塑料排水板联合真空预压的径向固结理论研究

基于椭圆柱排水体等效假定,结合真空度在塑料排水板内的线性损失规律,推导得出塑料排水板联合真空预压的径向固结解析解。基于所得解,对比分析塑料排水板的"形状效应"以及相关参数对地基固结的影响。进行实例验算,验证了解答的合理性。结果表明,在考虑塑料排水板的"形状效应"时,椭圆柱体等效法的效果最佳,且基于各周长等效法的计算结果与提出的椭圆柱等效结果较为接近;增大排水板的真空度衰减系数k_1和顶部真空负压p_0,可达到加快孔隙水的排出、加快地下水位的下降速率、降低最终地下水位线的效果;随着参数κ的增大,地基的整体固结速率减慢。     

基于排水体椭圆柱假定的井阻非线性竖井地基固结理论

 目前针对塑料排水板处理地基的固结计算均采用传统的砂井固结理论,即需要将扁长的塑料排水板等效为圆柱体之后再进行分析,这种简化过程可能会带来一定误差。为此,将塑料排水板视为与其形状较为接近的椭圆柱体进行研究。首先给出椭圆柱坐标体系下的塑料排水板处理地基固结基本方程,并基于椭圆柱坐标系理论得到超静孔隙水压和固结度的解析解,讨论新的解答与已有经典解答之间的关系,认为经典的排水板周长等效法高估了排水板的排水固结效果。此外,讨论了排水板渗透系数kw随深度线性衰减和随时间指数衰减变化时的固结解析解,通过计算讨论认为,渗透系数随时间的变化对排水板地基固结影响最显著,参数A2值越大,排水板地基固结速率越慢,以致于排水板严重淤堵而丧失排水能力。     

考虑变井阻的增压式真空预压固结解析模型及解答

增压式真空预压法是一种新兴的软土改良技术，目前对其固结理论的研究成果相对较少，基本处于起步阶段。基于等体积应变假设和达西定律，提出一种包含增压管和排水板的新型等效固结解析模型。首先，针对地基固结过程中排水板出现的淤堵问题，考虑排水板的变井阻效应，假定其渗透系数随时间指数衰减并随深度线性衰减；另外，考虑排水板涂抹作用以及土体内径–竖向渗流，推导了该模型的固结控制方程及其解析解。通过理论退化和比较研究，并与实际工程案例进行对比分析，验证了该解答的合理性。最后，通过参数敏感性分析对增压式真空预压固结特性进行研究，分析各种参数对固结性状的影响。结果表明：与传统真空固结解析模型相比，增压法能更有效地促进土体固结；考虑排水板的变井阻效应时，地基的固结进程将会减缓，且时间井阻因子αb的变化对地基后期固结速率的影响很大；单元内排水板布置越密集、土体泊松比越大、增压管影响区域越小或增压压力越大，均会导致地基固结加快。     

基于椭圆柱等效的排水板非线性径向固结理论

通过引入土体的e-lgσ'和e-lgk_h非线性模型,在椭圆柱等效模型的基础上,结合真空负压沿排水板深度的线性分布模式,分别采用差分算法和线性简化的方法推导得出径向固结的数值解和简化解析解。基于所得解,分别对比分析数值解与简化解、孔压定义的固结度U_p与变形定义的固结度U_s之间的差异,以及压缩指数与渗透指数的比值c(Cc/Ck)和真空荷载对固结性状的影响,最后将解答与试验结果进行对比,验证解答对固结度预测的适用性。结果表明:当c=1时数值解与简化解基本一致,当c1或c1时两者具有一定差距;在不同的c值下,同一时刻对应的U_s总比U_p大,随着c值的增大,土体固结变慢;对于由孔压消散描述的固结过程,当c1时,真空荷载越大,土体固结越快,当c=1时,真空荷载的变化对固结快慢无影响,而当c1时,真空荷载越大,土体固结越慢。     

真空预压–电渗联合作用下软黏土非线性大变形固结模型

基于分段线性差分法,建立轴对称真空预压–电渗耦合非线性大变形固结模型,可用于分析真空预压、电渗任意组合形式下的软黏土大变形固结问题。模型充分考虑固结过程中土性参数的非线性变化并采用FORTRAN编写计算程序,分别采用小变形解析解和大变形固结试验验证模型的准确性,模型在小变形情况下与现有解析解吻合,大变形情况下与室内试验实测值接近。开展真空预压与电渗不同组合形式下的工况分析,结果表明:真空预压–电渗联合作用的固结效果优于单一工法;不同真空预压与电渗组合形式下,土体最终平均沉降量接近,电渗滞后工况对降低能耗效果最好;为进一步降低能耗,工程中应将真空预压–电渗组合工况的固结度控制在80%以内。     

真空—堆载联合预压复合地基径向固结理论研究

针对真空―堆载联合预压复合地基径向固结问题,建立空间轴对称变形计算模型,采用位移法求出土体的变形和体积应变,给出真空―堆载联合预压复合地基径向固结解。通过退化分析和与已有解的比较研究,验证了此解的正确性和合理性。基于此解,通过算例计算,绘制出各参数对复合地基固结速率影响的曲线图。研究表明:桩径比n和扰动区影响因子s越小,复合地基固结速率越快;扰动区土体径向渗透系数越大,复合地基固结速率越快;桩土弹性模量比越大,复合地基固结速率越快;长细比越小,复合地基固结速率越快;径向时间因子越大,复合地基径向固结度越高。     

真空加载方式对排水板滤膜淤堵影响试验研究

塑料排水板滤膜的淤堵直接影响到真空预压排水固结法加固新近吹填淤泥的效果,而真空荷载加载方式是引起排水板滤膜淤堵的主要原因之一。通过室内模型试验,采用5种不同等效直径的排水板滤膜并通过控制不同的真空加载方式加固新近吹填淤泥。试验过程中对泥面沉降、排水板和土中不同深度的真空度进行监测,试验结束后测试排水板滤膜的渗透系数。试验结果表明:真空荷载不分级或者分级梯度太大,对真空预压排水固结法加固新近淤泥效果较差,排水板滤膜容易产生淤堵;排水板滤膜的等效孔径越小其淤堵情况越严重。排水板滤膜发生淤堵后其渗透系数将降低,对土体的排水固结产生不利的影响。作者建议:在真空预压处理新近吹填淤泥的工程实践中,宜选用较大的排水板滤膜等效孔,真空荷载宜采用分级加载方式,且每级真空梯度不宜过大。     

真空联合多级堆载下考虑井阻时空变化的竖井地基固结解析解

考虑到竖井地基固结过程中排水体井阻随时间和深度不断变化，基于等应变假定，针对堆载为多级加载及真空预压为单级加载的工况，引入竖井涂抹区渗透系数不同的变化模式，建立真空联合堆载预压下竖井地基固结模型，通过边界条件齐次化后设解的形式并解常微分方程获得固结模型对应的解析解。通过与已有的解析解及工程实测值对比，验证解析解的正确性并利用该解答开展固结性状分析。结果表明：变井阻理论更能够为最终负压沿深度线性衰减假定提供理论支撑；竖井排水能力衰减系数的增加及最终排水能力的减小都会使某时刻真空度随深度的衰减程度增大；竖井排水能力衰减系数越大，竖井固结速率越慢；最终排水能力越大，竖井固结速率越快；基于该固结模型在工前选取合适的真空加载值进行真空联合堆载预压能同时兼顾土体固结速率和经济效益。     

塑料排水板加固地基固结沉降特性研究

对采用塑料排水板加固软土地基的工程实例进行计算,比较了采用不同排水板间距、不同堆载预压时间、不同堆载高度时地基土层的总沉降、固结度、残余沉降情况,分析了不同地基加固参数对固结度和残余沉降的影响程度,并对地基加固参数的合理选取给出了建议。     

竖向排水井地基粘弹性固结解析理论

本文基于Merchant模型分别推导了自由应变条件和等应变条件下打穿软粘土层的竖向排水井地基粘弹性固结问题解析解,同时考虑了井阻和涂抹作用的影响.并将两种形式的解编制成相应的计算程序,据此对粘弹性参数的变化进行了大量的计算,探讨了竖向排水井地基粘弹性固结的一般规律.最后利用本文的解对杭甬高速公路试验段塑料排水板软基沉降进行了计算,结果表明粘弹性固结理论较现有线弹性固结理论更加接近实际.     

真空预压法加固软基三维有限元计算

为预测广州南沙港区某真空预压加固工程的效果与影响范围,针对真空预压加固大面积软基的变形特点,提出将单井涂抹效应对涂抹区内土体渗透性的弱化作用转化为减小单井影响区内土体水平向渗透系数的方法,既可减少三维有限元计算的前处理工作量,又可考虑砂井地基的涂抹效应,讨论了网格尺寸对固结度的影响。并视土体为多孔介质,采用比奥固结理论,结合依托工程,建立了较大规模的三维模型并进行有限元计算分析,计算结果与现场监测数据吻合得较好。     

真空-堆载联合预压加固软基简化计算方法

在深入了解真空预压机理和砂井地基固结理论的基础上 ,根据固结度等效的原则 ,推导了与单井固结理论等效的成层均质地基等效渗透系数 ,从而将复杂砂井地基转化为无砂井成层地基 ,以达到简化计算的目的 ,并结合真空-堆载联合预压的加固特点 ,提出一种简化的真空-堆载联合预压法的有限元计算方法 ,并结合工程实例对简化方法的可靠性进行了研究 ,通过与实测资料和常规砂井地基有限元计算值对比 ,表明该简化方法具有较高的准确性 ,可方便地应用于工程设计和实践。     

软土地基真空预压加固的有限元分析

以平面应变比对固结理论为基础,通过比较前人研究成果,选择考虑竖向排水体涂抹效应的砂井地基等效计算方法,根据真空预压加固软土地基的机理,运用有限元分析,提出一种真空预压计算方法。由于真空预压过程中地下水头将降低,因此在计算中采取降低地下水头的方式等效模拟地基表面施加的的真空度。结合工程实例给出算例,对该方法的可靠性进行研究。通过将计算结果同实测资料进行对比,验证了该方法的准确性。     

A simple solution for prefabricated vertical drain with surcharge preloading combined with vacuum consolidation

Prefabricated vertical drains (PVDs) with the surcharge preloading and vacuum consolidation has become considerably popular for ground improvement projects. A simple solution that incorporates the fundamental embankment features, such as the average degree of consolidation and excess pore pressure, are essential for the design of soft ground improvements by PVDs with vacuum preloading. However, most of the solutions for vertical drains with vacuum consolidation require numerical simulations, whose implementation tends to be laborious. In contrast, a simple solution for vacuum consolidation under time-dependent loading has not yet been proposed. In this study, a simple solution that can be easily incorporated into a conventional spreadsheet is derived for PVDs with vacuum preloading by applying the Laplace transform technique. The proposed solution accounts for several actual construction conditions, such as initial surcharge load, vacuum pump trial period, variations of radial permeability, and time-dependent loading. The results obtained from this proposed approach were validated with those from the finite element method and field data from the case study of the Cai Mep International Terminal project in southern Vietnam. The derived solutions, including the excess pore pressures and average degrees of consolidation, were in good agreement with the predicted and observed data.     

A pilot test on a membraneless vacuum preloading method

A membraneless vacuum preloading method is proposed in this paper for soft soil improvement. The method offers several advantages over the conventional vacuum preloading in which membrane is used to create the airtight condition and sand blanket layer to distribute vacuum. To assess the effectiveness of the proposed method, a pilot test was conducted at a land reclamation site in Tianjin, China. The ground settlement and the pore water pressure (PWP) at different elevations in soil were measured. After vacuum preloading, the average water content of the soft soils reduced by approximately 12% and the undrained shear strength increased twofold. The average degree of consolidation at the end of the vacuum preloading achieved 85.1% based on the settlement data and 84.5% based on the PWP data. The pilot test data have shown that the proposed method exhibits similar efficiencies to the conventional vacuum preloading method.     

真空预压法有限元计算比较

 按照固结度等效的原则,提出将单井涂抹效应对涂抹区内土体渗透性的弱化作用转化为减小单井影响区内土体水平向渗透系数的方法,并讨论单元尺寸对固结度的影响。该计算方法既可减少前处理工作量,又可考虑涂抹效应。结合广州南沙港一真空预压工程,建立三维模型进行有限元计算分析,并通过实测资料和目前常用的平面应变简化方法对计算值进行比较和讨论。研究结果表明,J. C. Chai方法将砂井地基转换为均质地基,所得结果偏保守,而“砂墙”等效方法通过砂井间距来调整渗透系数,会使计算结果偏大,三维有限元法比平面应变方法更能反映砂井地基的固结变形。     

Prediction of embankment behavior of regulating reservoir with foundation improved by vacuum consolidation method

The present paper addresses the numerical prediction of the behavior of a ground and a reservoir dyke with a retaining wall at the site of a regulating reservoir whose soft soil foundation is improved by using both the usual embankment preloading and vacuum consolidation. To evaluate the settlement, the lateral deformation, and the dissipation of pore pressure during vacuum preloading, a numerical analysis using an elasto-plastic FEM for soil–water coupled problems, incorporating the SYS Cam-clay model, is carried out in two dimensions. However, a change in the soil parameters during the vacuum preloading leads to a less accurate computation. To account for the uncertainties in the input parameters of the constitutive model for the improved ground, an inverse analysis approach is adopted. The particle filter is used to identify the compression index of the clay layers and the coefficient of permeability of the organic soil layer based on the measured settlement at the bottom of the preloading embankment during the vacuum consolidation. The reservoir dyke with a retaining wall is constructed on an improved foundation after removing the preloading embankment, and an attempt is made to predict its performance after construction by an elasto-plastic FEM for soil–water coupled problems using the identified parameters.     

试抽试验在真空预压软基中的应用

真空预压法是处理软土路基常用方法,一直广泛应用于软土路基工程。长期以来,国内学者主要研究真空预压法固结模型、数值及相似模型试验,很少关注真空预压设备的密封性、真空泵的数量、真空度及沉降,而上述特征恰是建设方、施工方关注的重点。以某高铁铁路路基试验段为依托,综合考虑经济、安全因素,采用真空试抽试验反馈分析了真空泵数量与膜下真空度关系、淤泥土层中真空度演变规律及断面沉降演化规律,提出了确保工程安全稳定的建议。其分析结果为后续路基稳定性研究奠定了基础。     

Experimental study on a dredged fill ground improved by a two-stage vacuum preloading method

The vacuum preloading method has been wisely chosen among many ground-improvement methods considering the time limit of many projects and the characteristics of reclaimed soil. However, the loss in vacuum with soil depth, the clogging around prefabricated vertical drains (PVDs), and the deteriorative consolidation of the deep soil layer, among other factors, create a large challenge to vacuum preloading for dredged marine clay fill. Thus, this study proposes a two-stage vacuum preloading method and focuses on its feasibility and effectiveness. Contrasting laboratory tests are performed in two identical experimental tanks with dredged marine clay fill from the Wenzhou land reclamation site in China. In one tank, the one-stage vacuum preloading method is used to serve as a baseline for this study. In the other tank, use of the two-stage vacuum preloading method is proposed for consolidation; it comprises two stages. In the first stage, the dredged marine clay fill is conditioned by vacuum preloading using half of the PVDs, where the dissipation of the excess pore water pressure tends to be steady. In the second stage, vacuum preloading is activated using all the PVDs. The results show that a better consolidation effect is achieved with the proposed method in terms of the settlement, vacuum pressure, pore water pressure, water content, vane shear strength, and soil particle microstructure after soil consolidation.     

软土地基孔隙水压力降低引起的压缩分析

根据土力学基本原理,分析孔隙水压力降低引起的土体压缩方式,并对真空预压地基的分层沉降现场实测值和理论计算值进行分析;同时,定量分析真空预压地基在不同深处单位压缩量。研究结果表明:(1)若按有效应力原理水土压力分开计算、地基土体中孔隙水压力降低时,土体竖向总应力维持不变而侧向总应力减小;(2)土体中孔隙水压力在相对压强小于0的范围内降低时,将引起土体等向压缩,在相对压强大于0的范围内降低时,将引起土体单向压缩;(3)南沙港区的真空预压地基若其沉降按欠固结计算时,计算值与实测值接近,自重应力欠固结引起的沉降是抽真空期间地基总沉降主要部分;(4)真空预压的加固深度随排水体深度增加而增大,土层压密效果随地层深度增加而减弱,故降低孔隙水压力法在加固软土地基中,设计排水体深度时应考虑最佳加固深度。