FURTHER EXAMINATION ON CHARACTERISTICS AND FORMATIOM MECHANISM OF DAGUANGBAO LANDSLIDE

Abstract Daguangbao landslide in Anxian county Gaochuan town is located on the upper plate of the Causative Fault of wenchuan earthquake. Its slide distance is 4.5 km, and its maximum width of accumulation is 2.2 km. The area is 7.8 km2, and estimated volume is 750 million m3. It is the largest landslide among individual landslides triggered by Wenchuan earthquake. Comparing with the other 178 large landslides in earthquake disaster area, Daguangbao landslide has the same characteristics as falls and slide disaster triggered by strong earthquake. They include shake debacle, unstability of debacle and slide mechanism, super-dynamic characteristics, large-scale and high-speed projectile, and long-distance movement．But it has itself standout trait, namely, a long-large sliding surface 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 Vwith downstream rock beddings. (2) Stage of friction decrease and slope shattering-sliding. Sliding bed at downstream boundary (the main controlling sliding surface) began shattering-cracking, relaxation, cutting expand-bulk enlarge. Strolling 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-slidingshaking- 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. Key words Daguangbao landslide, character of landslide, model of geomechanics, formation mechanism, shattering expand and surper press