The Tendency Of Rock-avalanche Deposit To Transform Into Debris Flow

Debris flow and landslide are two kinds of geological disasters widely distributed in mountainous areas of China,and they are one of the most important factors restricting local economic development.Landslide is closely related to debris flow.Usually,after landslide(especially large-scale landslide event),a large number of loose debris will be formed and deposited on the slope or valley,which will greatly increase the tendency of subsequent debris flows in this area.Although for past years great achievements have been made in close connection with the transformation of loose deposited debris generated by landslide into debris flow,there hitherto exist unknown fields which are giving rise to scientist’s attentions.(1)There is a lack of quantitative evaluation on the slurrying-persistence ability of loose deposited debris.(2)The mechanism of debris liquefaction in the formation of debris flow is still not fully understood.(3)The discrimination of the tendency of loose deposited debris into debris flow is not accurate enough.Hence,the research on the transformation of loose deposited debris from landslide into debris flow can deepen and improve people’s understanding of this kind of disaster and provide insights into the hazard assessments and designs of protective measures.Taking the Touzhai valley watershed where a large rock avalanche event(also known as large high-speed and long-runout landslide-debris flow event)occurred in 1991/09/23,we investigate the tendency of Touzhai rock avalanche deposit to transform into debris flow and the mechanism of debris liquefaction and the slurrying-persistence ability based on field meticulous investigation and monitoring,laboratory test and multi-type multi-scale laboratory test,as well as theoretical analyses.The main research results in this thesis are as follows:1.The Touzhai valley watershed has steep terrain,abundant rainfall,and is seriously affected by the mountain microclimate.The rainfall increases significantly with the increase of altitude.The annual rainfall near the source area is 1726.8 mm(monitoring data from October 2013 to September2014).About 900×10~4 m~3 loose debris is deposited in Touzhai valley along the way.The debris has the characteristics of wide grading and poor sorting.The debris(<5μm)is mainly composed of clay minerals such as montmorillonite and kaolinite.The edges and corners of the deposited debris are obviously passivated,and the shape is sub-circular to sub-angular,and some of the debris have dissolution pores inside,and the outside is surrounded by saprolite crust,but fresh debris still accounts for more than 80%of the entire deposition.Sufficient loose deposited debris,steep topographic environment and sufficient rainfall provide perfect basic conditions for the development of debris flow in Touzhai valley.2.Based on the results of slurring tests,excess pore water pressure monitoring experiments and rheological and slump tests show that the debris flow index(Id)of Touzhai(TZ)rock avalanche debris and Dongchuan Jiangjia valley(JJG)debris flow are similar(It means that the two types of debris have similar tendency to form debris flow).Both TZ rock avalanche debris and JJG debris can be remolded into high liquefaction degree debris-water mixture slurry,but the ability of TZ rock avalanche debris slurry to maintain a high degree of debris liquefaction is stronger than that of JJG debris flow slurry.Under the same solid volume concentration,the yield stress of TZ rock avalanche debris slurry is higher than that of JJG debris flow slurry,indicating that once TZ rock avalanche debris is transformed into debris flow,its ability to transport boulders will be stronger than that of JJG debris flow.3.Grain size distribution(wide grading,poor sorting),fine debirs mineral composition(main clay minerals are montmorillonite and kaolinite),debris geometry(sub-circular-sub-angular),and weathering state(dissolution pores inside the debris and the surface covered by saprolite crust)of Touzhai rock avalanche debris contribute to the improvement of its slurrying and long-runout persistence ability,thus increasing the tendency of Touzhai rock avalanche deposit to be transformed into debris flow.4.TZ debris slurry has shear thinning behavior similar to JJG debris flow slurry,which means that once TZ debris forms debris flow and begins to propagate along the valley,the internal resistance of the slurry will decrease rapidly.This characteristic is conducive to the long-runout propagation of TZ debris slurry in low-gradient channel.5.The results of ring shear test,slope rheology and pore water escape experiments and permeability tests show that the liquefaction degree of saturated debris-water mixture is mainly related to be agitated degree.The strong agitation will change the original contact behaviour between the particles,separate the particles from each other,enhance the combination of fine debris and saturated water,and form an interstitial fluid with greater density.The pore space generated by the separation of particles caused by the agitation is quickly filled by the interstitial fluid,and the contact stress released by the separation of the particles is borne by the interstitial fluid,resulting in the development of high pore fluid pressure inside the soil,which in turn liquefies the soil.6.Propagation and deposition behaviors of static saturated soils and debirs slurries show significant differences.The propagation capacity(propagation speed,propagation distance and inundated area,etc.)and the ability of the formed debris-water mixture to trap and hold pore water of static saturated soil after failure are significantly lower than those of debris slurry.The propagation and deposition behaviors of saturated soils are closer to that of granular flow,while debris slurries are closer to viscous debris flow.The process of the Touzhai rock avalanche debris transforming into debris flow can be regarded as the process of transforming static saturated soil into debris slurry.7.In the initial stage of the static saturated soil after the failure,the particles are in contact with each other,and the frictional resistance between the particles dominates the interior of the debris-water mixture.As the propagation continues on the rough and steep bottom,the particles collide and separate from each other,and the degree of liquefaction of the debris-water mixture gradually increases.During the propagation process of the debris slurry,the particles are suspended on the interstitial fluid,and the inertial force(collision between particles)is the main force inside the slurry.The collision between particles will increase the degree of liquefaction and further promote the long-runout propagation of slurry in low gradient channels.8.With the increase of slope and slope length,the propagation and deposition behaviors of static saturated soil and debris slurry after failure are the more similar,that is,the higher the tendency of saturated debris to debris slurry,which is closely related to the liquefaction mechanism of debris(the increase of slope and slope length will lead to the enhancement of agitation experienced by saturated soil after failure).However,when the slope and slope length is too large,grain‐size segregation will occur during the propagation process of the debris-water mixture,that is,the coarse particles gradually gather to the flow front when they propagate on the steep slope,which will hinder the further propagation of the liquid slurry at the back after propagating to the low gradient channel.