基于非等温管道流竖井地基热排水固结模拟

针对新技术热排水固结法,采用非等温管道流模拟竖井中U型导热管的传热过程,考虑温度对竖井扰动区和未扰动区渗透性的影响,在COMSOL Multiphysics有限元软件中进行二次开发,建立了竖井地基热排水固结法的有限元模型。以热排水固结软基处理原型试验为例,重点分析了模型耦合、部分耦合和不耦合情况下软土地基的固结度。结果表明,相对于传统排水固结法的不耦合模型,部分耦合模型下因温度产生的孔压增量延缓了地基固结的发展,固结速率有所减慢;耦合模型下温度虽也产生一定的孔压增量,但温度有效地改善了竖井涂抹区土的渗透特性,地基的固结速率加快,固结周期缩短,与试验结果一致。     

考虑井阻及涂抹作用的非饱和土竖井地基固结解析解

以往的非饱和土竖井地基研究中未同时考虑竖井的井阻和涂抹作用,大部分按理想竖井进行研究,然而井阻和涂抹作用是影响非饱和土竖井地基固结的重要因素。针对这种情况,本文基于Fredlund非饱和土一维固结理论及等应变假设,引入变量将超孔隙压力耦合控制方程组转化为等价的线性偏微分方程组,考虑涂抹和井阻条件,并采用分离变量法和待定系数法,推导出了瞬时荷载下同时考虑井阻和涂抹作用的非饱和土竖井地基等应变固结解析解。将所得解析解进行退化,与既有的饱和土竖井地基等应变固结解析解对比,验证了本文解析解的正确性,并应用典型算例分析了井阻和涂抹作用对竖井地基固结的影响。结果表明,井阻因子G、井径比N、涂抹系数α及涂抹半径与竖井半径比S这四者任何一个值减小,非饱和土竖井地基的固结速度都将变快;井阻因子G小于0.1时,建议实际工程中不考虑井阻作用的影响;当涂抹半径与竖井半径比S大于5时,涂抹作用对竖井地基固结的影响与S=5时无明显差异。实际工程中建议提高非饱和土竖井地基的透水能力并减少施工扰动,以降低井阻和涂抹作用对非饱和土竖井地基固结影响。     

考虑温度效应的单井固结解析解

在现有竖井固结理论的基础上,考虑涂抹效应对地基固结的影响,研究了温度对渗透系数的影响并给出了渗透系数与温度的关系式,得到温度效应影响下竖井地基单面排水的固结解析解,并针对宁波软黏土选取参数进行算例分析。结果表明:温度对竖井地基固结性状有显著影响,同一时刻,温度越高地基的固结度越大;相同温差条件下,温度作用对土体固结的影响会随着温度的升高而逐渐减弱;在温度相同的条件下,井径比越小,土体固结越快;在温度条件相同的情况下,涂抹效应会导致土体固结速度减小。     

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。     

考虑变井阻和涂抹效应的真空预压三维有限元分析

真空预压工程中塑料排水板等竖井通水量随地基深度和固结时间而变化,即变井阻效应已经证明是实际存在的。另一方面,在真空预压工程三维有限元分析中,由于竖井和涂抹区在有限元模型中的划分使得三维有限元模拟较为困难。为真实模拟真空预压工程实际情况,在有限元分析中引入变井阻数学模型,并给出变井阻无量纲参数A_1、α_2。另外,为降低三维有限元建模难度,选用合适的网格单元简化竖井区,并用等效方法简化涂抹区,实现了考虑涂抹、变井阻效应的三维有限元分析。以天津港某围海造陆工程为例,建立三维有限元模型进行分析计算,对比分析计算值与实测数据,验证了程序的合理性,为以后软基加固工程三维有限元模拟提供了一种有效方法。     

基于流-热耦合模型的土石坝渗流热监测研究

为有效应用基于分布式光纤测温的渗流热监测技术,需研究待监测区的背景温度场特征。为此,建立了土石坝饱和-非饱和瞬态渗流场与温度场耦合模型（流-热耦合）,模型考虑了热对流、热传导和热扩散效应,温度边界按周期性气温考虑,且相关参数按非线性考虑,如流体黏度的热效应、导热系数受含水率影响等,其仿真结果更接近土石坝温度场的真实状态。基于典型算例,讨论了与大气温度相关的周期性波动温度场特征。计算结果表明：温度场受坝体渗流和气温的影响,库水及气温是两个重要热源,饱和带的温度受库水渗流控制,非饱和带主要受气温控制,具有季节波动特征;在土石坝心墙部位若发生集中渗漏,渗漏通道附近岩土体的温度受库水影响;若在心墙上敷设分布式光纤传感器,很容易捕捉到渗漏点位置及渗漏发生的时刻。渗流热监测技术在理论上可以反映渗流场的时空分布特征,在资料分析中还应关注由气温波动引起的温度异常。     

竖井地基热排水固结理论初探

为了加速软土地基固结排水过程,在传统竖井排水固结法的竖井中增设U型导热管,对U型导热管内的水进行预加热,并使其在管内循环,实现管-土之间热传递,以改善土的渗透性状。针对这一新的软基处理技术,本文首先通过研究渗流固结过程中的温度影响因子,建立了渗透系数与温度之间的关系。其次,基于理想竖井地基固结理论,建立基于温度修正的理想竖井地基固结度解析解。最后,分析了温度因素对理想竖井地基固结度及固结时间的影响规律。结果表明:同一时间因素下,温度越高,理想竖井地基的固结度越高,且温差越大,固结度差值越大;温度较高时到达某一固结度所需时间比温度较低时到达同一固结度所需时间少。在固结度较小时,温度因素对固结时间的影响并不显著;而固结度较大时,则固结时间差别较为显著。     

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。     

热-水-力耦合作用下非饱和土变形特性的弹塑性模拟

在已有的非饱和土水-力耦合模型基础上,耦合考虑温度影响,建立了一个热-水-力耦合作用下非饱和土弹塑性本构模型。该模型以土骨架平均应力、修正吸力和温度为应力状态变量,以土骨架应变、饱和度和熵为应变状态变量。通过引入与温度相关的屈服面(LY、TY)以及相应的硬化规律来考虑温度对土体变形的影响。利用建立的模型,对文献中不同吸力和温度条件下的等向压缩和三轴排水剪切试验进行预测,预测结果表明,该模型能够较好地定量描述热-水-力耦合作用下非饱和土的变形特性。     

考虑径向渗透系数变化的真空预压竖井地基固结解析解

针对真空预压条件下竖井地基固结问题,考虑竖井地基扰动区土体径向渗透系数变化的3种模式(扰动区渗透系数为常数、线性变化、抛物线变化)和竖井井阻随时间变化等因素影响;建立数学计算模型,采用解析解法,推导了考虑径向渗透系数因施工扰动而变化的真空预压竖井地基固结问题的解析解。基于此解,编制了计算程序,绘制出了考虑扰动区土体径向渗透系数变化和竖井井阻随时间变化影响的真空预压竖井地基固结曲线图。研究表明:井阻变化率对固结速率有较大影响;在土体扰动区径向渗透系数变化的3种模式中,渗透系数为抛物线变化时固结速率最快,渗透系数为线性时次之,渗透系数为常数时固结速率最慢。     

Effect of Soil Disturbance on Consolidation By Prefabricated Vertical Drains Installed in a Rectangular Pattern

In this paper, the effect of soil disturbance caused by installation of prefabricated vertical drains (PVDs) on the rate of consolidation is studied by two-dimensional finite element analysis. A transition zone lying between the highly disturbed smear and the undisturbed zones is considered in the analysis. The hydraulic conductivity in the transition zone is assumed to vary linearly from a low value in the smear zone to the original in situ value in the undisturbed zone. The analysis uses the actual band shape of the PVD and the rectangular shape of the unit cell. A parametric study is carried out to investigate the effects of the degree of soil disturbance, the size of the smear and the transition zones, the PVD spacing and the mandrel size and shape. Design guidelines, where the smear and transition zones are replaced by an expanded smear zone producing the same effect, are provided so that existing analytical solutions considering only the smear zone can be used for analysis and design. A comparison with experimental results shows that consideration of the transition zone is important for correct estimation of the degree of consolidation.     

涂抹区重叠竖井地基固结特性研究

在软基处理工程中,经常出现竖井打设变密而地基固结效率降低的现象。鉴于此,建立了重叠涂抹区内土体水平向渗透系数的分布函数,给出了涂抹区重叠时竖井地基超静孔压和平均固结度的解析解。通过分析不同工况下竖井地基固结度随竖井间距的变化情况,探究了竖井间距减小而地基固结效率不增反减的成因。最后,探讨了涂抹作用和井阻作用对竖井最小临界间距的影响。结果表明：相邻竖井涂抹区重叠是竖井地基中出现竖井最小临界间距的根本原因。涂抹作用越大,则竖井最小临界间距越大;具体表现为当地基扰动程度增大时或涂抹区半径增大时,竖井最小临界间距随之增大。井阻作用越大,则竖井最小临界间距越小;具体表现为当竖井渗透系数减小时、竖井长度增大时或竖井半径减小时,竖井最小临界间距随之减小。     

Trial Embankment Analysis to Predict Smear Zone Characteristics Induced by Prefabricated Vertical Drain Installation

In this study, FLAC finite difference software has been adopted to simulate the performance of the ground improved using prefabricated vertical drains assisted preloading, considering smear zone characteristics. The numerical code has been applied to predict smear zone properties employing a back calculation procedure using the results of several case studies. The construction of a trial embankment is proposed as a reliable method to predict the smear zone characteristics. The proposed back calculation method is applied to estimate the minimum required degree of consolidation and consequently the minimum required preloading time, resulting in a reliable estimation of the smear zone permeability and extent. Three preloading case studies considering both conventional preloading and vacuum assisted preloading have been simulated to verify the numerical code and to conduct the parametric study using the back calculation procedure. According to the results, the properties of the smear zone can be back-calculated reliably, when at least 33 % degree of consolidation due to trial embankment construction is achieved.     

A new simplified Hypothesis B method for calculating the consolidation settlement of ground improved by vertical drains

Vertical drains are widely used in soft ground improvements to accelerate the consolidation process. This paper develops a new simplified Hypothesis B method for calculating the consolidation settlement of a soil layer improved by vertical drains under the instant and ramp loadings. As a comparison, the traditional Hypothesis A method is also used to calculate the settlement. Then, a fully coupled finite element consolidation analysis is utilized to examine and verify this simplified method and Hypothesis A method. For the instant loading, Carrillo‐Barron method and Zhu‐Yin method are used to obtain the average degree of consolidation for vertical drain system. Typical parameters, such as over‐consolidation ratio (OCR), smear zone, and space ratio of vertical drains, are considered. It is found that the calculation results from the new simplified method in this study agree well with finite element simulations, and relative errors are in the range of 0.1% to 12.3%. Comparatively, there are obvious differences between the calculated results from Hypothesis A method and finite element results. Carrillo‐Olson method and Zhu‐Yin method are utilized to obtain the average degree of consolidation for the vertical drain system to consider the ramp loading. Equivalent time is determined from half of the construction period to calculate the creep compression under the ramp loading. The accuracy of this simplified Hypothesis B method using both Carrillo‐Olson method and Zhu‐Yin method is acceptable with the relative errors less than 9.4%.     

Analysis of settlement and lateral deformation of soft clay foundation beneath two full-scale embankments

In 1986, the Malaysian Highway Authority constructed a series of trial embankments on the Muar Plain (soft marine clay) with the aim of evaluating the effectiveness of various ground improvement techniques. This study investigates the effect of two such ground improvement schemes: (a) preloading of foundation with surface geogrids and synthetic vertical drains and (b) sand compaction piles. The paper is focused on the finite element analysis of settlements and lateral displacements of the soft foundation. In scheme (a), the numerical predictions are compared with the field measurements. In scheme (b), only the numerical analysis is presented and discussed in the absence of reliable measurements due to the malfunctioning of the electronic extensometer and inclinometer system during embankment construction. The current analysis employs critical state soil mechanics, and the deformations are predicted on the basis of the fully coupled (Biot) consolidation model. The vertical drain pattern is converted to equivalent drain walls to enable plane strain modelling, and the geogrids are simulated by linear interface slip elements. The effect of sand compaction piles is investigated considering both ideal drains and non-ideal drains, as well as varying the pile stiffness. © 1997 by John Wiley & Sons, Ltd.     

Three-dimensional finite element analysis of spatially variable PVD improved ground

A stochastic approach that investigates the effects of soil spatial variability on stabilisation of soft clay via prefabricated vertical drains (PVDs) is presented and discussed. The approach integrates the local average subdivision of random field theory with the Monte Carlo finite element (FE) technique. A special feature of the current study is the investigation of impact of spatial variability of soil permeability and volume compressibility in the smear zone as compared to that of the undisturbed zone, in conjunction with uncoupled three-dimensional FE analysis. A sensitivity analysis is also performed to identify the random variable that has the major contribution to the uncertainty of the degree of consolidation achieved via PVDs. The results of this study indicate that the spatial variability of soil properties has a significant impact on soil consolidation by PVDs; however, the spatial variability of soil properties in the smear zone has a dominating impact on soil consolidation by PVDs over that of the undisturbed zone. It is also found that soil volume compressibility has insignificant contribution to the degree of consolidation estimated by uncoupled stochastic analysis.     

砂井地基平面问题转换及其应用

考虑砂井地基的井阻和涂抹区效应，建立了轴对称砂井和平面砂墙之间渗透系数以及几何尺寸的关系式，使砂井地基等效成平面砂墙处理。采用修正剑桥模型的二维平面应变Biot(比奥特)固结有限元对一高速公路塑料排水板路基的沉降预测，显示结果与实测沉降结果比较接近。     

Analytical solutions for consolidation aided by vertical drains

Soil disturbance caused during the installation of vertical drains reduces the in situ hydraulic conductivity of soft deposits in the immediate vicinity of the drains, resulting in a slower rate of consolidation than would be expected in the absence of disturbance. Experimental investigations have revealed the existence of two distinct zones, a smear zone and a transition zone, within the disturbed zone around the vertical drain. The degree of change in the hydraulic conductivity in the smear and transition zones is difficult to assess without performing of laboratory tests. Based on the available literature, four different profiles of hydraulic conductivity versus distance from the vertical drain were identified. Closed-form solutions for the rate of consolidation for each of these four hydraulic conductivity profiles were developed. It is found that different variations of the hydraulic conductivity profiles in the disturbed zone result in different rates of consolidation.