THEORETICAL CALCULATION MODEL AND MODEL TEST ON THE FAILURE CHARACTERISTIC OF KARST ROOF UNDER ROCK-SOCKETED PILE

Based on the isolated similar design method, the model test research on the failure characteristic of karst roof under a rock-socketed pile was carried out with the change of roof thickness and cave diameter, and a formula was also presented. It shows that:1) The thickness of karst roof impacts the load transfer path of the rock-socketed pile, the thicker the roof, the wider the transfer range, and the larger the volume of a shear zone. When the thickness of the roof is no more than the diameter of the pile, punching failure will happen around the free face of the roof, and the stability of the roof around the free face play a vital role. The thinner the roof, the smaller the volume of the tapered block. When the thickness of roof is greater than the diameter of the pile, but littler than two times of the diameter, the upper shear failure happens firstly, and then induces the punching failure around the free face. When the thickness of the roof is greater than two times of the diameter of the pile, the punching failure around the free face happens firstly, and induces the upper shear failure latterly, which will happens within the projection of the pile in the condition that the diameter of the cave is less than the transverse width of the shear zone. The load-settlement curve is of a rapid-falling style with a thinner roof, while the load-settlement curve is of a slow-change style with a thicker roof. 2) The shear deformation of the roof is significant when the diameter of the cave is less than three times of the pile diameter (the thickness of the roof is greater than two times of the pile diameter), the punching failure happens after the upper shear failure of the roof, and the load-settlement curve is of a slow-change style. The greater the diameter of a cave, the larger the volume of the tapered punching block, and the load-settlement curve is of a rapid-falling style when the diameter of the cave is greater than three times of the pile. 3) It is of great risk to calculate the safe thickness of the karst roof under the pile tip using the punching failure formula. However, the punching-shear failure model is providential, and related to the strength of the karst roof, the integrity of rock, the diameter of the pile, rock-socketed depth, the construction method and technologies. This approach can be used to calculate the safe thickness of the cave roof quickly in site.