| A large-scale debris flow occurred in Dongyuege(DYG)Gully of Gongshan County,Nujiang Prefecture in August 18,2010.More than 100,000 cubic meters of debris flow deposits were rushed out from the No.2 branch of DYG,instantly flooding nearby farmland,houses,transportation and power facilities.High-speed moving debris flow rushed into the main stream of Nujiang River,even causing short-term blockage of the main stream of Nujiang River hundreds of meters wide.The debris flow lasted for nearly half an hour from its occurrence to its end,resulting in more than 90 deaths and a direct economic loss of about 140 million yuan.It is a large-scale debris flow disaster with the largest scale,the strongest destructive force,the largest number of deaths and the most serious loss of economic property in Yunnan in recent years.By studying the topographic and geomorphological characteristics of DYG gullies,it is found that the whole gully is very narrow,and the width of gully mouth is different and interlaced.In the downstream area of the valley,there is a low gradient gully bed with a length of about 3 km and a slope of only 3-5 degrees.Usually,the migration distance of large-scale debris flows in low-slope gullies is restricted by riverway topographic fluctuation,the type of rock mass in the gully bed,and the vegetation cover of the valley,and the migration distance is limited.However,the debris flow that occurred in DYG gully moved 3 km long over the low gradient channels.In recent years,such high-density debris flows,which migrates huge deposits,rarely occur in low gradient channels for long distances,highlighting the uniqueness of debris flows in DYG gully.In the preliminary study on the mechanism of high-speed and long-distance migration of debris flows in the DYG gully,it is found that mineral composition in fine grains of debris flows play an important role in the long-distance maintainance of slurry,but when the mineral composition of clay is changed,whether the reconstructed debris can be slurried or not,and whether the rheological behavior of slurry is affected have become the focus of this paper.Considering that quartz is the main rock-forming mineral,widely distributed in the surface rock and soil,and quartz powder is easy to purchase,it is convenient to be used as experimental raw materials,so quartz is used instead of clay minerals to study this problem.In this paper,a series of experiments on excess pore water pressure and rheological parameters of debris flow samples and reconstructed samples mixed with quartz powder in Dongyue were carried out,by comparing the original debris flow samples and the reconstructed slurries replacing clay mineral components in DYG with laboratory experiments and data analysis,the following conclusions are drawn:(1)The DYG gully is characterized by steep terrain,narrow channel and large slope gradient,abundant branching ditches,high vegetation coverage and thick loose deposits.On the eve of the occurrence of debris flow,there was more rainfall in the area,and developed rock mass fissures and joints in the upper reaches.These factors constitute the necessary conditions for the occurrence of debris flow,such as landform,provenance,geological structure,rock mass structure and rainfall infiltration,which are the fundamental reasons for the subsequent occurrence of super-large debris flow.(2)On the premise that the gradation is basically the same,the reconstructed debris is reconstructed by substituting the clay component in the original debris flow with the framework mineral quartz,and the excess pore water pressure experiment of reconstructed debris is carried out.Comparing the reconstructed debris excess pore water pressure curve with the original debris flow chart,it is found that the reconstructed debris also has the process of accumulation,maintenance and dissipation of excess pore water pressure,which proves that the reconstructed debris replaced the clay mineral composition can still form slurry.(3)During the experiment,it was found that the excess pore water pressure of reconstructed slurry lasted shorter and dissipated faster than that of original slurry.Because of the change of the whole structure of debris flow,the range of slurry density of reconstructed slurry became narrower and slurry capacity was weaker,which reflected from the side that the migration capacity of reconstructed slurry was weaker than that of original debris flow.(4)Based on the data analysis of relative excess pore water pressure,volume of water released,liquefaction rate and dissipation rate of pore water pressure obtained from excess pore water pressure experiment,it is found that the volume concentration of reconstructed slurry,the upper limit particle size and the upper limit particle size of quartz powder are the main factors affecting the physical and mechanical parameters of debris flows.(5)In the experiment of rheological parameters measurement,when the volume concentration of slurry increases to the upper limit at the same shear rate,the corresponding shear stress will increase sharply,indicating that the higher the volume concentration,the greater the impact on the shear stress of slurry.Fitting the curves of yield stress and viscosity coefficient varying with volume concentration shows that the two rheological parameters increase exponentially with volume concentration.(6)By comparing the rheological parameters of reconstructed slurries with different upper limit particle size,different quartz powder particle size,different volume concentration and different mineral composition,the main factors affecting the rheological parameters of reconstructed slurries are slurry volume concentration,clay mineral composition,upper limit particle size of slurry and upper limit particle size of quartz powder.They have great influence on the viscosity and thixotropy of slurries.Macroscopically,it is also very important for the migration and maintenance of debris flow.(7)In this paper,a large number of experiments and data analysis show that the reconstructed debris composed of single mineral(quartz)can form debris flow slurry,but compared with the original slurry of debris flow,the reconstructed slurry has smaller the range of the slurrying,weaker pore water pressure maintenance ability,and poorer migration and suspension ability,which proves that clay mineral composition(mica,chlorite)plays an important role in the slurrying and rheological behavior of debris.It has certain guiding significance for the prevention and control of debris flow disasters. |
