黏滞性软土的流变特性及非线性模型研究

发布时间：2018-04-19 23:22

本文选题：软黏土 + 流变性　；参考：《暨南大学》2015年博士论文

【摘要】：软黏土广泛分布在我国沿海、沿江等经济发达地区,其具有含水率高、孔隙比大、压缩性强、黏滞性等特点,这使得在软黏土地基上修筑结构物时,其沉降问题尤其是长期沉降问题就显得尤为突出。软黏土的变形过程与时间密切相关,这既包括超静孔隙水压力消散、土体体积压密的固结过程,还包括由于其黏滞特性所引起的蠕变变形,两者互相影响,共同决定着土体的长期沉降。因此,如何基于土体变形的时间效应,建立起可以反映软黏土蠕变特性的本构模型并实现蠕变-固结的耦合作用,是工程界和学术界共同探讨的热点问题。本研究使用室内试验、理论分析及数值模拟等方法对软黏土的固结、蠕变特性及控制方程的建立进行了综合研究,主要研究内容及成果如下:(1)开展了一维K0侧限固结试验、三轴固结不排水剪切试验、三轴固结不排水蠕变试验,分析了软黏土变形的时效性特征和变形机理,并为进一步弹黏塑性本构模型的建立提供了试验基础。试验成果表明:1)土体的次固结系数随着加载时间的增长及预压作用而减小;且次固结系数随荷载的增加而增大,但其增加幅度依赖于土体的固结状态,次固结系数随荷载变化曲线会在与pc有关的某一范围处出现转折点。次固结系数与压缩指数之间呈较好的线性关系,其比值在0.028~0.049之间。2)当上部荷载较小时,土体的三轴固结不排水蠕变变形随时间会逐渐减弱并最终趋于稳定;而当荷载达到一临界强度时,土体的蠕变变形会在加载一段时间后随时间快速发展,土体发生剪切蠕变破坏。3)在应变控制模式下,土体的不排水剪切强度会随着剪切应变速率的提高而增加,且其率相关性可以很好地用速率参数表征。(2)基于一维K0侧限固结试验的成果,在Bjerrum等时间线理论的基础上验证了将蠕变变形当作与时间相关塑性变形这一假设的合理性,并分别从蠕变效应及应变率效应两个角度推导了一维K0侧限条件下的非线性弹黏塑性模型,给出了模型参数的物理意义及获取方法。将所建立的非线性弹黏塑性模型引入一维Terzaghi固结理论,建立了一维K0侧限条件下的蠕变固结耦合控制方程,并通过Crank Nicolson有限差分格式对控制方程进行了求解。通过对一维K0侧限固结试验的模拟及与Imai和Leroueil的试验结果对比,验证了所建模型的合理性。计算结果表明,随着土样高度的增加、土体黏滞特性的增强,本文模型与假设A计算所得的主固结完成时的变形量之间的差值也会越来越大。(3)基于改进的GJY型K0固结仪开展了一维柔性等比侧限固结试验,分析了土体在允许一定侧向变形时的压缩特性,并将试验结果与K0侧限条件下的成果进行了对比分析。结果表明,柔性等比侧限条件下的压缩曲线、压缩指数及次固结系数的发展规律均与K0侧限条件下的一致,但由于侧向变形的产生,其压缩指数约为K0侧限条件下的1.08~1.16倍,次固结系数约为K0侧限条件下的1.1~1.3倍。在柔性等比侧限固结试验的基础上,根据达西定律、有效应力原理及连续性原理推导出柔性等比侧限条件下的一维固结方程,且与建立的非线性弹黏塑性模型进行耦合,得到柔性等比侧限条件下的蠕变固结耦合控制方程,并给出了模型参数的意义及确定方法。通过对柔性等比侧限压缩试验结果的模拟,验证了模型的合理性,且模拟结果表明,柔性等比侧限条件下土样厚度、次固结系数对主固结完成时应变值的影响均较K0侧限条件下的大,本文对单层地基在柔性等比侧限条件下计算所得的主固结完成时的应变值约为K0侧限条件下的1.17倍。(4)为拓宽蠕变固结共同作用的路径,本文还基于室内蠕变试验成果建立了含分数阶导数的Merchant经验模型,并对一维K0侧限固结试验及三轴固结不排水蠕变试验成果进行了拟合,给出了模型参数的取值范围。该模型可以回避蠕变固结相互作用的复杂机理,从宏观上预测实际工程的长期变形。计算结果表明,与传统的Merchant模型相比,分数阶导数Merchant模型能更好地描述软黏土的蠕变变形,尤其是对于模拟试验曲线拐点处的变形具有明显优势,且能更好地反映土体变形随时间发展的非线性化程度。模型参数分析表明,在各级荷载作用下阶数α的变化范围最小,而黏滞性系数η1的变化范围最大,但从精度上总的来说可以满足工程计算的要求。(5)基于Perzyna过应力理论,引入参考面的概念建立了一般应力条件下的三维非线性弹黏塑性模型,并考虑了初始各向异性的影响。通过将所建立的本构模型引入至Biot固结理论,实现了三维条件下的蠕变固结耦合,并推导出了应力增量及增量形式的雅克比矩阵的表达式,编制了Abaqus有限元软件中的UMAT子程序,对软基上堆载问题进行了模拟。计算结果表明,由于土体的蠕变作用,即使外荷载保持不变,当排水条件较差时,孔隙水压力随时间也会有一定幅度的增加,并对地基的变形和长期变形产生实际影响。
[Abstract]:Soft clay is widely distributed in the coastal areas of China, along the river and other economically developed areas. It has the characteristics of high water content, large pore ratio, strong compressibility, viscosity and so on. This makes the settlement problem especially the long-term settlement especially when the structure is built on the soft clay foundation. The deformation process of soft clay is closely related to the time, which is not only closely related to the time of the soft clay. The consolidation process including the dissipation of excess pore water pressure and the consolidation process of soil volume compaction, including the creep deformation caused by its viscosity, both influence each other and determine the long-term settlement of soil. Therefore, based on the time effect of soil deformation, the constitutive model which can reflect the creep properties of soft clay is established and the creep is realized. The coupling effect of consolidation is a hot issue in the engineering and academic circles. This study uses laboratory tests, theoretical analysis and numerical simulation to study the consolidation of soft clay, the creep properties and the establishment of the control equation. The main research contents and results are as follows: (1) one dimension K0 side limit consolidation test and three axis are carried out. The consolidation undrained shear test, the three axis consolidation undrained creep test, analyzed the aging characteristics and deformation mechanism of the soft clay deformation, and provided an experimental basis for the establishment of further elastoplastic constitutive model. The test results showed that the secondary consolidation coefficient of soil decreased with the increase of loading time and preloading, and secondary consolidation. The coefficient increases with the increase of load, but its increase depends on the consolidation state of the soil. The secondary consolidation coefficient appears to be a turning point at a certain range related to the PC. The coefficient of secondary consolidation has a good linear relationship with the compression index, and its ratio is.2 between 0.028~ 0.049) when the upper load is smaller, the soil mass is lower. The undrained creep deformation of three axis will gradually weaken with time and eventually tends to stability, and when the load reaches a critical strength, the creep deformation of the soil will develop quickly after loading for a period of time, and the soil has shear creep failure.3). Under the strain control mode, the undrained shear strength of soil will change with the shear strain. The rate correlation can be increased and its rate correlation can be well characterized by rate parameters. (2) based on the results of one dimensional K0 side limit consolidation test, the rationality of the creep deformation as a time dependent plastic deformation hypothesis is verified on the basis of the Bjerrum equal time line theory, and the creep and strain rate effects are from the angle of creep and strain rate respectively. The nonlinear elastic viscoplastic model of one dimension K0 side limit is derived, the physical meaning and the method of obtaining the model parameters are given. The nonlinear elastic viscoplastic model is introduced into the one-dimensional Terzaghi consolidation theory, and the coupling control equation of the creep consolidation under the one dimension K0 side limit is established, and the finite difference scheme of the Crank Nicolson is adopted. Through the simulation of the one-dimensional K0 side limit consolidation test and the comparison with the experimental results of Imai and Leroueil, the rationality of the model is verified. The calculation results show that the viscosity of the soil increases with the increase of the height of the soil sample, and the deformation amount of the model and the false A are calculated by the model and the false A. The difference between them is also getting bigger and bigger. (3) based on the improved GJY K0 consolidation instrument, a one-dimensional flexible equal side limit consolidation test is carried out. The compression characteristics of the soil in a certain lateral deformation are analyzed, and the experimental results are compared with the results under the K0 side limit. The development law of the compression index and the secondary consolidation coefficient are all consistent with the K0 side limit, but because of the lateral deformation, the compression index is about 1.08~1.16 times of the K0 side limit and the secondary consolidation coefficient is about 1.1~1.3 times of the K0 side limit. On the basis of the flexible equal side limit consolidation test, the effective stress principle is based on Darcy's law. And the continuity principle is used to derive the one-dimensional consolidation equation under the flexible equivalent side limit condition, and coupled with the nonlinear elastoviscoplastic model established, the coupling control equation of the creep consolidation under the flexible isometric side limit is obtained, and the significance and the determination method of the model parameters are given. The model is proved to be reasonable, and the simulation results show that the influence of the soil sample thickness and the secondary consolidation coefficient on the strain value at the completion of the main consolidation is larger than that under the K0 side limit condition. In this paper, the strain value of the single layer foundation under the flexible equivalent side limit condition is about K0 side limit under the condition of the main consolidation. 1.17 times. (4) in order to widen the path of the common action of creep consolidation, based on the laboratory creep test results, the Merchant empirical model with fractional derivative is established, and the one-dimensional K0 side limit consolidation test and the three axis consolidation undrained creep test results are fitted, and the range of the model parameters is given. This model can avoid vermicular creep. The complex mechanism of the consolidation interaction is used to predict the long-term deformation of the actual project from the macroscopic point of view. The calculation results show that the fractional derivative Merchant model can better describe the creep deformation of the soft clay compared with the traditional Merchant model, especially for the deformation at the turning point of the simulated test curve, and it can be better reflected. The nonlinear degree of soil deformation developed with time. The model parameter analysis shows that the variation range of the order number alpha is the least, and the viscosity coefficient ETA 1 has the largest variation range, but it can meet the requirements of engineering calculation in general. (5) based on the theory of Perzyna over stress, the concept of reference surface is introduced. The three-dimensional nonlinear elastic viscoplastic model under the general stress condition and the effect of initial anisotropy are taken into account. By introducing the established constitutive model to the Biot consolidation theory, the coupling of creep consolidation under three dimensional conditions is realized, and the expression of the Jacobi matrix of stress increment and incremental form is derived, and the finite Abaqus is drawn up. The UMAT subroutine in the meta software is used to simulate the loading problem on the soft foundation. The calculation results show that the pore water pressure increases with time, and the deformation and long-term deformation of the foundation can be influenced by the creep of the soil.

【学位授予单位】：暨南大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TU447

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