| There are a large number of debris flow gullies in the northern and western mountainous areas of Beijing.Due to the the monsoon climate and mountain ranges,Beijing is prone to heavy rainfall in the annual 7~8 month,which leads to the frequent occurrence of mountain torrents and debris flows.Therefore,it is crucial to investigate and study the debris flow disaster in Beijing.Funded by the Beijing Municipal Science and Technology Commission,detailed geological investigation,data collection,test of physical and mechanical properties and simulation of debris flow in Nanjiao gully were carried out,which provided evidence for early warning research of debris flow in Beijing.At the same time,it is of great academic value and practical value to study the erosion process and the mesoscopic mechanism in the start-up procedure of debris flow.Geological survey for Nanjiao gully was carried out in Fangshan District of Beijing.Based on the information of geomorphology,rainfall and disaster history of Nanjiao gully,the accumulation of the source in the main and the branch gully and the structure of the rock mass were investigated and analysed.The characteristics and the risk of debris flow were evaluated.The shear strength parameters of the soil with different water content in Nanjiao gully were obtained by direct shear test.With the aid of a large scale simulation flume,the erosion and destruction mechanism of debris flow start-up procedure in a flood process was studied.Based on the small scale flume,the optimal combination of rainfall intensity,gradient and moisture content was carried out by orthogonal test,and the mechanism of debris flow start-up was analysed.The stress-strain curves of soil with 1.5%,5%and 11%water content were fitted by PFC to determine the optimal simulation parameters.Simulation of debris flow starting tests was carried out with soil micro-parameters under different moisture content according to the fitting results.In the process of simulation,velocity of debris flow,output of material source,motion trajectory of particle and porosity change of congeries were monitored in real time and analyzed in depth to reveal the microscopic mechanism of debris flow initiation.The main conclusions were as follows:(1)Nanjiao gully was the large,diluted and slope erosion debris flow gully,which had strong ability to cause disaster.But it had been in a recession and belonged to moderate risk.There were many early formed slope deposits and loose deposits in the gully and joint was developed,which provided abundant source for the occurrence of debris flow and had great threat to residential areas.The soil particle with size in the range of 60~200mm was more,accounting for about 47.15%of the total mass,which had a certain transporting capacity.The soil was dominated by primary minerals,and clay minerals were mainly illite and kaolinite.(2)The stress-displacement curves of soil in Nanjiao gully were strain softening type,which could be roughly divided into three stages:elastic deformation stage,plastic deformation stage and failure stage.With same water content,the greater the normal stress was,the greater the yield strength was,the higher the shear strength was and the lower the softening degree was.With same normal stress,as water content increasing,the yield strength,shear strength,residual strength and resistance to deformation of soil decreased,and softening degree increased.(3)With joint action of rainfall and a flood process,the failure mechanism of Nanjiao source was rainfall,runoff,surface erosion,erosion at toe of slope,gully erosion and erosion termination.Gully erosion occurred before the arrival of flood peak.The toe of slope reached saturation with water content of 25%~30%and damaged first at 3 kinds of rainfall.Gully erosion damage had occurred when the soil was not saturated with directly heavy rain.The soil erosion damage was a random paroxysmal process,and the pore water pressure curve also had paroxysmal characteristics.When the gully erosion destruction occurred,the flood flow volumes of the intermittent antecedent rainfall&rainstorm,rainstorm and light rain to rainstorm were 1.2m~3/h,2.2m~3/h and 1.92m~3/h,respectively,which were less than the peak discharge of debris flow,and it spent 10min,14min and 20min respectively.(4)When the gradient was 5 degrees and 8 degrees,the outfall was prone to clogging and siltation at ten years'raininess.The outfall was not easy to be blocked,but prone to be passing and slippery at hundred years'raininess.When the raininess was large,debris flow with surface erosion occurred mainly.Water content of surface soil changed at four stages,rapid rise?gentle rise?rapid rise?saturation.The water gathered at the toe of slope,where the pore water pressure was high.The grandient was the decisive factor of the formation time of the trench and the output of the source.The raininess was the decisive factor of the cumulative rainfall during the formation of the trench.Moisture content had the greatest impact on the amount of runoff,and had the least influence on the formation time of gully and cumulative rainfall.(5)The large direct shear test could be simulated accurately by the grain flows program.The main failure modes of soil at 1.5%moisture content were erosion and siltation at toe of slope and retrogressive erosion.The main failure modes at 5%moisture content were erosion at toe of slope and tension destruction at trailing edge.The main failure modes at 11%moisture content were tension destruction at trailing edge and whole slippery.During the starting process of debris flow,the velocity of debris flow first increased,then decreased,and tended to stability.(6)The evolution models of debris flow initiation were established.The stress condition and the critical velocity of particles under rainfall conditions were obtained.The velocity of the particle with diameter of 1m and 2m was calculated and the critical speed of the two kinds of particles with stage of erosion at toe of slope was 2.2m/s and 3.1 m/s,respectively. |
PDF Full Text Request