addition,the shear strength of the slope is further reduced.In the middle and late period,hydraulic erosion is the main action,which produces more landslide material sources,so the amount of material sources increases continuously,moving downstream,and eventually debris flow break out,forming a huge destructive power.3.The PFC-2D particle flow program was used to establish the two-dimensional particle model of debris flow,and the mesoscopic parameters were obtained by numerical biaxial experiment for parameter calibration.The position vector cloud diagram,velocity vector diagram,contact force vector diagram,particle displacement vector diagram and maximum erosion and deposition distance of debris flow under simulated continuous heavy rainfall were obtained.The results showed that the furthest erosion and deposition distance of debris flow was 3625 m,which seriously threatened the life and property safety of residents in the middle reaches of the gully.The parameters of each monitoring point were consistent with the debris flow stress value.4.It is planned to lay 1# gravity type blocking dam in A-A’ section,2# gravity type blocking dam in C-C’ section,drainage channel in D-D’ section to 24-24’ section,revetment wall engineering in 24-24’ section to G549 of the national road. |
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