【作者】 卢萌盟；

【导师】 谢康和；

【作者基本信息】 浙江大学 ， 岩土工程， 2009， 博士

【摘要】 与砂井地基相比,复合地基具有桩体压缩模量较大、直径较大、受荷后应力向桩体集中以及桩体发生侧向膨胀等特点,因此,复合地基固结虽然类似于砂井地基固结但又与之不同。本文针对现有复合地基固结理论不能合理充分地考虑复合地基这些特点的不足,基于复合地基桩、土共同承担外部荷载的原则,提出了一个新的关于复合地基固结的初始条件,并对复合地基施工扰动引起的土体渗透系数变化、附加应力随深度和时间变化、桩体固结压缩、土体非线性以及桩、土发生侧向变形等复杂情况下的复合地基固结问题进行了较详细深入的研究,主要的工作和创新成果如下:1.给出了能同时考虑扰动区土体渗透系数变化、附加应力既随时间变化又随深度变化的复合地基固结普遍解,以及荷载单级施加、附加应力沿深度线性变化时超静孔压和平均固结度的具体表达式,并对地基的固结性状和桩-土应力比的发展规律进行了分析。2.鉴于传统桩周流量连续的假定与等应变条件矛盾,对现行桩周流量连续假定提出了修正,建立了新的能够考虑桩体压缩的复合地基固结控制方程,给出了瞬时加荷和附加应力沿深度均匀分布情况下复合地基超静孔压和平均固结度的表达式,将其与以往解答进行了对比,分析了复合地基的固结性状。结果表明:采用修正的桩周流量连续条件给出的地基平均固结度小于传统桩周连续假定给出的平均固结度,而且,随着井径比的减小,两者之间的差值变大。3.为考虑由于桩径较大而实际存在于桩体内的径向渗流,建立了能考虑桩体内的径、竖向组合渗流的复合地基固结控制方程,引入两个新的边界条件,并给出了相应解析解。对比分析表明:本章的解答可以合理地退化到五种简单情况下的地基固结解答;不考虑桩体内径向渗流将高估复合地基固结度速率,而且,井径比越小,高估值越大。4.通过引入广为认同的土体孔隙比和有效应力以及孔隙比和渗透系数的对数关系,考虑了土体固结过程中压缩模量非线性增长和渗透系数非线性减小的特征,给出了一种复合地基非线性固结解析解,并对复合地基的非线性固结性状进行了分析。结果表明:考虑非线性时按应力和按变形定义的两种固结度不相等,并且按变形定义的固结度一般大于按应力定义的固结度;当压缩指数小于渗透指数时,不考虑土体的非线性特征会低估地基的固结度,而当压缩指数大于渗透指数时,不考虑土体的非线性则会高估地基的固结度;对于按应力定义的固结度来说,当压缩指数小于渗透指数时,随着附加应力的增大,地基固结速度加快,而当压缩指数大于渗透指数时,附加应力增大,地基固结减慢;对于按变形定义的固结度而言,不论压缩指数大于还是小于渗透指数,复合地基固结速度总是随着附加应力增大而加快。5.在竖向变形相等的条件下,给出了能考虑桩体和土体在固结过程中发生侧向变形的复合地基固结解析解,并对该解和不考虑桩、土侧向变形的现有解答进行了比较。结果表明:考虑桩体的侧向变形会降低地基的固结速率。最后分析了桩体和土体的变形模量和泊松比对地基固结的影响以及桩-土应力比随时间的发展规律,结果表明:桩、土的泊松比越小,地基固结越快;桩、土的弹性模量之比越大,变形模量之比越大,则复合地基固结越快;考虑了桩、土的侧向变形后,复合地基的桩-土应力比虽然随固结逐渐增大,但与现有理论预测的不同,该应力比不会增长到桩-土模量比的值。本文工作使复合地基固结理论与实际更接近。更多还原

【Abstract】 Comparing with sand drain well, the columns in composite ground have a higher compression modulus and a larger diameter together with the characteristics of stress concentration from soil to column and the lateral expansion of column after loading applied. Therefore, although the consolidation theory for composite ground is similar to that for sand drain ground, it is also different from the latter. Aiming at the difficiency that the previous consolidation theory can not consider the abovementioned characteristic of composite ground in a comprehensive and rational way, a new initial condition more suitable for the consolidation of composite ground is derived based on the principle that the external load is always undertaken by the column and the surrounding soil together. Furthermore, detailed studies are made on the consolidation of composite ground under a series of complicated conditions, such as the variation of horizontal permeability coefficient of soil due to the column construction, the depth-and time-dependent characteristic of stress increment, the consolidation and compression of column, the non-linearity of soil and the later difromations of soil and column in the process of consolidation et al. The main original works and innovative achievements are as follows:1. A general analytical solution is obtained for the consolidation of composite ground considering simultaneously the variation of horizontal permeability coefficient of soil, the coupled variation of stress increment with depth and time together. Furthermore, detailed solutions are obtained for excess pore water pressure (EPWP) and average degree of consolidation (ADC) under the particular situation that the stress increment is linearly changed along the column depth and the erternal load is applied in a single-stage way. Finally, the consolidation behavior of composite ground and the development of the column-soil stress ratio are analyzed, respectively.2. In light of the contradiction between the tradional flow continuity assumption at the column-soil interface and the equal strain condition, the traditional flow continuity assumption is modified to eliminate this contradiction. New equations governing the consolidation of composite ground are proposed to account for the column consolidation and compression. Solutions are obtained for EPWP and ADC with an instantly applied and uniformly distributed stress increment along column depth. Then the present solution is compared to some previous available solutions and the consolidation behavior of composite ground is investigated. The results show that ADC predicted by the present solution is less than that predicted by the previous solution using the traditional flow continuity assumption. In addition, the difference in ADC predicted by these two types of solution increases with a reduction of radius ratio.3. To incorporate the radial seepage within the column due to the large diameter of column, new governing eqations for consolidation are proposed to consider the combined flows in radial and vertical directions within the column. The solutions of the new governing equation are obtained by introducing two new boundary conditions. The results by comparisons show that the present solution can be degenerated to five simple particular cases in a rational way. Ignoring the radial flow within the column will over-estimate the consolidation rate. Furthermore, the less the radius ratio is, the greater the over-estimated value is.4. By incorporating the well-known logarithm relationship in effective stress and void ratio as well as that in void ration and permeability coefficient, the characteristics of the nonlinear increase in the soil’s compressive modulus and the non-linear decrease in the soil’s permeability during consolidation are considered. Based on the non-linear characteristics of soil, an analytical solution for the consolidation of composite ground is developed. Then the non-linear consolidation behavior of composite ground is analyzed and the results show that ADC in terms of stress is not equal to that in terms of deformation and furthermore the latter is always greater than the former. When the soil’s compressive indices is less than the permeability indices, ignoring the soil’s non-linearity will under-estimate the consolidation rate; however, when the soil’s compressive indices is greater than the permeability indices, the reverse is true: ignoring the non-linearity will over-estimate the consolidation rate. For ADC based on stress, when the soil’s compressive indices is less than the permeability indices, the increase in the stress increment within the ground leads to an acceleration of the consolidation rate; however, when the compressive indices is larger than the permeability indices, the increase in the stress increment causes a reduction in the consolidation rate. For ADC in terms of deformation, whether the compressive indices is less or larger than the permeability indices, an increase in the stress increment within the ground always accelerates the consolidation rate.5. Under the condition of equal vertical deformation, an analytical solution is derived for the consolidation of composite ground by incorporating the lateral deformations of column and soil occurring in the process of consolidation. The present solution is compared to the solution ignoring the lateral deformations of column and soil. The results show that considering the lateral deformation will reduce the consolidation rate. Finally, analyses are performed on the influence of Poisson’s ratios and Young’s moduli of column and soil to the consolidation behavior of composite ground and the development of column-soil stress ratio is investigated. The results show that the less the Poisson’ ratios of column and soil are and the larger the Young’ modulus ratio of column to soil is, the greater the consolidation rate is. When the lateral deformations of column and soil are considered, the column-soil stress ratio will keep increasing during the earlier stage of consolidation, but unlike that predicted by traditional theory, it will not increase up to the value of compression modulus ratio of column to soil.The present work makes consolidation theory of composite ground closer to the practical engineering.更多还原