软土的弹塑性损伤模型在BiOt固结有限元中应用

在Biot固结理论基础上，引入弹塑性损伤本构关系，建立了固结分析有限元方程。对某试验路堤进行了损伤模型计算与弹塑性模型计算的结果比较。结果表明：损伤模型的计算结果大于弹塑性模型的计算结果；损伤随着加荷时间的增长而增大；路基中心线和路基坡脚下的土损伤程度较大。     

面向对象方法在Biot固结有限元程序中的应用

面向对象程序设计方法应用于有限元程序编制带来了相当大的优越性。但迄今为止，这种方法在岩土工程中的应用实例很少。至于用面向对象方法进行土力学中常见的Biot固结有限元分析软件的开发，尚未见报道。现将面向对象有限元方法引入Biot固结有限元程序编制中，用C＋＋语言编制中一个Biot固结有限元程序实例。和结构化程序设计方法相比较，采用面向对象方法进行Biot固结有限元程序的编制，使程序的可维护性、可再生性、可扩充性和程序开发的效率都得到了很大的提高。指出将面向对象程序设计方法引入了Biot固结有限元程序的编制等岩土工程的数值计算领域，是今后的发展方向。     

真空-堆载预压处理高速公路软基的有限元计算

根据高速公路路基的变形特点，提出了真空-堆载联合预压法处理软基的三维Biot固结有限元计算模型，将砂井按照实际尺寸划分单元，避免了平面计算方法中的砂井处理的问题，较以往的平面数值方法更接近实际情况。结合修正剑桥模型，对某真空-堆载联合预压试验段进行了计算、分析，结果令人满意。     

竖井地基的粘弹—粘塑性固结及有限元解

用1个带双屈服面的粘弹-粘塑性模型来描述软土的流变性状，并结合Biot固结理论对软土地基的固结沉降进行有限元分析。对竖井预固结地基进行计算，以预压所产生的沉降量及所引起的附加有效应力分别大于设计载荷所引起的最终沉降量和附加应力为终止预压的依据，计算取得了合理结果。     

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。     

地基固结沉降随机有限元计算和可靠度分析

基于比奥（Biot）固结理论，采用邓肯-张本构模型，考虑随机场的影响，首次给出了地基固结沉降随机有限元分析方法，并基于随机有限元对点固结沉降和差异固结沉降的可靠度进行了分析。     

真空预压侧向变形影响因素的研究

依据大量工程实测资料以及以比奥 (Biot)固结理论为基础的有限元分析结果 ,对固结系数、排水板打设间距、真空度、预压面积等因素对采用真空预压处理地基的侧向变形的影响进行了研究 ,并通过分析证明了巴隆轴对称固结理论单井假设的合理性     

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。     

广州饱和软土固结过程微孔隙变化的试验分析

软土的孔隙特征及其变化规律对软土的压缩性和渗透性有重大的影响,软土孔隙特征随荷载变化规律的试验研究为揭示软土排水固结过程与变形规律,建立基于孔隙压缩规律和渗流机制的排水固结模型提供有力依据。利用美国全自动压汞仪,对不同压力下固结的广州番禺淤泥土样的孔隙及其尺度分布进行测试,根据测试结果对土样孔隙尺度分布特征及其随固结压力的变化规律给出了定量分析。试验结果表明：淤泥土中介于0.4～2.5 μm范围的颗粒间及团粒内的小孔隙所占比例最大,而大于10 μm的大孔隙和小于30 nm的超微孔隙所占比例均很小;较大的孔隙更易于被压缩而湮灭或被分裂成微小孔隙;固结压力将显著改变淤泥土的孔隙尺度及其分布特征,以致改变土体的压缩性和渗透性。在固结前期( 200 kPa)孔隙尺度较大,压缩系数和渗透系数较大并随固结压力增加而快速减小;在固结后期( 200 kPa)孔隙尺度小,压缩系数和渗透系数小,且随固结压力增加的变化趋于平缓。     

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。     

未打穿散体材料桩复合地基固结简化解研究

利用平均固结度普遍解,将未打穿散体材料桩复合地基转化为等效双层地基,推导出未打穿散体材料桩复合地基固结简化解,给出其平均固结度计算简化公式。根据简化解、现有解和有限单元法编制程序,绘制考虑贯入比、桩体渗透系数、土体扰动区渗透系数和土体扰动区影响因子等因素对固结的影响曲线图,分析了各因素对未打穿散体材料桩复合地基固结性状的影响,并对3种计算方法所求出的平均固结度进行了比较研究,进而探讨了简化解的适用性。研究结果表明,无论是在单面排水还是在双面排水条件下,简化解求出的平均固结度总体上是介于现有解和有限单元法二者之间,尤其在固结过程的“中后期”（即固结度≥50%时）。从实用角度来看,简化解的计算结果具有足够精度,且简便、实用;在各种影响因素中,散体材料桩长度和桩体渗透系数对固结过程的影响最大。     

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.     

Finite element analyses of negative skin friction on a single pile

This paper presents finite element analyses of negative skin friction on a single pile under various conditions. Negative skin friction is a common problem if a pile is designed in a highly compressible soil. There are two most important parameters in estimating the load caused by negative skin friction: (1) the distribution and magnitude of skin friction and (2) the location of the neutral plane. The neutral plane is the location where the pile and soil settle the same amount or have no relative displacement. Negative skin friction is a very complex phenomenon influenced by many factors. In this paper, a two-dimensional axisymmetric model is built in the finite element program, ABAQUS. The model is first verified with a known case history. A systematic parametric analysis is performed to investigate the influence on both the neutral plane and the magnitude and distribution of negative skin friction along the pile length of various influencing factors, including the consolidation time, the properties of pile/soil interface, the lateral earth pressure coefficient, pile-soil limiting displacement, the intensity of surcharge, and soil stiffness. Based on the analyses, it is found that the location of the neutral plane is significantly influenced by the consolidation time and the stiffness of bearing layer. The distribution and magnitude of negative skin friction is influenced mainly by the pile/soil interface, soil compressibility, and the surcharge intensity. Based on the field measurements from literature and this investigation, a simple design procedure is proposed for estimating the pile load caused by negative skin friction.     

Thin flow element and the problem of ill-conditioning

The problem of ill-conditioning in anisotropic and heterogeneous flow regions using plane and axisymmetric conditions is studied. A new concept, conditioning ratio, which covers all the possible factors significantly affecting the problem of ill-conditioning (e.g. aspect ratio, heterogeneity and anisotropy) is proposed. It has been shown that there is an upper limit for this conditioning ratio, beyond which the stiffness matrix of the modelled region will be ill-conditioned. Using this limiting value and considering the fact that flow behaviour in a single fracture is, basically, independent of the value of permeability normal to the plane of the fracture, k₁, a criterion is established to calculate the value of k₁ for thin elements. By adopting this criterion the problem of ill-conditioning could be avoided, regardless of the value of the aspect ratio. The reliability of the proposed criterion is examined by using thin elements with very large aspect ratio (up to 10⁶) in modelling the pumping problem from a well fully intersecting a vertical fracture. The comparison of the numerical results with the available analytical solutions is found to be very satisfactory.     

A finite element–based approach for predictions of rigid pile group stiffness efficiency in clays

This paper presents a finite element parametric study of several variables that affect the stiffness efficiency of rigidly capped pile groups with a view to developing a solution for preliminary design purposes. Previous empirical solutions from linear elastic work had identified a significant dependence of stiffness efficiency on pile group size and group spacing, and in this study, the effect of the pile length-to-diameter ratio, the compressibility of a stiff bearing stratum beneath the pile group and the depth below ground level to the stiff bearing stratum are also considered. Pile groups in a soft clay/silt are modelled using PLAXIS 3D Foundation in conjunction with a soil model that captures the stress dependency of soil stiffness. The trends from the soft soil study have been formulated into a set of equations which can be used to predict the stiffness efficiency of pile groups. This new approach captures more variables than previous simpler empirical prediction methods and performs better when applied to a database of 29 published pile group case histories.     

Consolidation analysis for partly saturated clay by using an elastic–plastic effective stress–strain model

The theory of consolidation is extended to partly saturated clay soils, and formulated for finite element analyses. This formulation couples the effects of both stress and flow. It takes account of variations of this permeability of the soil and compressibility of the pore fluid with changes in void ratio, and the non-linear stress–strain behaviour of soil. The Cam Clay model is revised to model the stress–strain behaviour of compacted soils. The compressibility of pore fluid is derived using Boyle's Law and Henry's Law, taking into account the effect of surface tension. An empirical equation is developed for permeability of pore fluid. An example of settlement of a footing on partly saturated soil is described and discussed.     

条形埋置基础固结分析计算图表

基于比奥固结理论,用有限元方法研究了有限饱和土层中条形埋置基础的固结沉降特性。仔细分析了影响固结度的3个参数：基础埋深与土层厚度的比值;土层厚度与基础宽度的比值;土骨架的泊松比。根据分析结果,提出了归一化无量纲时间,并给出了计算条形埋置基础固结度的计算图表。这些图表为基础设计提供了便捷准确的计算方法。运用文中图表计算固结度的最大误差不超过3 %。     

An expression for the permeability of anisotropic granular media

There are many expressions proposed for the permeability of isotropic media based on flow channel and pore size distribution concepts, but there are no such expressions for anisotropic media. In this paper an expression for the permeability of an anisotropic medium is proposed, which has been verified in the laboratory. The mechanism behind fluid flow through soil was investigated using microscopic computer simulations to propose an expression for macroscopic permeability. The soil was assumed to be a spatially periodic porous medium, and the Navier-Stokes equation was solved using the FEM with appropriate boundary conditions for several different arrangements of the porous medium. The basic variables influencing flow through soil at the microscopic level were identified as specific surface area, void ratio, particle shape, material heterogeneity and the arrangement of particles in a porous medium. A sensitivity analysis was carried out to obtain an expression for the permeability in terms of the above variables. The corresponding macroscopic variables for the above microscopic variables are average specific surface area, average void ratio, anisotropy, tortuosity due to material heterogeneity, and the arrangement of particles respectively. An expression for the directional permeability is proposed in terms of these variables for the most common occurrence of particles in a porous medium. For the verification of the proposed equation, the permeability values of a fine-grained sand were measured at different void ratios and were compared with those predicted by the proposed equation. The results show that the predicted permeability values from the proposed equation are very close to the measured values.     

干湿-冻融循环条件下膨胀土的压缩及渗透特性变化规律

北疆供水一期工程穿越膨胀土区域,历经多次干湿-冻融循环后力学特性衰减严重,易产生渠坡局部浅层滑坡和冻胀破坏等现象。为深入研究其劣化机制,通过干湿-冻融循环条件下的压缩试验、渗透试验和SEM微观扫描试验,从宏-细-微观多角度分析其压缩和渗透指标的变化规律。研究结果表明：随干湿-冻融循环次数的增加,膨胀土整体压缩性增大,其压缩曲线可分为拟弹性段与拟塑性段;随循环次数的增加,回弹指数呈波动趋势,压缩指数与循环次数呈指数正相关,与细-微观裂隙呈线性正相关。黏土颗粒在循环作用下组成“团聚体-孔隙-填充颗粒”形式的较松散的临时结构,絮凝结构增加,各向异性减少;土样承受竖向压力时,膨胀土孔隙间距减少,压缩性较大;压力超过固结屈服应力时,团聚体颗粒扁角化、极角频率增加、孔隙压密,压缩性逐渐稳定。渗透系数在循环过程中变化分为缓慢、迅速、稳定3个阶段;渗透系数在第5次循环变化较大,第7次循环后逐渐稳定,与循环次数及表面裂隙率呈正相关趋势。渗透系数与各项微观参数的灰色关联度均大于0.65,微观孔隙率是最主要的影响因素;循环作用下微观孔隙发育明显,形成新渗流通道,渗透系数与微观孔隙率呈线性正相关。     

不同粒径级砂土渗透特性试验研究

渗透性是砂土的重要工程性质之一,影响砂土渗透性的因素有很多,比如土体密实程度、土颗粒自身特性、流体性质等,其中孔隙率与颗粒粒径是两个重要影响因素。而以往基于混合粒径的天然砂土的研究很难分别对这两个因素进行独立的研究。基于此,首先开展了单一粒径级砂土的常水头渗透试验,分别研究了同一粒径级砂土渗透系数随孔隙率的变化和同一孔隙率下不同粒径级砂土渗透系数随均值粒径的变化规律。试验结果显示,渗透系数随着孔隙率的增加而线性增加、随均值粒径二次方的增加而线性增加,其中均值粒径的影响较大,其变化能导致渗透系数量级上的差异。在此基础上,开展了多粒径混合砂土的渗透试验,讨论了曲率系数、不均匀系数等级配参数对渗透性的影响,从而将单一粒径级的研究成果推广到天然砂土,最终拟合出渗透系数与相关影响因素的经验公式,以便工程实践参考使用。