FURTHER EXAMINATION ON CHARACTERISTICS AND FORMATIOM MECHANISM OF DAGUANGBAO LANDSLIDE

Daguangbao landslide in Gaochuan town Anxian county is located on the upper plate of the CausativeFault of wenchuan earthquake. Its slide distance is 4.5 km, and its maximum width of accumulation is 2. 2 km. Thearea is 7.8 km2 , and estimated volume is 750 million m3. It is the largest landslide among individual landslides trig-gered by Wenchuan earthquake. Comparing with the other 178 large landslides in earthquake disaster area, Da-guangbao landslide has the same characteristics as falls and slide disaster triggered by strong earthquake. They in-clude shake debacle, unstability of debacle and slide mechanism, super-dynamic characteristics, large-scale andhigh-speed projectile, and long-distance movement. But it has itself standout trait, namely, a long-large sliding sur-face more than 1 km long and 100 ～250 m wide,which is in a class by itself. On the basis of the study last year,the author has further progressed the site investigation many times, and has finished mapping and sampling analysis.The preliminary view is that Daguangbao landslide occurring process is one-time completion. Composition of sliding zone is very complex. It is mainly composed of stronger weathering argillaceous limestone of Sinian, and its part stratum contains Devonian shawozi sub-group ( Ds) and its associated phosphate ore. Its formation mechanism can be divided into the following 3 stages. (1) Stage of slope shattering cracking. Rear fracturing boundary and upstream fracturing boundary were shaped under strong earthquake, and they composed huge wedge " V" with downstream rock beddings. (2) Stage of friction sharply decrease and slope shattering-sliding. Sliding bed at downstream bound-ary (the main controlling sliding surface) began shattering-cracking, relaxation, cutting expand-bulk enlarge. Stroll-ing scrape effect made the friction decrease sharply, and made the huge wedge slide with high speed along bedding strike, like "pulling drawer". (3) Stage of shattering-sliding—shaking-accumulating. Landslide body rushed into Hongdongzi gully and was barred strongly by faced mountain, so it thrust up more than 500 m, and then its surficial loose accumulation with large inertia began fastly backing and shaking, accumulating, shaking level. Debris flows whose inertia force did not decrease imported landslide and enlarged ejection body. At last,it stopped at Dapianqiao bridge after flowing 1 km towards downstream of Hongdongzi gully.