Excess Pore Pressure And Its Formation Mechanism In Debris Flow

At 1:30 a.m.on August 18,2010,a catastrophic debris flow broke out in Dongyuege(located on the left bank of the Nu River)in Gongshan County,Nujiang Autonomous Prefecture,Yunnan Province,with outflow of about 60×104 m~3.The whole process of debris flow lasted for nearly half an hour.It caused a short blockage of the Nu River main stream that was more than 100 meters wide.The backwater height reached more than 10 meters,eventually the debris flow killed 96 people,causing direct economic losses of 140 million yuan.The total catchment area of the Dongyuege is about 46.7 km~2,and the length of the main channel is about 13.5 km.The overall drainage pattern has the characteristics of a narrow and long leaf-like shape.The slopes on both sides of the gully are steep,and the cross section is mainly shown as“V”character,and the cutting depth is large with typical features of sudden rises and falls in terrain.Debris flows with a migration distance of more than 3 km are common,and those travelling over 5 km also occasionally occurred,but such debris flows that can travel at an average gradient of 9°over 11-km channel like the Dongyuege debris flow have not been reported.In order to study the basic characteristics of debris-flow formation in Dongyuege and formation mechanism of excess pore pressure,this paper combines detailed field surveys and measurements,grain size analysis,and laboratory experiments based on the Dongyuege debris flow,and draws the following conclusions:(1)The main channel of Dongyuege is narrow and long,the slope is large and straight,and the section is of“V”shape,alternating in width,which promotes the rapid migration of debris flow.The branch ditches on both sides of the main channel are developed and the vegetation is developed.No.1 and No.2 and No.3 branch ditches have transported a large number of solid loose deposits for the movement of debris flow,of which branch No.2 is the main provenance area.Based on the field investigation of the relevant parameters of debris flow in Dongyuege,combined with relevant norms and empirical formulas,it was determined that the“8.18”Dongyuege debris flow is classified as ultra large—low-frequency—viscous—gully shape—storms and ice-melt debris flow.(2)According to the analysis of fabric characteristics,the wide gradation characteristics of sediments in Dongyuege debris flow are very obvious.There are widespread distributions ranging from 0.04 microns to several meters in size,and the maximum grain size can differ by 9 orders of magnitude.Dongyuege debris flow has a good grading and is dominated by coarse-grained particles.From the start-up area of debris flow to the accumulation area,the grain gradation is getting better and better,and the sortability is getting worse.(3)The slurrying of Dongyuege debris flow is limited by solid volumetric concentration(SVC).The lower SVC limit,,is the SVC boundary between a hyperconcentrated streamflow and a debris flow.The upper SVC limit,,corresponds to the SVC boundary between a debris flow and a granular flow.The difference betweenand(known as debris flow index,)can reflect the slurry forming ability of debris.(4)Peak value()and rate of dissipation(R)of relative excess pore pressure for Dongyuege debris flow are dependent on SVC.increases with SVC.With increasing SVC,R initially decreases,and then subsequently increases.Hence,an optimum SVC()exists that allows the dissipation rate of excess pore pressure in Dongyuege debris flow to be minimized,thereby facilitating the long runout of debris flows.For different sediments with the same maximum grain size,R value is controlled by the sorting and composition of debris.(5)The long runout distance of the Dongyuege debris flow over the low gradient channel is related to the internal high excess pore pressure and its strong persistence capability.However,not all sediments can be reconstituted into an integrated homogeneous slurry.Poor sorting is a prerequisite.The layer-lattice silicates can be the typical clay minerals including kaolinite,montmorillonite and illite,and also the unrepresentative clay minerals such as muscovite and chlorite.Small woody debris can also contribute to the slurrying of sediments and maintenance of debris flows in well vegetated mountainous areas and thus extend debris-flow mobility.(6)Compared with Dongyuege debris flow,Jiangjiagou debris flow slurry has a very strong ability to maintain relative excess pore pressure over a long period of time,and its ability to release water is extremely poor;in terms of the relative excess pore water pressure peak,under the same volume concentration conditions,the peak value of relative excess pore pressure in the Dongyuege debris flow is generally larger than that of Jiangjiagou.This means that Dongyuege debris flow itself can share more solid weight to the fluid inside the debris flow slurry,that is,Dongyuege debris flow has a stronger handling capacity than Jiangjiagou debris flow.(7)Seen from the rheological relationship between the Jiangjiagou debris flow and Dongyuege debris flow,with the increase of volume concentration,the shear stress,yield stress and viscous coefficient of debris flow slurry all show an increasing trend.In addition,under the same volume concentration conditions,the shear stress,yield stress,and viscous coefficient of the Jiangjiagou debris flow slurry are significantly higher than those of the Dongyuege debris flow slurry.The physicochemical properties of debris from debris-flow slurry are important factors that cause the differences in the rheological properties of debris flows in Jiangjiagou and Dongyuege.