不同条件下桩侧阻力端阻力性状及侧阻力分布概化与应用

根据24组51根桩竖向静载试验的侧阻力端阻力测试结果分析表明,桩侧土层性质与分布、桩长径比、后注浆效应是影响侧阻性状与分布的主要因素。软土中的桩其侧阻发挥正常,分布模式不受长径比影响;碎石土、砂土侧阻在桩顶以下约5d深度范围呈现应变软化,随深度增加逐渐演变为应变硬化导致桩身下部侧阻发挥滞后或发挥值显著降低,侧阻分布模式异化;土愈硬、长径比愈大,侧阻分布模式异化愈明显;后注浆对侧阻的增强与分布模式的影响,碎石土、砂土远甚于其它类土。工作荷载下的侧阻分布模式可概化为正梯形、倒梯形、橄榄形、灯笼形、蒜头形、峰谷形六种模式。端阻比随侧阻增强,随长径比增大而降低,随荷载水平提高呈非线性增长,给出了工作荷载下端阻比经验参考值。将每种侧阻概化模式分解为桩长l、kl的矩形、三角形分布侧阻单元,据此可查表确定供沉降计算的Mindlin解附加应力系数。计算结果表明,除正梯形和蒜头形分布外,其附加应力积分值按Geddes正梯形分布假定计算附加应力比实测侧阻概化分布大15%~74%,侧阻分布重心愈高差异愈大。

Behaviors of shaft resistance and tip resistance of piles under different conditions and conceptualization and application of distribution of shaft resistance

According to the analysis of the results of the vertical static load tests on shaft resistance and tip resistance of 51 piles in 24 groups, the soil properties and vertical distribution, ratio of length to diameter of piles and post-grouting effect are the main factors affecting the characteristics and distribution of the shaft resistance. In soft soils, the shaft resistance of piles plays a normal function, and its distribution pattern is not affected by the ratio of length to diameter. In gravel and sand, the shaft resistance below ground about 5d depth exhibits strain softening and gradually evolves into strain hardening, which leads to the result that it lags or decreases significantly with the increasing depth. The harder the soils, or the larger the ratio of length to diameter, the more obvious the dissimilation of distribution pattern of the shaft resistance. The effect of post-grouting reinforcement on the shaft resistance of piles in the gravel and sand is far higher than that in other soils. The distribution modes of shaft resistance under working loads can be conceptualized into six types: trapezoid shape, inverted trapezoid shape, olive shape, lantern shape, garlic shape, peak and valley shape. The tip resistance ratio decreases with the increasing shaft resistance and the increasing ratio of length to diameter, and it increases nonlinearly with the increasing head loads. The values of the tip resistance ratio under working loads are given. Each conceptual mode for the shaft resistance is decomposed into rectangular and triangular distribution units associated with the pile length l and kl. Then the additional stress coefficient of Mindlin solution for settlement can be determined according to the above results. The calculated results show that except regular trapezoid shape and garlic shape distributions of shaft resistance, the integral value of the additional stress using the Geddes mode is about 15%~74% larger than that of the conceptualization shape. The higher the center of gravity of the shaft resistance, the greater the differences between the Geddes mode and the measured.