Model test on bearing characteristics of piles in layered rock strata above karst caves

To investigate the vertical bearing characteristics of foundation piles under the coupling effect of layered rock strata and underlying cavities, model load tests were conducted on foundation piles under conditions where the thickness of the cave roof was three times the pile diameter and the dip angles of the rock strata were 0°, 30°, 45°, and 60°, respectively. Based on the test results, theoretical calculation method for the pile top ultimate load was proposed. The results indicate that:(1) The failure of foundation piles overlying cavities involves interlayer slippage of the roof, tensile cracking of the rock slab, and final punching failure. The failure body exhibits a truncated cone shape with a trumpet-shaped rupture surface. As the dip angle of the rock strata increases, the width x of the failure body decreases, while its length y remains largely unchanged. (2) The load-displacement curve of the test pile is generally steep and can be divided into four stages: a nearly linear phase, a load fluctuation phase, a load steep-drop phase, and a residual load phase. The load fluctuation phenomenon primarily results from slippage of the rock layers or tensile cracking of the rock slab under the action of pile-end loading. (3) As the dip angle of the rock strata increases from 0° to 60°, the bending-tensile yield load of the test pile increases nonlinearly. In contrast, the punching failure load decreases nonlinearly when the dip angle increases from 0° to 45° and 60°. (4) Based on the punching shear failure mode of the surrounding rock layer determined from tests, an algorithm for calculating the failure load at the pile top is established using the energy conservation principle of the punching body. By using an elastic plate model under concentrated force, a pile top load algorithm is proposed based on the bending-tension yield criterion of the karst cave roof. The discrepancies between the proposed values from the punching model and the bending-tension model and the experimental values range from -5.96% to 8.52% and from -18.56% to 23.95%, respectively. The ultimate pile top load should be taken as the smaller value between the results from the punching and bending-tension models. The research achievements can provide scientific guidance and reference for the design of foundation piles in karst areas.