Abstract:

To explore the bearing mechanism and failure mode for rock layer of pile tip of bridge pile groups with underground karst cave, laboratory model tests of single pile and pile groups with different numbers of piles were carried out. The ultimate bearing capacity and failure modes for rock layer of pile tip of pile groups with different numbers of piles were captured. The failure surface was divided into two parts according to the characteristics of the failure mode for the rock layer with underground karst cave, and a calculation method for ultimate bearing capacity was developed by combining with limit analysis method. The theoretical calculation values match well with the experimental values, which verifies the rationality of the proposed method. Meanwhile, the relationship between the ultimate bearing capacity for the rock layer has been analyzed and it can provide reference for bridge pile foundation construction in karst area. The experimental and theoretical calculation results indicate that: (1) When the rock layer of pile tip of pile groups with underground karst cave is overall destroyed, the whole destroyed body can be regarded as a large pier foundation  similar to destroyed body of single pile. (2) When the pile spacing is smaller, the ultimate bearing capacity for the rock layer increases with the increase of the length of the outer envelope line of outer foundation pile. Furthermore, when the length of outer envelope lines are the same, the arrangement of internal foundation piles has no effect on the ultimate bearing capacity for the rock layer. (3) The coefficient of pile group effect increases with increasing the pile spacing, and the critical pile spacing is 5d−6d (d is the pile diameter).

Key words: bridge pile group, ground with underground karst cave, ultimate bearing capacity, limit analysis, pile group effect coefficient