Analytical solution for vertical load transfer of cast-in-place piles considering shear-induced volumetric contraction across shaft-rock joints
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摘要: 当钻孔灌注桩深入软岩地层时，其竖向荷载传递行为与桩岩界面的粗糙度具有高度相关性，具体表现为孔壁与桩身的接触面在荷载作用下错动而剪胀并引起侧向约束（法向应力）的增加。现有基于Patton模型及其广义模型的桩岩界面建模方法可较好地模拟峰前剪胀过程中法向应力的增长过程；但其忽略了一个重要事实：当剪胀达到一定程度时，孔壁粗糙体将由于局部应力增加而破坏，此时由于剪胀过程所积蓄的系统内能将随粗糙体的断裂或破坏被迅速释放，在宏观上表现为桩岩界面的体积剪缩并引起法向应力的减小。为此，综合已有室内试验的观测结果并基于单边受压楔体的上限解，确定出三角形粗糙体破坏时将分离出一个新的三角形岩屑并沿着本文定义的剪缩角斜向滑动。在考虑粗糙体几何特征和运动学原理的基础上，利用能量准则求解了剪缩角以及峰后剪缩过程中的滑动摩擦力。随后，修正了已有的界面剪切模型并利用室内直剪试验的结果验证了桩岩界面剪切全过程的应力-位移预测曲线，并据此得到基桩竖向荷载传递解析解。参数分析表明半波长、剪胀角和岩石内摩擦角是影响剪缩角和单位侧阻力发挥的主要因素。Abstract: In soft rock strata, the vertical load transfer behavior of cast-in-place piles is significantly influenced by the roughness of shaft-rock joints. It is particularly pronounced at the interface between the shaft and the surrounding rock, where dislocations occur under loads, leading to shear dilation and an increase in the lateral constraint (normal stress). The existing models, such as the Patton's model and its generalized form, can well predict the normal stress at the pre-peak, but they ignore a critical aspect: in specific, the potential destruction of the asperity when shear dilation reaches the critical state due to increasing local stress, leading to the rapid release of accumulated energy. This destruction is macroscopically represented as the volume shear contraction of the shaft-rock joints, causing a decrease in the normal stress. This study identifies that a newborn debris will be separated from the original rock asperity and obliquely slides after the asperity fails based on the upper-bound solution of a unilaterally compressed wedge and the existing laboratory observations. Considering the kinematic principles, the energy principle is used to determine the shear contraction angle and the sliding resistance at the post-peak. The modified shear model is verified using the observations of the existing direct shear tests. On this basis, the analytical solutions for the distribution of axial force are obtained. The parameter studies reveal that the half chord-length, shear dilation inclination and internal friction angle of rock have a strong impact on the shear contraction angle and unit shaft resistance.
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图 1 灌注桩桩岩界面粗糙体破坏前后体积变化示意图
Figure 1. Sketch of volumetric change of asperity of shaft-rock joints at pre-failure and post-failure stages
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图 2 混凝土-岩石粗糙体峰前剪胀和峰后剪缩阶段运动学机制
Figure 2. Kinematics sketch of concrete-rock asperity at pre-peak shear-dilation and post-peak shear-contraction stages
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图 3 剥离楔体与软岩粗糙体基座几何示意图
Figure 3. Geometric sketch of potential newborn scrap on remaining asperity
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图 4 剪应力模型预测结果与实测结果对比
Figure 4. Comparison of shear stress among proposed, experimental and existing solutions
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图 6 桩身轴力分布试验值与理论值对比
Figure 6. Comparison of axial load distributions between proposed solutions and experimental observations
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图 7 泥质页岩地层桩身轴力分布试验值与理论值对比
Figure 7. Comparison of axial load distributions between proposed solutions and experimental observations of clay shale
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图 8 界面粗糙体几何参数与抗剪强度指标对剪缩角的影响
Figure 8. Effects of roughness parameters and shear strength parameters of asperity on shear contraction angle
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图 9 粗糙体半波长和剪胀角对单位侧阻力发挥的影响
Figure 9. Effects of half chord-length and shear dilation inclination of asperity on mobilization of unit shaft resistances
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图 10 软岩内摩擦角和黏聚力对单位侧阻力发挥的影响
Figure 10. Effects of internal friction angle and cohesive of soft rock on mobilization of unit shaft resistances
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