长短桩复合地基变形及承载特性研究

Research on the Deformation and Bearing Characteristics of Composite Foundation with Long and Short Piles

研究目的：路堤荷载下长短桩复合地基的工作机理十分复杂,有关设计、计算的研究成果较少,为更好地推广这种复合地基,须深入研究该复合地基的工作性状。本文通过现场试验,分析长短桩复合地基沉降特性,并采用数值模拟的方法探讨桩长变化对复合地基沉降和承载特性的影响。经对比分析,验证模型的正确性和参数的合理性。研究结论：（1）路堤荷载下长短桩复合地基沉降过程分为三个阶段：快速发展阶段的沉降占总沉降的59.2%～62.5%;缓慢发展阶段为静置期前45 d,沉降占总沉降的28.1%～33.3%;稳定阶段为静置期45 d之后的沉降,约占总沉降的7.5%～9.45%;（2）路基基底沉降沿横断面呈近似盆状分布,中心线处最大,坡脚处最小;长桩设计时应综合考虑短桩长度、下卧层刚度等因素,短桩设计时在满足沉降及承载力的前提下可依据工程造价选取最经济的桩长;（3）长短桩的桩身应力随着深度的增加先增大后减小,峰值出现在桩顶下一定深度,长桩桩身应力随短桩长度的增加而减小,且能全桩长发挥承载作用;（4）长桩桩土应力比和荷载分担比随长桩长度的增加先迅速增加然后趋于平稳,随短桩长度的增加逐渐减小;（5）该研究成果可为路堤荷载下长短桩复合地基的设计提供一定的理论依据。

Research purposes： Under embankment load, the working mechanism of long and short plies composite ground is very complicated and the research results of the design and calculation are fewer. In order to promote this composite ground, it is necessary to study the behaviors of this composite ground. The settlement characteristics of the long and short piles composite ground are studied with field tests, then the effects of pile length on the settlement and bearing characteristics of composite ground are studied using numerical simulation. The accuracy of the model and the rationality of the parameters are verified through comparing analysis. Research conclusions： （ 1 ） Under embankment load, the settlement process of long and short piles composite ground is divided into three stages： the rapid development stage in which the settlement is 59.2% to 62. 5% of the total settlement; the first 45 days in the rest period is the gradual development stage in which the settlement is 28.1% to 33.3% of the total settlement; the last stage is the stable stage of settlement in which the settlement is about 7.5% to 9.45 % of the total settlement. （2） The subgrade base settlement along the cross - section has a bowl - like distribution, the largest value is at roadbed centerline, the minimum value is at the foot of the slope; short piles length and substratum stiffness should be considered when the long piles are designed. Meeting the settlement and bearing capacity,the short piles are designed on the basis of the project cost. （3） At first the stresses of both long and short piles increase then decrease with the pile depth, the peak appears at a certain depth near the top of the pile. The stresses of long pile decrease with increasing of the short pile length, and the whole pile length play a bearing role. （4） At first the pile soil stress ratio and load sharing ratio of long piles increase rapidly, then tend to stable, and they decrease with the increase of short pile length. （5）The research results can provide theoretical basis for the design of composite ground with long and short piles under embankment load.