软黏土层一维有限应变固结的超静孔压消散研究

根据土力学固结理论计算分析软黏土层固结过程的超静孔隙水压力值,确定软黏土体固结过程的强度增长,对排水固结法处理软土地基至关重要。软黏土层固结过程中土体变形较大时,有限应变固结理论和小应变固结理论计算分析软黏土固结所得结果差异较大。利用非线性有限元法及程序,通过对软黏土层固结工程算例的计算结果分析,研究了有限应变固结理论和小应变固结理论计算分析软黏土层一维固结超静孔压值消散的差异;探讨了软黏土体一维固结过程中,几何非线性、土体渗透性变化和压缩性变化对超静孔隙水压力消散的影响。研究结果表明,当土体的变形较大时,有限应变固结理论计算出的超静孔压要比小应变固结理论得到的值消散的更快。考虑土体固结过程中渗透性的变化时,超静孔压消散变慢;可用软黏土渗透性变化指数ck 反映渗透性变化对超静孔压消散的影响,渗透性变化指数ck值越小、超静孔压消散越慢。固结过程中软黏土压缩性的大小及变化也影响超静孔压的消散,可用软黏土的压缩指数cc反映固结过程中压缩性的大小及变化对超静孔压消散的影响,软黏土的压缩指数cc越小,固结过程软黏土层中的超静孔压消散越快。     

考虑幂函数渗透压缩变化的双层砂井地基固结性状

采用Barron轴对称固结及大变形固结问题的某些简化与假定,推导建立了砂井地基大变形固结控制方程,利用建立的双层砂井地基大变形固结方程及编制的计算程序,通过引入软土渗透系数、有效应力与孔隙比之间的幂函数关系k =ced与e=a( )b,对瞬时加载下双层砂井地基固结性状进行算例计算。结果表明：（1）双层软土幂函数渗透关系及压缩关系中诸参数对双层砂井地基固结性状有重要影响：随着两层软土幂函数渗透关系中参数c1、c2的增加（渗透性增加）、或幂函数压缩关系中参数a1、a2的增加,各土层水平径向与竖向孔隙比减小更快,沉降发展速率与超静孔压消散速率也相应增加,且沉降发展速率快于孔压消散速率。（2）两层土在分界面处的孔隙比及平均超静孔压均出现明显的突变,将沿深度分布曲线分成形状不同的两段,表现出不同的固结性状。     

变荷载下双层地基一维非线性固结近似解析解

目前考虑土体非线性压缩及渗透特性的双层地基非线性固结解均假定土体固结系数保持不变,能反映固结系数变化的双层地基非线性固结解还很鲜见。引入经典的e- 和e- 非线性关系描述土体的非线性压缩、渗透特性,在假定双层地基上、下土层压缩指数与渗透指数比值 相等且不等于1的基础上,得到变荷载下考虑土体固结系数变化的双层地基一维非线性固结近似解。该解答在 1条件下可退化为已有的 1时双层地基一维非线性固结解。基于此解探讨了双层地基上、下土层参数的相对比值对非线性固结性状的影响。结果表明：单面排水条件下 越小,下层土与上层土的相对压缩性越低、相对渗透性越高,则双层地基非线性固结速率越快;减小 值,增加双层地基中压缩性小、渗透性高的土层的厚度,会加快地基的固结速率。     

考虑起始水力坡降的软土大变形非线性固结分析

软黏土中渗流存在起始水力坡降的现象已逐渐为人们所认识,但变荷载下考虑起始坡降的软土大变形非线性固结理论还很鲜见。考虑土中渗流存在的起始水力坡降及非线性压缩渗透特性,在拉格朗日坐标系中建立变荷载下以超静孔压为变量的软土大变形非线性固结模型并给出其有限差分解。在此基础上,通过与达西定律下大变形固结半解析解对比分析,验证了解的可靠性。最后着重分析起始水力坡降对软土大、小变形固结性状影响的差异。结果表明,无论采用小变形假定还是采用大变形假定,考虑起始坡降后超静孔压的消散速率及最终沉降量均比达西定律下小。大、小变形假定下起始坡降均会引起超静孔压不能完全消散的现象,且大变形假定下超静孔压残留值要小于其小变形假定下的残留值,致使大变形假定下土层最终沉降量要比其小变形假定下大。起始水力坡降和几何变形假定均会影响固结性状,且起始坡降值无疑影响更明显,故在软黏土中渗流存在的起始水力坡降在固结计算中不容忽视。     

考虑起始坡降双层地基的大应变非线性固结

天然软土成层分布特性及土中渗流存在起始水力坡降的现象已被人们熟知。但变荷载下能同时考虑黏土中起始水力坡降、软土非线性压缩渗透特性及大应变特性的双层地基固结理论还鲜见报道。在拉格朗日坐标系中建立以超静孔压为变量的双层软土地基大应变非线性固结模型并给出其有限差分解。通过与考虑起始水力坡降的单层地基大应变非线性固结数值计算结果对比,验证了差分解的可靠性。着重分析了上、下土层起始坡降无量纲参数R1、R2对双层地基固结性状的影响,分析在大应变与小应变假定下双层地基超静孔压消散及固结沉降变形的异同。结果表明：上层土无量纲参数R1对双层地基固结性状的影响程度较下层土无量纲参数R2显著;大应变假定下双层地基渗流前锋的下移速度要快于小应变假定下的移动速度;大应变假定下考虑起始水力坡降的双层软土地基超静孔压消散速率要比小应变假定下快,且大应变假定下考虑起始水力坡降的双层地基最终沉降量要比小应变假定下大。     

变荷载下基于指数渗流双层地基一维固结分析

在土中渗流服从指数形式的前提下,建立了变荷载作用下双层地基的一维固结控制微分方程。利用有限差分法求得孔隙水压力的数值解,并通过与解析解对比对其可靠性进行了验证。对双层地基在指数形式渗流时不同参数下的固结性状进行分析,结果表明：单面排水条件下,双层地基中上层土渗流指数的大小对固结速率起决定性作用,而下层土渗流指数大小对固结速率的影响很小;如果上、下两层土体的压缩性不同,则地基按变形定义的平均固结度和按孔压定义的平均固结度不再相等;地基中下层土与上层土的相对压缩性越低、相对渗透性越高,则地基的固结速率越快;增大压缩性小、渗透性高的土层相对厚度,会加快双层地基的固结速率。     

考虑非达西渗流的双层软土地基大变形非线性固结分析

现有的双层软土地基大变形固结理论均假定土中渗流遵循达西定律,但软黏土中渗流在低水力坡降下会出现偏离达西定律的现象已逐渐为人们认识。综合考虑双层软黏土中的非达西渗流、软土的非线性压缩渗透特性及实际中的变荷载作用,在拉格朗日坐标系中建立以超静孔压为变量的软土大变形非线性固结模型,并给出其有限差分解。在此基础上,通过与已有的非达西定律下单层地基大变形固结数值解计算结果相对比,以验证其差分解的可靠性。最后着重分析上、下层非达西渗流参数m_1、i_(11)及m_2、i_(12)对固结性状的影响,并分析了在大、小变形不同几何假定下对双层地基超静孔压消散及固结沉降影响的异同。结果表明:上层土参数m_1、i_(11)对固结性状的影响要比下层土参数m_2、i_(12)显著;m_2、i_(12)的增大会引起上层土超静孔压消散速率的加快,但双层地基的固结速率会减慢;大变形假定下双层软土地基的固结速率要比小变形假定下快,但两种几何假定下双层地基的最终沉降量是相等的。     

考虑变渗透系数的饱和黏土一维流变固结分析

为进一步探讨饱和黏性土的弹黏塑性和变渗透性对一维固结进程的影响,引入考虑时间效应的统一硬化（UH）本构模型描述饱和黏土的弹黏塑性,并用Taylor的经验关系式表示其渗透系数与孔隙比的关系,修正了太沙基一维固结理论,并利用有限差分法对方程进行求解。通过与一维流变固结试验结果对比,验证本文计算方法的有效性以及UH模型的适用性。在此基础上,讨论了UH模型参数、压缩指数与渗透指数的比值以及荷载强度等对固结过程的影响。计算结果表明:饱和黏土的黏滞性使得在固结初期地基内部出现了孔压升高现象,并减缓了固结中后期地基孔压的整体消散,增大了地基的沉降量,并且这种现象随着初始超固结参数的增大而更明显;另外,随着压缩指数与渗透指数比值的减小,地基固结进程会加快,但地基最终沉降量不受影响;同时,在固结后期,当压缩指数不大于渗透指数时,较小荷载强度对应的地基内孔压消散较慢,然而,当压缩指数大于渗透指数时,荷载强度对孔压消散的影响则相对很小。     

考虑井阻时空变化的竖井地基非线性固结解析解

软土固结过程中展现出明显的非线性压缩和渗透特性,同时竖井的淤堵效应常导致井阻在固结过程中随深度和时间不断演化,但目前能考虑井阻随时空演化的竖井地基非线性固结解析解还很鲜见。通过引入孔隙比与有效应力及孔隙比与渗透系数间的半对数模型描述了土体的非线性固结特性,建立了能同时考虑井阻随时空变化和涂抹影响的竖井地基非线性固结模型,并采用分离变量法获得了固结模型的解析解。将特定参数下固结解的计算结果与实测数据、已有的竖井地基固结解答进行了对比分析以验证其可靠性。最后,对竖井地基的非线性固结性状开展了大量计算分析。结果表明：竖井渗透系数随深度线性衰减越明显则地基固结速率越慢;外荷载一定时,随着软土压缩指数c_c与渗透率指数c_k之比的增大,竖井地基固结速度减慢;在c_c /c_k值不变的情况下,外荷载增加,地基固结速率加快。在涂抹区的3种径向渗透系数变化模式中,抛物线变化模式下的地基固结速度最快,线性变化模式下的地基固结速度次之,恒定模式下的地基固结速度最慢,且这种性状并不因为考虑井阻变化或土体非线性固结特性而发生改变。     

基于非达西渗流的软土一维非线性固结半解析解

在土中渗流遵循非达西渗流定律的前提下,考虑软土在固结过程中的非线性固结特性,根据饱和土体一维固结的连续条件,推导出基于非达西渗流的软土一维非线性固结控制方程。利用半解析方法对其进行求解,并与差分计算结果进行对比,验证半解析方法的可靠性。最后,着重分析非达西渗流与达西定律之间非线性固结性状的差别,以及不同自重应力分布方式对固结速率的影响。结果表明,考虑非达西渗流下的非线性固结速率比达西定律下要慢,且指数和临界水力坡降越大,非线性固结速率越慢。而且,作用的外荷载越小、地基土层越厚,非达西渗流下非线性固结速率的减慢愈明显。自重应力均匀分布下的非线性固结速率要比自重应力线性分布下慢,但随着荷载的增大、土层的变薄,两者之间的差别会越来越小。     

Study of the 1-D consolidation behavior of two-layered soft soils: Parametric studies using a rheological model with viscoplastic body

The governing equations for the coupled processes of consolidation and creep of two-layered soft soils are established. The Nishihara rheological model is adopted to simulate the elasto-viscoplastic characteristics of soft soils, disregarding the effects of the soil self-gravity. A semi-analytical theory combined with numerical and analytical methods is introduced to solve the governing equations of the one-dimensional rheological model. The computational procedure and the approximate solutions for two-layered soft soils subjected to surface loading are obtained for two drainage conditions. The solutions and the computational procedure are used to study the effects of the two layers and constitutive parameters on rheological consolidation behavior of soft soils. It can be concluded that two layers affect the rate of excess pore water pressure dissipation and settlement development. The parametric studies show that when the parameters of the upper layer remain constant, increases in the permeability and elastic modulus in the lower layer accelerate the dissipation of the excess pore water pressure, and meanwhile increases in the viscosity coefficient and viscoplastic limit slows down the dissipation of the excess water pressure.     

Electro-osmotic consolidation of soil with variable compressibility,hydraulic conductivity and electro-osmosis conductivity

In present study, the non-linear variations of soil compressibility, hydraulic and electro-osmosis conductivities were analyzed through laboratory experiments, and incorporated in a one-dimensional model. The analytical solutions for excess pore water pressure and degree of consolidation were derived, and numerical simulations were performed to verify its effectiveness. The results indicated that both the non-linear variations of hydraulic and electro-osmosis conductivities showed remarkable impacts on the excess pore water pressure and degree of consolidation, especially for soils with relative high compressibility. A further comparison with previous analytical solutions indicated that more accurate predictions could be obtained with the proposed analytical solutions.     

A study on one‐dimensional consolidation of layered structured soils

The governing equations for one‐dimensional consolidation of layered structured soils under time‐dependent loading are established. Using simplified k‐σ′ and m_v‐σ′ models, n‐layered structured soils are transformed into (n + 1) or (n + 2)‐layered soils in which the thickness of upper and lower layers are gradually changing. The approximate solutions for the governing equations are then obtained under two types of boundary conditions, and the computer program is developed. Based on the solutions and computer program, the consolidation behavior of layered structured soils with soft interlayer is studied. It is shown that the permeability and compressibility of the soft interlayer have the greatest influences on the rate of settlement and rate of the dissipation of excess pore water pressure. Copyright © 2015 John Wiley & Sons, Ltd.     

考虑渗透系数变化的非饱和土固结性状分析

基于Fredlund非饱和土一维固结理论,研究了有限厚度的表面透水透气、底面不透水不透气的线弹性和黏弹性非饱和土地基在加荷随时间指数性变化时的一维固结特性。分别得到了两类地基在固结过程中同时考虑液相、气相渗透系数非线性变化和仅考虑液相渗透系数变化两种情况下的半解析解答。利用典型算例进行计算,分析了不同情况下两类地基中超孔隙水、气压力消散以及地基固结度随时间的变化规律,并与不考虑渗透系数变化时的半解析解计算结果进行了对比。结果发现：固结过程中渗透系数呈非线性变化;只考虑液相渗透系数变化时,超孔隙气压力的消散变化不大,超孔隙水压力的消散加快;气相渗透系数变化对超孔隙气的消散产生明显影响,对超孔隙水压力消散影响不大。同时考虑液相和气相渗透系数变化时,土体中超孔隙水、气压力的消散均有明显变化,土体固结速度也相应加快;分析结果对非饱和土固结的进一步研究具有重要意义。     

Consolidation analysis of saturated multi‐layered soils with anisotropic permeability caused by a point sink

This paper presents the analytical layer‐element method to analyze the consolidation of saturated multi‐layered soils caused by a point sink by considering the anisotropy of permeability. Starting from the governing equations of the problem, the solutions of displacements and stresses for a single soil layer are obtained in the Laplace–Hankel transformed domain. Then, the analytical layer‐element method is utilized to further derive the solutions for the saturated multi‐layered soils in the transformed domain by combining with the boundary conditions of the soil system and continuity conditions between adjacent layers. The actual solutions in the physical domain can be acquired by the inversion of Laplace–Hankel transform. Numerical results are carried out to show the accuracy and stability of the proposed method and evaluate the influence of sink depth and anisotropic permeability on excess pore pressure and surface settlement. Copyright © 2011 John Wiley & Sons, Ltd.     

Stress history effects on 1‐D consolidation of soft soils: a rheological model

Stress history plays an important role in controlling the consolidation behavior of soft clays, but few models exist that can provide quantitative estimate of its influence. In this paper, the Gibson–Lo rheological model is used to simulate the coupled processes of drainage and creep of soft soils that takes stress history into account. A hybrid combination of analytical and numerical methods is adopted to solve the governing equations of consolidation with the nonlinear rheological model. The methodology is applied to a saturated soft soil subjected to surface loading. The soil profile is separated into normally consolidated and overconsolidated layers by a boundary that is allowed to move. Comparisons of the model predictions and its simulations are used to evaluate the effects of stress history, model parameters, and loading pattern on consolidation behavior. It is shown that stress history influences the location of the moving boundary, variations of the profiles of excess pore water pressure dissipation, stress and deformation‐based average degrees of consolidation. Parametric studies conducted show that when soil is stiffer, the excess pore water pressure dissipates much more quickly, and thus the soil consolidates much faster especially at the early stages. The results also show that soil viscosity influences the deformation‐based average degree of consolidation at the latter stages. The consolidation process of soil layer under linear loading is shown to lag behind those under instantaneous loading: the longer the loading period is, the smaller the average degrees of consolidation are no matter how they are defined. Copyright © 2012 John Wiley & Sons, Ltd.