天然地基、墩基础和桩基础承载力分析

Bearing Capacity Analysis of Natural Foundation,Pier Foundation and Pile Foundation

工程实践中经常遇到场地地表附近存在较好土层的情况,采用桩基方案埋入深度需大于6m,桩长较长;如采用墩基方案,墩基承载力按照天然地基承载力选取,需较大直径墩头;由此出现了墩基承载力按桩基还是天然地基的争议。本文对天然地基、墩基和桩基进行理论、规范规定、工程实例和数值模拟研究,结果表明:长度小于5m的短桩、墩基可采用太沙基、简布公式对其极限端阻力进行估算,并采用静载实验进行验证;采用超短桩或墩基是可行的,其承载力按天然地基进行设计过于保守;桩/墩长径比在0~10之间时,承载力呈线性变化,无突变;太沙基、简布理论与有限元数值模拟极限端阻力差值较小,长径比小于5的墩基侧阻可偏于安全的不予考虑。

In engineering practice,it is often encountered that there is a good soil layer near the surface of the site,so the pile foundation scheme should be adopted,and the length more than 6 m is required;If the pier foundation scheme is adopted,the bearing capacity of pier foundation is selected according to the bearing capacity of natural foundation,and the pier head with larger diameter is required;Therefore,there is a dispute about whether the bearing capacity of pier foundation is based on pile foundation or natural foundation.In this paper,the natural foundation,pier foundation and pile foundation are studied by theory,specification,engineering examples and numerical simulation.The results show that the ultimate end resistance of short pile and pier foundation with length less than 5 m can be estimated by Terzaghi and Janbu formula,and verified by static load test;It is feasible to adopt ultrashort pile or pier foundation,and its bearing capacity is designed according to natural foundation,which is too conservative;When the length diameter ratio of pile/pier is between 0 and 10,the bearing capacity changes linearly without mutation;The difference of ultimate end resistance between Terzaghi’s theory,Janbu’s theory and finite element numerical simulation is small,and the lateral resistance of pier foundation with length diameter ratio less than 5 can be ignored.