复杂地层大直径深嵌岩桩荷载传递及抗力研究

文章以重载作用下某公铁两用长江大桥大直径深嵌岩桩为研究对象, 针对目前对嵌岩桩桩侧摩阻力的研究, 传统的做法认为在构造上桩-岩接触界面是一个平面, 提出了嵌入深厚岩石地层的桩基础在桩-岩界面会形成几何形态差异较大的粗糙面, 需要考虑界面粗糙度(joint roughness coefficient, JRC)及混凝土初凝期与围岩侧壁作用压力 $ P_{hi} $对灌注桩承载力的影响, 通过室内混凝土-岩样界面直剪试验获取考虑三维JRC与 $ P_{hi} $的影响下的桩-岩接触界面参数, 建立嵌岩桩竖向荷载下数值计算模型, 分析其桩-岩接触界面侧向抗力、重载作用下桩基的传力机理及极限承载力。结果表明: 大直径深嵌岩桩在承受上部重载作用时, 嵌岩段桩身侧向抗力沿桩深位置是变化的, 不同于桩基规范认为同一地层桩侧摩阻力是不变的假定; 在桩-岩界面成型中, 混凝土浆液自重应力随成桩深度增加而增加, 可大大提升桩-岩界面特性深度效应, 进而提升桩基容许承载力; 提出考虑桩基挤压效应和桩-岩界面特性深度效应的侧阻发挥系数修正公式。该研究成果对重载作用下复杂地层深嵌岩桩基础的设计具有一定参考意义。

This study focuses on large diameter deep embedded rock piles of a rail-cum-road Yangtze River bridge under overload conditions. Traditional research approaches regarding the lateral friction resistance of rock-embedded piles assume that the pile-rock interface is a plane. However, it is proposed that in deep embedded rock foundations in thick rock strata, the pile-rock interface actually forms a rough surface with significant geometric variations. Therefore, it is necessary to consider the joint roughness coefficient (JRC) and the pressure exerted by the initial setting of the concrete against the surrounding rock (referred to as $ P_{hi} $) to evaluate the bearing capacity of the cast-in-place pile. By conducting indoor direct shear tests on concrete-rock interface samples, parameters for the pile-rock interface considering JRC $ ^{3D} $ and $ P_{hi} $ are obtained, and a numerical calculation model for the vertical load of rock-embedded piles is developed to analyze the lateral resistance at the pile-rock interface, as well as the load transfer mechanism and ultimate bearing capacity of the pile foundation under overload conditions. The following conclusions are drawn: when subjected to upper load conditions, the lateral resistance of the rock-embedded segment of large diameter deep embedded rock piles varies with depth, which differs from the assumption in pile design codes that the lateral friction resistance remains constant across the same stratum; during the formation of the pile-rock interface, the self-weight stress of the concrete slurry increases with the depth of the pile, significantly enhancing the depth effect of the interface. characteristics and, consequently, improving the allowable bearing capacity of the pile foundation. A modified formula for the mobilization coefficient of the lateral resistance, taking into account the squeezing effect of the pile foundation and the depth effect of the interface characteristics, is proposed. The research results may provide reference for the design of the large diameter deep embedded rock piles in complex strata under overload conditions.