The Research Of Mechanism That The Loose Materials Transform Into Debris Flow After The Earthquake-considering The Influence Of Slope, Discharge, Initial Moisture Content

Various geological disasters happen after “5.12” earthquake, especially debrisflow that affects seriously. It is significantly different that debris flow happens in theearthquake region after the earthquake from happens in non-earthquake region beforeearthquake. Debris flow presents new characteristics after the earthquake, whichbrings the severe challenge to control. Therefore, it has an important significance toresearch. The mechanism that the loose materials transform into debris flow isresearched by laboratory model tests. Through lots of flume experiments, obtain thestartup mode of debris flow, the erosion of gully and the influence of slope, dischargeand the initial moisture content. The main research findings are summarized asfollows:(1)A fully instrumented laboratory model test system for debris flow is designed.Through three groups of repeatability tests, the results show that the system has agood reliability.(2)The startup modes of debris flow can describe as:1).shallow landslide. Thesurface of model will be eroded when the test starts. A gully will form behind themodel. And a dam will form in front of model. Once exceeding the critical steadystate of the dam, the dam will failure and the preliminary debris flow occurs.2).Thephenomenon of Damming and breaching. We can describe the process of debris flowas “erosion-instability-damming-breaching”. After shallow landslide, there exists anunobstructed gully on the surface of the test model. And the water will continuouslyerode the gully. It causes the gully being deeper, and triggers sides of slope beinginstability. Finally, the gully is obstructed because new loose materials slide into thegully. As the dam failures, new debris flow will happen again. The phenomenon ofdamming and breaching happens many times after the shallow landslide. (3)In order to understand the process of erosion, we use3D laser to scan thegully at different moment. We obtain the three-dimensional model of eroded gully, thecurve forms of eroded gully and the channel curves at the same crosssection. Through analysis of erosion curves, we know that vertical erosion is the mainform behind the test model when the test starts. As there is nothing at the bottom ofgully, then the water will horizontally erode the soil materials at the sides of gully. Butthe erosion of gully is different in front of the test model. Vertical and horizontalerosion is the main erosion patterns.(4)The impact of slope and discharge is considered in the tests. The criticaldischarge that makes the loose materials transform into debris flow decreases as theslope of model increases. Only considering the impact of the slope, as slope becomesgreater, the rate of erosion will be faster. Only considering the impact of discharge, asdischarge becomes greater, the rate of erosion will be faster and the scale of debrisflow will be larger. When both considering the impact of slope and discharge, we findthat the rate of erosion is faster as slope and discharge become greater.(5)The impact of initial moisture content is considered. The initial moisturecontent can influence coefficient of permeability and mechanical properties of soil. Itcauses different patterns of debris flow. When the soil is dry or the initial moisturecontent is10%, the process of debris flow involves shallow landslide and thephenomenon of damming and breaching. The difference is the way that the slopebeing instability. When the soil is dry, a large number of loose materials slide intogully when the slope is instability. As existence of the matrix suction because the soilof test is unsaturated, the soil collapses into the gully when the slope is instability. Theinitiation mode of debris flow is significantly different when the moisture content ofsoil is5%.At first of the test, all of water infiltrations into the model and the surface ofthe test model doesn’t form a gully. When the model is saturated after a period of time,the water starts to seep out at the front edge of the model. Then the soil at front ofmodel creeps and many cracks emerge from front of model to back of model. Finally,the whole model collapses. Then, a large number of loose materials with thewater pour out. Comparing to other test with different initial moisture content, thescale of debris flow is larger when the initial moisture content is5%.