苏坡店沟“7·10”泥石流破坏都汶公路毛家湾大桥机理

The Failure Mechanism of Maojiawan Bridge by “7·10” Debris Flows

2013-07-10四川省汶川县岷江干流沿线群发性泥石流(简称"7·10"泥石流)对都(江堰)汶(川)公路(G213线)和都汶高速桥梁、路基、隧道造成重大破坏,G213线和都汶高速共16处中断,尤以G213线破坏严重。苏坡店沟毛家湾大桥是"7·10"泥石流冲毁桥梁、造成交通中断的典型模式。在野外调查、采样和数据分析的基础上,确定苏坡店"7·10"泥石流的重度18.5 k N/m2,流速9.3 m/s,峰值流量232.5 m3/s,剪应力13.3 k Pa。高速、大流量泥石流沿陡峻的主沟排泄过程中,强烈侵蚀和起动沟床,左侧桥基基础强烈淘蚀悬空,形成1.65 m宽的临空面;在泥石流的作用下,桥基沿主沟发生5 m位移变形,导致桥梁垮塌,交通中断。经计算,桥墩下部土体支撑面积为42.8 m2,基底最大压力达545 k Pa,大于规范规定允许承载力,并且从抗倾覆角度计算也超出了规范要求,处于失稳状态。针对跨越高陡泥石流沟的桥梁,建议加强泥石流沟排导、预设足够的桥下净空高度、增加桥梁防护等措施来减少桥梁的破坏。

The catastrophic debris flows on July 10, 2013 resulted in huge destruction to Dujiangyan - Wenchuan highway （G213） in Sichuan Province of China. The highways were seriously destructed and blocked in 16 sites, which were induced by 7collapsed bridges and base, 2 of 7 destructed were .scoured by debris flow. Based on field investigation and analysis, the parameters were calculated, which were the density of 1.88 g/cm^3 , the velocity of 9.3 m/s, the peak flow of 232.5 m^3/s, and yield stress of debris flowslof 13.3 kPa. The debris flows scoured the foundation of the bridge down the steep gully seriously, the pier lost its stability and the bridge collapsed. The calculation indicates that： the soil below the pier is scoured away by the debris flow leading to the pier overhanging, and the lateral length reaches 1.65 m; on the other hand, the water in debris flows should run into the foundation soil, which reduces the soil＇ s strength. The pier failed when the maximum pressure （545 kPa） of the foundation exceeding the allowable bearing capacity, then the bridge destroyed. By the analysis of the failure model and mechanism of Maojiawan Bridge, it suggests that debris flow drainage channel; clearance under the bridge and special protective projects should be carried out for highway protection and traffic security.