Abstract:

To explore the bearing mechanism and failure mode of the rock stratum at the end of pile row crossing over underlying karst caves, laboratory model tests of pile row crossing an underlying karst cave were carried out. The ultimate bearing capacity and failure modes of the rock layer at the pile end were obtained for various pile row configurations. Based on the test phenomena and existing research, the failure modes of pile row overlying an underlying karst cave were proposed. A simplified method for calculating the ultimate bearing capacity of the rock layer beneath a single pile toe was developed using the limit equilibrium method. Furthermore, incorporating the test results, calculation methods were proposed for determining the ultimate bearing capacity of the rock layer beneath the toes of double-pile, triple-pile, and quadruple-pile rows overlying an underlying karst cave. The calculated results agreed well with the experimental values, validating the rationality of the proposed methods. Finally, the influences of roof thickness h and load eccentricity e on the bearing capacity reduction factor for pile row overlying an underlying karst cave were analyzed. Findings reveals that:1) Pile row foundations superimposed on underlying karst caves demonstrate four distinct failure modes, contingent upon the roof thickness, load eccentricity distance, and pile spacing. These modes encompass punching failure of the cave roof, composite punching failure involving the cave roof, eccentric load-induced punching failure, and general shear failure of the rock foundation. 2) When the eccentricity distance is small (1d, 2d, where d denotes diameter) and the roof thickness is thin, the bearing capacity reduction factor increases approximately linearly with roof thickness. Upon reaching a specific thickness, the variation trend becomes essentially consistent with that observed when the eccentricity distance is e≥3d. 3) The ultimate bearing capacity of triple-pile and quadruple-pile row overlying an underlying karst cave approximates that of intact bedrock when the roof thickness h= 4.3d. When h＞ 4.3d, the ultimate bearing capacity of triple-pile and quadruple-pile rows becomes independent of the load position, and the rock mass beneath the pile toes consistently undergoes overall shear failure.

Key words: underlying karst cave, pile row, limit equilibrium, ultimate bearing capacity, experimental study, failure mode