汶川地震触发大光包巨型滑坡基本特征及形成机理分析

BASIC CHARACTERISTICS AND FORMATION MECHANISM OF THE LARGEST SCALE LANDSLIDE AT DAGUNGBAO OCCURRED DURING THE WENCHUAN EARTHQUAKE

大光包—红洞子沟巨型滑坡是汶川地震触发的最大规模滑坡,其体积达7.42亿m3,堰塞坝高达690m,是我国已知的最大规模地震滑坡和最高的滑坡堰塞坝,也是目前全世界已知的为数不多的几个方量在5亿m3以上的超大规模滑坡之一,其高达690m的滑坡堰塞坝为目前世界最高的滑坡坝。滑坡位于发震断层上盘,距发震断裂——映秀—北川断裂不足7km。震前斜坡为三面切割的孤立型山脊,相对高差达1500m;斜坡岩层走向与坡面近于垂直,层面延展性极好,构成滑动面形成的基础。调查和分析表明,斜坡的临空条件和贯通性好的灰岩层面是滑坡产生的基础;而高强度和长持时强震地面运动是导致滑坡产生的根本因素。滑坡产生的机理和过程可分为以下4个阶段:即坡体震裂、松弛和解体阶段、高速溃滑阶段、震动堆积阶段、二次抛射和碎屑流堆积阶段。失稳高速下滑的坡体,形成了沿主滑方向长4.2km,宽2.2km的堆积体,高速流动的碎屑流越过下游侧风波岩山脊,沿红洞子沟形成了长1km的碎屑流堆积区。

The landslide at Daguangbao and Hongdongzigou site is the largest among all landslides occurred during the Wenchuan Earthquake. Its volume is up to 742 Mm^3 , and its debris dam height reaches 690 m. So it is also the most largest scale landslide triggered by earthquake among all investigated landslides in China. Additionally, it has the highest debris dam originating from landslide in China. It is even one of a few super large - scale landslides known all over the world, whose individual volume is over 500 Mm^3. The debris dam formed by this landslide can be the highest around the world. The landslide is located on the upper plate of the Causative Fault. It is nearly 7 km away from the triggering seismic fracture of Yingxiu - beichuan fault. The slope was an isolated ridge before the Earthquake. it＇s the ridge had its 3 sides cut. Its height difference reaches 1500 m. The strike of clinothen was almost vertical to the slope surface, the clinothen had perfect extension. They composed together the basic conditions of sliding surface formation. Our investigation and analysis show that the free surface conditions of the slope and the surface connectivity of limestone bedding plane were the base of landslide formation. And the ground movement resuited from the intensive quaking with large intensity and long duration was the radical triggering factor of the landslide. The mechanism and process of the landslide could be divided into 4 stages as follows： （ a） the stage of slope body shatter, relaxation and disintegration, （b） the stage of high speed sliding, 9c） the stage of accumulation with shock, and （ d）the stage of the second projectile and debris flow deposition. Because of destabilization and high speed sliding down, the slope mass composed a deposit body along main direction of sliding, 4.2 km long and 2.2 km wide. High -speed clastic flow crossed downstream Fengboyan, and formed a deposit zone of debris flow along Hongdongzigou, 1 km long.