软土地区预制桩单桩最终极限承载力估算方法

An Approach for Estimating the Final Ultimate-bearing Capacity of Driven Pile In Soft Clay

软土地区预制桩施工具有较强的挤土效应,通常表现为桩周一定范围内土体受到侧向挤压,土体孔隙比降低,抗剪强度增加。目前估算预制桩单桩极限承载力通常选用沉桩前土体参数,这和桩体最终受力状态相比有一定的差异,计算结果通常偏于保守。应用圆柱形小孔扩张理论分析了沉桩的挤土机理,并根据土体抗剪强度与土体密度的唯一性关系,建立了沉桩休止期后桩周土体抗剪强度增量的理论计算公式。应用该公式可估算出预制桩单桩最终极限承载力。经与工程实测结果相比,两者比较接近,说明这种计算方法有一定的工程应用价值。

The prefabricated pile, driven in soft clay, can squeeze the soil laterally in a certain scope around the pile, and the void ratio is reduced significantly; consequently, the shear bearing capacity is raised. But the soil parameter, which is used to estimate the capacity of pile, is reduced according to the soil state before the pile was driven. Then, a certain difference appears when the pile is working. The calculation result usually is partial to conservative. This paper analyzes the squeezing mechanism of the pile using the theory of cylindrical cavities expansion. And a formula, which is used to estimate the increment of shear bearing capacity of soil around the pile when excess pore water pressure from pile driving distributes completely, is presented based on the unique relation between the shear bearing capacity and the density of the soil. This formula can be used to estimate the final ultimate- bearing capacity of the pile. The calculated results by the proposed formula are in good agreement with those of the site measurement.