软土地基电渗固结理论分析与数值模拟

电渗固结是加速软土固结、提高地基承载力的有效技术。传统的电渗固结理论假设土体的物理力学、水力学和电学特性均匀稳定,其理论解答与试验结果差别较大。针对电渗固结处理过程,对土体位移场、渗流场和电场的耦合特征进行了理论分析,根据电荷守恒原理、水流连续原理和Biot固结理论,建立了电渗固结过程的多场耦合控制方程;考虑土体相关特性参数的非线性关系,开发了有限元软件用于分析电渗过程中电场强度、土体位移以及超静孔隙水压力的变化特征。计算结果与理论数据吻合较好,能够反映土体相关特性参数非线性关系对结果的影响。软件能够为电渗固结系统设计提供参考依据。     

高岭土电渗固结特性及数值模拟研究

电渗固结是促进低渗透性软土排水固结的有效方法。为了揭示不同电势梯度影响高岭土电渗固结的基本规律,在自制电渗试验装置上对高岭土进行电渗试验。试验过程中测量电流、排水量、沉降量以及有效电压随时间的变化,并进行单位排水能耗分析。基于电渗固结多场耦合控制方程,实现土体电渗固结全耦合分析的有限元数值方法,计算结果与解析解吻合良好,验证了程序的有效性。为预测不同电势梯度下土体沉降量随时间的变化关系,分别对0.5,1.0,1.5 V/cm 3种电势梯度电渗固结试验进行数值模拟分析,获得模型表面沉降量分布、阳极超静孔隙水压力时空发展规律、阳极位置固结度等曲线,计算结果和试验结果吻合良好,可为实际电渗试验提供理论指导。     

深基坑大变形耦合分析与数值模拟

深基坑开挖的时空效应既与深基坑工程支护设计、施工方案有关,又依赖于场地岩土介质的工程特性。依据Biot固结理论与有限变形的理论基础,采用经典的变分原理方法推导了基于大变形Lagrangian描述的U. L方法--Biot固结理论耦合系统方程及其有限元数值模拟方法。根据工程实例进行大小变形计算的比较以及深基坑开挖的时空效应分析,得出开挖时空效应显著、小变形计算的安全性不足的结论。     

边坡电渗模型试验及能量分析法数值模拟

采用导电塑料制成的电动土工合成材料(EKG)进行了15.5 d的边坡电渗加固试验,测定了电渗之后土体等含水率分布曲线。试验结果表明,电渗加固效果是随时间从阳极到阴极逐渐扩展的,因此,越靠近阳极接入点的土体加固效果越好,越靠近阴极末端的土体加固效果越差。提出了电渗能量分析法,该方法仅要求黏性土体在排水固结开始的初始时刻是处于饱和状态,而在排水固结进行的过程中土体可以是饱和的,也可以是非饱和的。提出了基于能量分析法的电渗过程数值模拟方法,并对试验过程进行了数值模拟,模拟结果与实测结果能够较为吻合。     

考虑有效电势变化的软土一维电渗固结理论

通过前人研究成果和电渗室内试验,发现了电渗中有效电势先随时间线性减小后保持不变的规律。将此规律作为基本假设,与Esrig一维电渗固结理论相结合,列出了工程中常见的阳极不排水、阴极排水情况下考虑有效电势变化的一维电渗固结方程。求解出了考虑有效电势变化的孔隙水压力和平均固结度的解析式。与实际情况相比,考虑有效电势变化的一维电渗固结理论解较Esrig理论解误差更小,将两理论进行了对比,求出了任意时刻排水量的误差减少量,为实际工程提供参考。     

砂土液化变形的有限元-无网格耦合方法

通过构造Biot固结理论u-p方程的无网格伽辽金-有限元耦合方法,对砂土液化变形问题进行了数值模拟。对于饱和砂土,采用Oka等提出的弹塑性本构模型,同时采用更新的Lagrange计算格式推导了控制方程。耦合方法能够发挥有限元和无网格各自的优点,既避免了由于单元变形扭曲而引起的计算中断,也可节约计算时间,算例验证了该方法在地震液化问题中的有效性。     

电渗-堆载作用下非饱和土轴对称固结特性分析

地基处理中,电渗常与堆载预压联合使用。基于Fredlund非饱和土三维固结理论和Esrig电渗固结理论及等应变假设,推导出电流场与渗流场耦合作用下,考虑径竖向渗流及涂抹效应的轴对称固结控制方程;利用Laplace变换、解耦技术及Laplace逆变换等方法,得到了时域内的半解析解。然后,将所得解分别退化至饱和土中考虑电渗-堆载联合和非饱和土中仅考虑堆载这两种情况与已有文献结果对比,验证了研究方法的可靠性。最后,通过算例研究有效电压、电-水力渗透系数比和涂抹系数对非饱和土轴对称电渗固结特性的影响。结果表明：超孔隙水压最终产生的负孔压与有效电压成正比;电渗-堆载联合下的非饱和土地基固结可分为两个阶段,分别主要受堆载和电渗影响;电渗联合堆载处理地基时的最终沉降是两种作用分别处理地基时的最终沉降之和,且对于低渗透性的土体,其固结速率将显著提高;此外,施加电压后涂抹系数在同区间内,超孔隙水压消散的速率差距比不考虑电压时的大,且涂抹系数越小,最终产生的负孔隙水压越大。     

电渗固结中的界面电阻问题

结合试验对电渗固结中的界面电阻问题进行了详细的分析和探讨,提出了界面电压降假定,在该假定的基础上,经过一系列理论推导,给出了一个简洁的界面电阻表达式,这将有助于我们对电渗固结的电流以及能耗做出更为准确的估计。     

沉积泥砂非线性大变形固结沉降计算模型

沉积形成的水底黏性泥砂自重固结过程表现出显著非线性大变形固结特征,应采用大变形固结理论进行泥砂沉积固结计算。基于软黏土一维非线性大应变固结理论,应用有效应力、渗透系数与孔隙比间扩展幂次函数固结本构关系,由达西定律、有效应力原理、连续介质方程等建立大变形固结控制方程,根据固结单元孔隙水渗流、单元变形与泥砂沉积层固结沉降耦合关系形成黏性泥砂大变形自重固结数值模型。泥砂自重作为固结荷载,数值模型假定沉积泥砂各向同性且固结沉降应变、孔隙水渗流仅发生于竖直方向,为一维单向沉积固结过程;采用泥砂沉降柱试验确定泥砂非线性扩展幂次函数关系参数。模型应用中,划分竖向固结单元,由沉积泥砂固结本构关系确定各固结单元有效应力及超孔隙水应力,通过超孔隙水应力时间维度上的消散过程及各固结参数间的耦合关系计算泥砂固结沉降。数值模型计算结果表明,沉积黏性泥砂自重固结初期表现为有效应力调整过程,初始有效应力与孔隙比根据固结本构关系匹配调整为扩展幂次函数关系;沉积泥砂应变与应力固结度存在20%左右误差,泥砂固结沉降发展快于超孔隙水应力消散过程,证明沉积泥砂固结沉降变形的发展与超孔隙水应力消散并非同步耦合。计算模型应用于室内沉降柱试验模拟淤积黏性泥砂自重固结沉降预测中,模型输出与试验结果符合良好。     

大尺寸三维电动-水力渗透协同固结淤泥能耗特性

为解决目前电渗应用的高能耗困境及大尺寸模拟困难等问题,提出针对低渗透性、高含水率土体的电动-水力渗流协同作用的三维固结方法。自行研制了一套阴极-集水-排水协同作用的多功能集排水系统,采用间歇式抽水代替连续抽水。采用三维电动-水力渗流固结系统对取自贡湖湾湿地和白旄堆场的2种不同太湖底泥进行了电渗试验研究,同时对白旄堆场太湖底泥进行了传统一维电渗固结试验,并对2种试验条件下的单位体积排水能耗和单位体积单位排水量能耗等关键指标进行了对比。结果表明：三维集水井设计可大大降低土体阴极附近电阻,间断性提高系统电流,提高排水效率,降低电渗总能耗;间歇式抽水可间歇性降低系统内总电流,利用电动-水力协同作用,保持系统渗流的连续性;三维电动-水力渗流固结系统的电流呈周期性减小—增大模式,并且降低速率较慢,尤其对于有机质含量较高的土体,电渗过程电流始终保持在一个较高的水平,提高了排水固结效率。三维电动-水力渗流固结系统的单位体积排水能耗和单位体积单位排水量能耗分别约为一维电渗系统的2/3和1/30。在高含水量软土固结排水中具有显著的节能效果。三维电动-水力渗流固结系统可以提高排水固结效率、大幅度降低能耗,为...     

应力历史对饱和软土固结系数的影响

为了探究不同固结状态下的饱和软土固结系数的变化规律,在太沙基固结理论的基础上,利用渗透系数和孔隙比的关系、孔隙比和固结应力的关系,分别推导出了在正常固结和超固结状态下固结系数（C_v）随固结应力变化的关系式,将关系式代入Terzaghi方程,进而获得考虑应力历史和固结应力影响的修正Terzaghi一维固结方程;通过室内固结试验和工程应用分析对固结系数关系式和修正的Terzaghi一维固结方程的准确性进行验证.结果表明,对于正常固结的软土,当固结应力小于前期固结应力时,固结系数随应力的增大而增大;当固结应力大于前期固结应力时,固结系数随应力的增大而减小.对于超固结状态的软土,固结系数随应力的增大而增大,最后趋于平缓.当上覆荷载较小时,修正前后的Terzaghi一维固结方程计算结果相近;但当上覆荷载较大时,修正后的Terzaghi一维固结方程计算的固结度明显滞后于修正前的计算结果.前期的应力历史和后期的固结应力对软土固结系数的影响是不容忽视的,修正后的Terzaghi一维固结方程更能真实反映土体的固结性状.      

A one-dimensional viscoelastic and viscoplastic constitutive approach to modeling the delayed behavior of clay and organic soils

Accurate modeling of the time-dependent behavior of geomaterials is of great importance in a number of engineering structures interacting with soft, highly compressible clay layers or with organic clays and peats. In this work, a uniaxial constitutive model, based on Perzyna’s overstress theory and directly extendible to multiaxial stress conditions, is formulated and validated. The proposed constitutive approach essentially has three innovative aspects. The first concerns the implementation of two viscoplastic mechanisms within Perzyna’s theory in order to distinguish between short-term (quasi-instantaneous) and long-term plastic responses. Similarly, elastic response is simulated by combining an instantaneous and a long-term viscous deformation mechanism. The second innovative aspect concerns the use of a bespoke logarithmic law for viscous effects, which has never been used before to simulate delayed soil behavior (as far as the authors are aware). The third concerns the model’s extensive validation by simulating a number of different laboratory test results, including conventional and unconventional oedometer tests with small and large load increments/decrements and wide and narrow loading/unloading cycles, constant rates of stress and strain tests, and oedometer tests performed in a Rowe consolidation cell with measurement of pore pressure dissipation.     

Theoretical Equations of Vertical and Radial Consolidation by Equating Degrees of Consolidation by Settlement Analysis and Dissipation of Pore Pressure

Degree of consolidation, U, is defined in two ways: (1) by settlement analysis and (2) by dissipation of pore pressure. Equations of vertical and radial consolidations are derived by equating the above two U. The new equations are compared with the existing equations of vertical and radial consolidations. It is shown that new equation of radial consolidation is exactly same as Barron’s equation of equal strain radial consolidation while in case of vertical consolidation it is similar to later part of Terzaghi’s equation of vertical consolidation. A few possible reasons and advantages of comparison are given. The theory of rapid loading methods is based on the equality of these two degrees of consolidations. It is concluded that in rapid loading methods the next load increment can be applied at any time in radial consolidation once the effects of initial compression are over. However, in vertical consolidation it can be applied only after 60% consolidation.     

Behaviour of piles in consolidating soil

The development of negative skin friction on circular piles in a consolidating layered soil is investigated by an elasto-plastic load transfer theory which accounts for slippage of the soil. The elastic load transfer theory is premised on the compatibility condition that the vertical displacement of the ‘pile’ is equal to the summation of the vertical displacement of the layered soil due to the consolidation of the upper soil layer and the vertical displacement in the ‘soil layers’ along the pile’s centroidal axis caused by a system of pile-soil interactive forces. Slippage of the soil is accounted for by imposing the shear strength of the clay layer as the limit of the pile-soil interface shear stress. The saturated upper clay is consolidating under a uniform surcharge in accordance with Terzaghi’s one-dimensional consolidation theory. The validity of the proposed solution is confirmed by comparison with field measurements. Extensive parametric studies with regard to the effect of pile-soil slip on pile behaviour are presented.     

变荷载下基于指数形式渗流的一维固结分析

建立了考虑指数形式渗流以及变荷载条件下的一维固结微分方程,采用相对稳定的Crank-Nicolson差分格式获得控制方程的差分解答并验证了计算程序的可靠性。结果表明,当指数大于1时,较小时间因子下固结速率比达西渗流快,较大时间因子下固结速率比达西渗流慢;而当指数小于1时,较小时间因子下固结速率比达西渗流慢,较大的时间因子下固结速率比达西渗流快。在土层厚度相同的情况下,指数大于1时作用于土层的荷载越小,固结速率越慢;基于指数形式渗流,传统一维固结理论中室内土样固结与实际地基土层固结之间的相似关系不再成立;加荷速率越快,则土层的固结速率越快。     

A practical evaluation of the surcharge preload period in staged construction subject to creep

Soft clay exhibits creep behavior, but simple methods of surcharge preload assessment generally do not take into account creep during primary consolidation. Because Yin–Graham’s model can predict both primary and secondary settlement, it is employed in this paper to obtain the formulae for critical settlement at the unloading time during surcharge preload and for final settlement at the end of the service life. Because “aged” soft natural clay exhibits obvious apparent preconsolidation pressure during a long-term sedimentary history as a result of creep effects and because the field permeability coefficient is considerably larger than the laboratory permeability coefficient, most field measurements indicate that the theoretical excess pore pressure based on Terzaghi’s theory is greater than the measured excess pore pressure even in soft natural clay with obvious viscous behavior. Because of the widespread application of the degree of consolidation in terms of effective stress based on Terzaghi’s theory in real preload projects, the analytical solution for the surcharge preload period subject to creep is derived through the combination of Yin–Graham’s model and Terzaghi’s theory for consolidation. Compared with the existing solution considering secondary settlement, the formula for the preload period presented in this paper is easily applied to assess the preload period using a chart. The case study described indicates that when the consolidation parameters of Terzaghi’s theory are calculated from field-measured excess pore pressure in preload tests, the surcharge preload period determined as described in this paper is suitable for preload design and performance.     

盾构掘进速度及非正常停机对地面沉降的影响

软土中盾构隧道施工不可避免地扰动周围地层,进而引起地面沉降,沉降过大时将危及邻近建（构）筑物的正常使用和结构安全。全面理解盾构隧道施工引起的地面沉降的影响因素及对沉降的准确预测,对于减少施工环境危害十分重要。考虑盾构压重后,引入Mindlin解计算盾构下卧土层中的附加应力,采用单向压缩分层总和法计算盾构下卧土层的总固结沉降,由盾构掘进速度及停机时间确定附加应力作用时间后,应用太沙基一维固结理论计算在该作用时间内的固结沉降,应用Peck公式建立了盾构下卧土层沉降与地面沉降的关系,并以杭州庆春路过江隧道地面沉降的实测验数据对上述理论进行了验证。分析表明,考虑盾构掘进速度及停机时间的地面沉降计算理论基本合理;盾构掘进速度及停机时间会对隧道施工引起的地面沉降产生显著影响;在其他施工条件相同的前提下,提高盾构掘进速度和减少停机时间有利于减少地面沉降。     

Observed Behaviour of Laterally Expanded Stone Column in Soft Soil

The paper describes the behavior of remolded kaolin clay reinforced by stone column as investigated in laboratory. The installation of stone column was simulated by performing lateral expansion at different rates within hollow cylindrical remolded kaolin specimens initially subjected to K0 consolidation path. These specimens were, then, subjected to classical consolidated undrained triaxial tests while recording excess pore pressure values. The objective of the experimental programme was to quantify the effects of consolidation stress and stone column on the undrained Young’s modulus and shear strength of kaolin clay. Obtained results showed a significant improvement in Young’s modulus when the cavity expansion ratio and the consolidation stress increase. It was also found that the increase in undrained shear strength of improved kaolin clay mainly occurred at lower consolidation stress. Another important finding of this study is that the ratio of undrained Young’s modulus to undrained shear stress increases when the consolidation stress decreases. Finally, the paper presents a model developed for the design of stone columns.