Experiment on bearing capacity of bridge pile foundations in karst areas and reasonable rock-socketed depth
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摘要: 为研究岩溶区桥梁桩基的承载特性, 依托平顶山市西斜立交桥实体工程, 进行了桩基静载试验, 通过在桩端和桩顶布设应变传感器和位移计, 测得了桩身内力, 分析了岩溶区桥梁桩顶荷载(Q)-沉降(s)规律; 考虑现有桩基设计的局限性, 结合静载试验结果, 采用不同函数模型预测了单桩竖向极限承载力; 基于岩-桩体系宽梁力学模型和溶洞顶板拉-弯破坏模式, 探讨了桩基嵌岩深度的计算方法, 提出了一种适于岩溶区桥梁桩基嵌岩深度的优化方法。研究结果表明: 各级荷载作用下桩基Q-s曲线呈缓变型发展, 当桩顶荷载较小时, 曲线基本呈线性, 当桩顶荷载大于6 000 kN时, 曲线逐渐变为非线性, 虽然桩已嵌入灰岩较深, 但仍表现为典型的摩擦桩承载性状, 当加载到8 400 kN时, 桩顶沉降为3.69 mm, 远小于0.03D (D为桩径) 或40mm的破坏标准, 桩端阻力为122.9 kN, 仅占桩顶荷载的1.6%, 桩的承载力尚有富余; 在静载试验全过程中, 桩的受力状态处于Kulhawy理论的第1阶段, 桩侧阻力和桩端阻力同步发挥; 双曲线模型拟合精度在0.99以上且预测值偏安全, 建议在同类工程中优先考虑采用; 在同时满足溶洞顶板安全厚度和桩基承载力与稳定性要求的前提下, 采用提出的计算方法可使桩的嵌岩深度减小2.4 m。Abstract: To study the bearing characteristics of bridge pile foundations in karst areas, with relevance to Xixie Overpass Project in Pingdingshan City, the pile foundatiion static load test was carried out.By placing strain sensors and displacement meters at the pile top and bottom, the internal forces of pile were measured, and pile top load (Q)-settlement (s) law of bridge in karst area was analyzed.Considering the limitations of current pile foundation design and combiningthe static load test results, the vertical ultimate bearing capacity of a single pile was predicted using different function models.Based on the wide beam mechanical model of rock-pile system and tensile-bending failure mode of karst cave roof, the calculation methods for the rock-socket depth of pile foundation were discussed, and an optimized calculation method for the rocksocketed depth of bridge pile foundations in karst areas was proposed.Research result shows that under different load levels, the Q-s curve exhibits slowly-varying developments.When the pile top load is small, the Q-s curve is basically linear.When the pile top load is greater than 6 000 kN, the Q-s curve becomes nonlinear.Although the pile is embedded deeply into limestone, it still shows typical friction pile characteristics.When loaded to 8 400 kN, the pile top settlement is3.69 mm, far less than the failure criterion of 0.03D (D is the pile diameter) or 40 mm.The pile tip resistance is 122.9 kN, accounting for only 1.6% of the pile top load.Bearing capacity of pile is still sufficient.Over the entire static load testing process, the stress state of pile is in the first stage of Kulhawy theory.The pile tip and side resistances develop synchronously.The fitting accuracy of hyperbola model is above 0.99 and the predicted value is determined to be safe.Thus, the model is recommended for use in similar projects.The proposed calculation method can reduce the rock-socketed depth of pile by 2.4 m, while meeting the requirements for safe thickness of karst cave roof, as well as bearing capacity and stability of pile foundation.
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图 1 堆重平台与反力装置
1为桩基; 2为外基准梁; 3为内基准梁; 4为磁性表座; 5为位移计; 6为千斤顶; 7为球座; 8为托梁; 9为支墩; 10为横梁平台; 11为压载
Figure 1. Pile test platform and reaction device
图 2 位移计和应变传感器布置(单位: cm)
Figure 2. Layouts of displacement meters and strain sensors (unit: cm)
图 7 $\frac{Q_{\mathrm{s}}}{Q}$与Q的关系
Figure 7. Relationship between $\frac{Q_{\mathrm{s}}}{Q}$andQ
图 10 桩基静载试验Q-s、Qs-s和Qp-s曲线
Figure 10. Q-s, Qs-s and Qp-s curves of pile foundation static load test
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