SPECIAL DEFORMATION AND FAILURE MODE OF RIVER BANK SLOPE —— STUDIES ON MECHANISM OF DEFORMATION AND FAILURE OF BANK ROCK SLOPE AT SETI BRIDGE SITE，NEPAL
Abstract：Seti bridge，located in Pokhara city，Nepal，is one of the key bridges in main national highway network，connecting Kathmandu with Plkhara，and plays an important role in Pokhara city traffic. Its construction started in 1995 and was completed in September 1998. During floods in June 1995，three cracks appeared at left bank. Till now，the largest crack width is about 4 m，threatening the safety of the bridge. In March 2001，a group of seven Chinese scholars investigated the deformation and failure of bank rock slope. Investigation results show that the main deformation mechanisms of the Seti bridge river bank rocks are the combination actions of its peculiar conditions：special meteorological-hydrographic condition (intensive rainfall)，valley topographical features (deep-cutting and narrow valley) and special geological structures (banks consisting of bonded gravel with “easy-scouring” rock stratum underneath，as proved by new investigation results，2002)，and the erosion of the bottom rock strata to form a reentrant by strong water flow. The deformation and failure mechanisms of bank rock slope at Seti bridge site are very special，named as a fracturing-toppling-falling pattern. Under intense scouring actions，easy-scouring strata at the bottom of valley are hollowed out by rushing water. Tensile cracks will then develop in the relatively rigid rock mass overlaid，and induces toppling failure like cantilever due to gravity. The rock mass among the tensile cracks starts to fall. This pattern of deformation and failure can explain reasonably failure phenomena that appeared in deformation process of bank slope. At present，the inside crack at Kathmandu bank is 17 meters away from Seti Bridge，and the tensile cracks have the possibility to develope，and the crack are threatening the safety of Seti Bridge. Therefore，it is necessary to reinforce both of bank slopes with appropriate measures at bridge site.
