Abstract: Since the Jiweishan landslide in Chongqing in the front edge of the “key block” is of certain concealment, it is of significance for the early warning of landslide to study the failure property the rock bridge in the front locking section of “key block”. Based on the geological condition of Jiweishan, after prefabricating different length edge cracks, infilling soft materials and forming the joints of soft interlayer at the ends of the sample, direct shear tests were conducted on the prepared samples under different confining pressures. The propagation rules of crack, the characteristic information of the block spalling and the fracture condition of the rock bridge were analysed and the k value of changed rate of shear stress was proposed in a would-be sliding stage. The length of joint of the soft interlayer has a certain effect on the failure modes and the extent of block spalling, and the longer the end of rock bridge is, the smaller the k value of the growth rate of the shear stress is. The failure modes of the rock bridge at the end include shear failure, tensile-shear failure and tensile failure, however the extent of block spalling of the same joint is different under varied coalescent modes. The ratio of shear stress of spalling point to that of failure point is in the range of 79.5%-92.2%. The short joint is slower than the middle joint, but the long joint is faster than the middle joint in the time of would-be sliding stage. The three types of failure modes of the end rock bridge meet certain fracture conditions and present three stages and the crack growth stably. The conducted direct shear tests reveal the failure and characteristic information of rock bridge at the end and the fracture condition, and thus this study can provide a theoretical basis for the instability and failure estimate of the rock slope containing the front edge of the locking section which is called “key block”.

Key words: front edge of the locking section, shear test, terminal of rock bridge, block spalling, fracture condition