Study On The Accumulation Characteristics And The "Braking Effect" Of Daguangbao Massive Landslide

Generally, a strong earthquake always has a very complex dynamic environmentwhen it happened near the seismogenic fault. Therefore, high-speed remote landslidestriggered by the earthquake often have a more complex movement and accumulationprocess, and also have some kinds of rare dynamic characteristics only under theseismic conditions. Daguangbao massive landslide is the largest landslide triggered by"5.12" Wenchuan earthquake, which is also the largest known earthquake landslidesince the founding of new China. The large-scale of the landslide, strong destruction,the complex formation mechanism and the unique signs of accumulationcharacteristics are extremely rare in the world. On the basis of a great deal of the fieldinvestigation in the Daguangbao landslide area, the author preliminarily did lots ofresearch and analysis of the approximate location of the shear zone of landslide, andstudied the characteristics of the sliding mass in order to separate it into small pieceswith the fine description of each one. Besides, the author preliminarily calculated thevelocities of the various stages of landslide occurrence. In addition, with the physicalsimulation experiment verified the "braking effect" mechanism of Daguangbaolandslide, the author analysed its dynamic characteristics. The research work andresults obtain the following basic conclusions and the knowledge as follows:(1) By compared the latest measured topographic map (1:2000) of Daguangbaolandslide area detailedly with the original one (1:50000) before the Wenchuanearthquake, and arranged a total of15profiles of7vertical and8horizontal lines, theauthor detailedly analyzed the longitudinal structural characteristics of the landslideaccumulation body, and reasonablely assumed the approximate of shear zone ofDaguangbao landslide, located the bottom right side of the huangdongzi valley, whoseelevation is about1440~1600m.(2) Based on the detailedly field investigation, and taken the landslide elements, the characteristics of the sliding massive, lithology distribution of surface stone andthe topographic before the earthquake into consideration, Daguangbao landslide areacould be separated into5zones, which are zone-Ⅰ source area, zone-Ⅱ accumulation ofpost-collasp area, zone-Ⅲ main accumulation area, zone-Ⅳ upstream debris flowdeposition area, zone-Ⅴ downstream debris flow deposition area, zone-Ⅵ ejectionand overfall deposition area. Zone-Ⅰ also can be divided into3sub-districts as well aszone-Ⅱ, and zone-Ⅲ can be divided into5sub-districts. So, there is a total of11sub-districts in the landslide area. By describing the accumulation characteristics ofall the districts of the landslide detailedly, the result shows that the characteristic ofthe sliding mass is not random clutter, while there is still a certain amount ofregularity. For instance, the stone lithology of the accumulation is predominantlyDevonian shawozi group (Ds), and the other lithology also largely distributed in theaccumulation according to the original sequence.(3) The working group investigated511flat stones that scattered on the surfaceof the sliding massive, about the advantage occurrence of the stones. It shows someaccumulation law about the flat stones that overall, the advantage occurrence of theflat stones are not very prominent, obviously effected by the local terrain, though itstill has obvious advantage direction from the point of view each partition, which hasa close relationship with the topography of the position and the sources and causes ofthe debris-flow.(4) In addition, the working group surveyed106dumping trees that scattered onthe accumulation body about the tendency and the diameter e.g. at the same timewhen they were investigating the flat stones. The survey turns out that the overalltrend of the dumping trees is consistent with the landslide axis direction, which isbasic fitting the results of this analysis and field investigation. Meanwhile thedumping trees will also be affected by the local movement trends of the accumulationbody so that it can only reflect the local movement and accumulation characteristics.(5) The dynamic process of Daguangbao landslide can be classifed into4stagesas follows:(a)The cracking and the shattering of the substrate;(b) shearing failure ofthe toe of the landslide and the landslide was initiated;(c) the sliding mass travelledand collapse of high-speed;(d)the sliding mass slid over the valley and crest anddeposited. It was the elastic impulse acceleration effect caused by the earthquake atthe substrate that Daguangbao landslide initiated very fast. Landslide departure speedVeo1.63to2.82m/s was calculated based on the accumulated strain of the lockingsegment at the toe of the landslide. The sliding mass travelled in high-speed, and when it slid over the bottom of huangdongzi valley, its maximum speed V1’ was up to91.34m/s, which is the collision speed of the "braking effect". After collision, thevelocity of the main sliding mass was49.54m/s. So it can be seen that after the mainsliding mass had a strong collision caused by the "braking effect", the kinetic energy ithad halved in an instant, more than half of the energy loss. Besides, ejection speed inthe ejection and overfall deposition area had been calculated, and the velocity V0wasabout102.3m/s.In addition, the apparent coefficient of friction of Daguangbao landslide is about0.3014, a little bigger than that of the ordinary high-speed remote landslide, whichmeans that Daguangbao landslide has obviously been the strong blocking by themountain in front of it under the action of the "braking effect", which cost so much ofthe energy that it reduces the mobility and the movement distance of the landslidedebris flow.(6) The author exploratorily designed and tested a physical simulation experiment of"braking effect", using six different test materials, within two kinds of different speeds.It was a total of11times test on the experiment, which obtained various experimentalphenomena. Above analysis on the comparation of the model test results with the"braking effect" of Daguangbao landslide in reality, it turns out to be that manyexperimental phenomena in the model test are coupled with the actual brakingeffect. Therefore, these results of the physical simulation experiment basicly verifythe formation mechanism of "braking effect" of Daguangbao landslide.(7) The formation process of "braking effect" can be summarized in two stages:1)the stage of high-speed slope body collision and disaggregation;2) the stage ofreversing, onlap and accumulation. Stage1), mainly performed that the terrane at thelower part of the main slipmass broke and scrammed and the upper terrane took acertain degree of deflection deformation. Because of the free surface condition, thestrong-unloaded rackmass and the overlay of the top part rushed forward directlyalong the strike of the terrane. Stage2), mainly performed that the strong-unloadedrackmass and the overlay at the top part of the main slipmass moved forward, collidedand desegregated. And the high-speed clastic flows crossed dushanshu ridge andformed onlap accumulation. The upper terrane thrust up and climbed the dushanshuridge with the action of inertia. This made the original terrane stick up and form aseries of nappe structure. The topside terrane even directly crossed the ridge andformed the onlap accumulation.Furthermore, there are three key factors to explain the "braking effect" of Daguangbao landslide, which are the topographical condition in landslide area,characteristics of the rock mass structure of the sliding body, and the sufficientpotential energy difference. All these three key factors are definitely necessary for the"braking effect", and the topographical condition is the most important one.