【作者】 李海芳；

【导师】 龚晓南；

【作者基本信息】 浙江大学 ， 岩土工程， 2004， 博士

【摘要】 复合地基在工程中的应用不断得到发展，其沉降计算方法还不成熟。新近的研究成果大多是刚性基础下复合地基的试验和计算结果，只有少量的路堤和柔性荷载下复合地基的研究结果。高等级公路在软土地基的兴建，出现了桥面和桥头段沉降差引起的“桥头跳车”现象。加固软土路基的复合地基不同于刚性基础下设置垫层的复合地基，路堤为桩体的向上刺入提供了足够的空间，路堤荷载接近于柔性荷载，桩土应力比也将不同。目前还没有在柔性荷载下进行现场试验的方法，加强现场测试和计算分析是探讨路堤下复合地基工作性状的一种途径。 本文在假设的位移模式下，考虑桩土相互作用，通过力学推导，得到了路堤荷载下复合地基桩侧摩阻力、加固区桩间土压缩量和桩土应力比的解析表达式。根据桩侧摩阻力的解析解，给出了加固区压缩量的简化算法； 采用典型段法对复合地基沉降影响因素进行了三维有限元分析，避免了平面应变分析方法中对复合地基中桩体的不合理换算，以使计算模型在力学概念上更合理。在有限元分析的基础上，给出了基于天然地基沉降计算的复合地基沉降计算方法及相应图表。在进行复合地基沉降计算时，按通常方法计算路堤荷载下天然地基相应加固区和下卧层的压缩量。然后，确定7个修正系数，按推荐的方法计算复合地基相应区域的压缩量及总沉降。 对低强度桩复合地基处理桥头段软土路基进行了现场试验研究，对现场观测结果进行了分析，包括对载荷板及路堤下桩顶和桩间土的土压力及沉降、分层沉降、侧向位移和孔隙水压力观测结果的分析。现场原型观测表明，桩间土的沉降量大于桩顶的沉降量，填土中出现拱效应，填土荷载逐渐从桩问土向桩顶转移，桩土应力比逐渐增加。填土荷载下的桩土应力比的变化过程不同于刚性荷载板下的变化过程，其数值也远小于刚性荷载板下的桩土应力比。 由于采用了低强度混凝土桩复合地基，路基的沉降大大减少。现场测量结果表明，该复合地基的沉降量约为天然地基沉降量的1／3。更重要的是，采用了变桩长的方式协调桥台和路面不均匀沉降取得成功，路堤顶面的沉降基本上是均匀的。分层沉降观测结果和地表测量结果是一致的。分层沉降观测结果还表明，复合地基的主要压缩变形发生在加固区底部和下卧层顶部的一定范围内。更多还原

【Abstract】 Composite foundation is widely used in construction projects. However, its settlement computation method needs improving. Recent researches are mainly aim at the composite foundation under rigid base. Only a few of the research results published are about the composite foundation under embankment or flexible load. When a highway is built on soft clay, vehicles will bounce when they run through a bridge approach because of large settlement difference between a bridge and embankment. The composite foundation under an embankment is not same as that under rigid base in that an embankment provides enough space for pile stabbing into and embankment weight is a kind of flexible load so that their pile-soil stress ratios would be different. As no flexible field load test has been developed, prototype observation and numerical analysis are reasonable way to study composite foundation under embankment. With a suggested displacement model, this dissertation takes the interaction of soil and piles into consideration and derives an analytical-function of compressive deformation of stabilized layer of composite foundation under embankment. Analytical-functions are also derived of frictional resistance between pile and soil and of the pile-soil stress ratio. One simple method for computing the deformation is also derived, based on the analytical-function of friction resistance.Since piles are set into a composite foundation under an embankment it can not be directly analyzed in the plane strain way. To compute its settlement, a representative segment method is developed and used in settlement analysis of composite foundation under embankment. Compared with the traditional plane strain method that needs to unreasonably suppose piles as walls, the representative segment method is more reasonable. This dissertation, by FEM and ABAQUS program, discusses the normalized compressive deformation of stabilized layer and sub-layer of a composite foundation under an embankment and produces fit formulas.Based on the numerical analysis, a settlement formula is suggested and pertinent adjustment coefficients are produced. Composite foundation settlement can be computed from the pertinent natural foundation deformation and the adjustment coefficients. A composite foundation under embankment is given to show how to use the representative segment method in computing its settlement.Jialichen Bridge uses a composite foundation of low strength piles with different length along embankment to reduce settlement difference between the bridge and embankment. This dissertation is an analysis of the field observation results, including pile-soil stress ratio, settlement of the piles and soil, settlement in different depth, lateral displacement and pore pressure. The prototype observation shows that because settlement of soil is larger than that of piles, "arch effect" appears in the embankment so that its weight is gradually transferred from the soil to piles and pile-soil stress ratio increases.Thanks to the composite foundation, two-thirds of natural foundation settlement is removed. It is more significant that embankment settlement is basically even because of the piles with different length along the embankment. The observation of settlement in different depth shows that larger deformation is produced in the layers nearby the stabilized layer bottom.更多还原