大光包滑坡启动机制:强震过程滑带动力扩容与水击效应

A model of initiation of Daguangbao landslide:dynamic dilation and water hammer in sliding zone during strong seismic shaking

大光包滑坡是2008汶川地震触发的最大规模滑坡,也是世界上百年内罕见的巨型滑坡。滑坡发育在坚硬的碳酸盐岩地层、与发震断裂垂直距离5 km、与震中直线距离85 km,达11.59×108 m3的方量改变了近10 km2地形地貌,暴露后缘600 m近直立断壁、上游1.5 km拉裂边界、下游1.8 km剪滑光面及堆积600 m滑坡高坝,呈现出不同常理的滑坡现象,引起国内外持续争论和广泛研究。在前人研究基础上,总结7 a研究成果,认为深埋厚层软弱岩带(层间错动带)和强震过程是滑坡启动的背景条件。展开岩体宏–细–微观结构调查和测试,证实强震过程滑带的碎裂和扩容,揭示出扩容成因是地震动放大过程的振动冲击(动力扩容)。对滑带岩体强度测试,肯定由震裂导致的强度劣化不是大光包滑坡骤然启动的主因;水文地质调查表明,震前大光包山体具有丰富地下水及连接滑带的潜在通道,并且发现滑带扩容裂隙受到与水流相关的碎屑充填。强震过程中潜在通道垂向贯通、高位地下水瞬间挤入深埋构造带的扩容空间、可能产生水击效应,降低构造岩体有效应力,并导致岩带强度骤降。建立水击力计算的力学模型,计算结果表明在地下水头70~360 m范围内可激发水击力达20 MPa;从而得出,强震过程滑带动力扩容和水击效应可能使得滑坡骤然启动。最后,提出一种大光包滑坡启动的机制模型:强震过程→滑带地震动力破裂扩容→水击效应→滑坡启动。

Daguangbao landslide（DGB landslide） is the largest landslide triggered during Wenchuan earthquake 2008.It is also one of the most catastrophic landslides around the world during the past 100 a.DGB landslide is in the area of carbonate rock and about 5 km away from the seismogenic fault and 85 km from the epicenter.However,this landslide changed the landforms of a region about 10 km^2 with a volume of 11.59×108 m^3.Additionally,DGB landslide exposed a main scarp with an average height of 600 m（maximum height of 850 m）,an open tension flank in the north with a length of 1.5 km and a sliding surface in the south with a length of 1.8 km.The river in front of DGB Mountain was filled up with a dam of 800 m in height.These incredible landslide features had attracted the wide concerns.Based on the previous studies and our observations in the past 7 years,the geology setting and earthquake-induced ground shaking were presented in detail in this paper.The distribution and properties of the 112 large-scale landslides triggered by Wenchuan earthquake were studied,and it was found that a bedding fault under Mt.DGB and the amplification of seismic ground motion in DGB area were the precondition of DGB landslide.Petrologic studies on the slip zone of DGB landslide microscopically were carried out and macroscopically and experimental investigation of the rock properties was performed.The results indicate that the rock mass of slip zone dilates during the earthquake.Site observation indicates that the dilatation of slip zone is resulted from the impact loading during the earthquake due to the amplification of seismic shaking.The shear strength of the materials in the slip zone was estimated.The results indicate that the reduction of shear strength due to rock fragmentation could not result in a sudden failure of DGB landslide.Recently,the ground water in Mt.DGB was investigated.The site evidence indicates that Mt.DGB has rich ground water with the well-developed vertical drainage before the earthquake.It is hypothesized that the vertical drainage is connected instantly during the earthquake,and that the ground water swarms into the dilated bedding fault which induces water hammer within this layer.A model of water hammer force in open tension crack was established,and the water hammer pressure was calculated.The results show that the maximum pressure is 20 MPa when the height of water head is assumed in a range of 70–360 m.It is concluded that water hammer pressure reduces the shear strength suddenly,and that the landslide is initiated with the high initial velocity.Finally,an improved model of DGB landslide mechanism was proposed.