层状岩层中下伏溶洞基桩承载特性模型试验

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

    • 摘要: 为研究层状岩层与下伏溶洞耦合作用下基桩的竖向承载特性,开展了溶洞顶板厚度为3倍桩径,岩层倾角分别为0°、30°、45°和60°工况下基桩模型载荷试验,并基于试验结果提出了桩顶极限荷载的理论计算方法。结果表明:(1)下伏溶洞基桩的破坏包含顶板的层间滑移、岩板拉裂和最终的冲切破坏,破坏体呈圆台体,破裂面为喇叭形;随着岩层倾角的增大,破坏体的宽度x随着倾角的增大而减小,而其长度y随着倾角的增大变化很小。(2)试桩荷载-位移曲线整体呈陡变型,分为近线性变化阶段、荷载波动阶段、荷载陡降阶段和残余荷载阶段;曲线荷载波动现象的出现主要原因是在桩端荷载作用下,岩层出现滑移或岩板拉裂。(3)岩层倾角从0°逐步增大至60°,试桩的弯拉屈服荷载逐步非线性增大;岩层倾角从0°增大至45°和60°,试桩的冲切破坏荷载逐步非线性降低。(4)根据试验确定的桩周岩层冲切破坏模式,建立了基于冲切体能量守恒的桩顶破坏荷载算法;采用集中力作用下弹性板模型,提出了基于溶洞顶板弯拉屈服准则的桩顶荷载算法;冲切模型、弯拉模型的计算值与试验值误差分别为-5.96%~8.52%和-18.56%~23.95%,桩顶极限荷载应取冲切与弯拉结果中的小值。研究结果可为岩溶地区基桩设计提供科学指导与参考。

       

      Abstract: 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.

       

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