FORMATION MECHANISM OF WENJIAGOU HIGH-SPEED AND LONG-RUNOUT DEBRIS AVALANCHE TRIGGERED BY WENCHUAN EARTHQUAKE

The seismic geological context,morphology,formation mechanism and evolution of Wenjiagou high-speed and long-runout debris avalanche are introduced. The debris avalanche was located at Wenjiaogou syncline block between Yingxiu-Beichuan fault and Guanxian-Anxian fault of which severe activity provides precondition for the debris avalanche. It is 1360m high from the peak of debris avalanche source area to Wenjiagou valley mouth; seismic load shows remarkable amplification effect at the ridge due to the terrain of towering hill with deep valley at the foot,and directly results in the landslide. The peak values of ground motion acceleration in 3 directions in the ridge are aEW=2.4g,aNS=2.3g,aUP=1.2g(g: the acceleration of gravity). The Guanwushan formation(D2gn)limestone slope has layered structure with progressive strength. About 50m below the slope surface is loose structural Qel+dl~upper part of fresh rock,which is cut into initial landslide mass results from the tension-compression and shear coupling effect due to seismic P-wave and S-wave. The sliding mass shear out from edge of the 8th platform; the maximum traveling velocity of sliding mass is accelerated up to 93~122m s-1 in Wenjiagou valley upstream. Trapped air inside of the valley is compressed by upper part of landslide mass,the dry debris flow,at 2 turnings of Wenjiagou valley; distinct air cushion effect is generated. Besides,the bottom layer of debris flow is liquefied; effective dynamic friction coefficient is decreased with increasing of shearing velocity; all of these effects lead to the high-speed and long-runout traveling of debris flow. Simultaneously,the valley slope is scraped and crossed; the trees are cut as debris flow passes. After Wenchuan earthquake,up to Sept. 2009,rainfall has triggered several debris flows,which indicates the continuity and long-term nature of seismic geohazard.