上海浅部黏土渗透系数及其各向异性

为研究上海浅部黏土的渗透系数及其各向异性,沿水平及竖直方向对上海浅部主要黏土层进行了不同压力下的固结试验,联合使用时间平方根法和时间对数法获取了试样的渗透系数,探讨了渗透系数与孔隙比及渗流方向之间的关系,并通过电镜扫描,从微观角度分析了水平和竖直方向渗透系数存在差异的原因。研究结果表明：上海浅部主要黏土层的渗透系数均随着孔隙比的增加而增大。对于单组试验来说,渗透系数k与孔隙比e在e lgk坐标系中呈现很好的线性关系,且渗透变化指数Ck大致呈现Ck = 0.5e0的规律;但对于整体试验而言,渗透系数与孔隙比在e lgk坐标系中大致呈现出曲线关系。通过扫描电镜观察,揭示了沉积形成的絮状微观结构是竖直和水平渗透系数差异较小的主要原因。     

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。     

NaCl溶液饱和膨胀土的压缩特性及其微观机制

孔隙溶液浓度及组份改变会影响膨胀土颗粒间作用力,改变微观孔隙结构,从而影响土体的物理力学特性。为此,以宁明膨胀土为研究对象,采用不同浓度NaCl溶液制备泥浆预固结重塑样开展一维压缩试验和压汞试验,研究化学作用对重塑天然膨胀土的压缩性和微观孔隙结构的影响规律。结果表明：随着渗透吸力增加,颗粒间水化能力降低,物理化学力使土颗粒由分散状态转变为集聚体状态,形成了集聚体内孔和集聚体间孔,土体表现出双峰孔隙分布特征。预固结样（压力为20 kPa）的初始孔隙比随渗透吸力增加而减小,进而导致固结屈服应力增加;但是渗透吸力对压缩性影响不大,压缩指数和回弹指数基本不变。此外,利用固结系数计算了土体的渗透系数,随着竖向压力增加渗透系数降低;当竖向压力小于200 kPa时,随着渗透吸力增加,渗透系数先增加后减小,但是竖向压力超过200 kPa后,渗透系数变化不大。分析发现,渗透吸力增加导致大孔隙增加,渗透系数增加,但同时密实度增加会导致渗透系数降低,低竖向压力下渗透性受密实度和微观孔隙结构变化耦合作用控制。     

Nonlinear consolidation in randomly heterogeneous highly compressible aquitards

Severe land subsidence due to groundwater extraction may occur in multiaquifer systems where highly compressible aquitards are present. The highly compressible nature of the aquitards leads to nonlinear consolidation where the groundwater flow parameters are stress-dependent. The case is further complicated by the heterogeneity of the hydrogeologic and geotechnical properties of the aquitards. The effect of realistic vertical heterogeneity of hydrogeologic and geotechnical parameters on the consolidation of highly compressible aquitards is investigated by means of one-dimensional Monte Carlo numerical simulations where the lower boundary represents the effect of an instant drop in hydraulic head due to groundwater pumping. Two thousand realizations are generated for each of the following parameters: hydraulic conductivity (K), compression index (C _c), void ratio (e) and m (an empirical parameter relating hydraulic conductivity and void ratio). The correlation structure, the mean and the variance for each parameter were obtained from a literature review about field studies in the lacustrine sediments of Mexico City. The results indicate that among the parameters considered, random K has the largest effect on the ensemble average behavior of the system when compared to a nonlinear consolidation model with deterministic initial parameters. The deterministic solution underestimates the ensemble average of total settlement when initial K is random. In addition, random K leads to the largest variance (and therefore largest uncertainty) of total settlement, groundwater flux and time to reach steady-state conditions.     

高岭-蒙脱混合黏土渗透各向异性模型研究

完善的渗透各向异性模型有助于指导地下建筑施工期和运营期的排水注浆方案的选定,为了建立完善的渗透各向异性模型,利用三轴渗透仪对高岭-蒙脱混合黏土开展一系列渗透试验,并从宏观和微观两个角度尝试建立渗透各向异性模型。研究结果表明：随着膨润土掺入量增加,混合黏土中小孔隙含量不变,中孔隙含量增多,而大孔隙含量减少;随着固结应力的增大,混合黏土中孔隙均减少,且孔隙最大孔径减小;宏观参数孔隙比和液限孔隙比与渗透各向异性比之间的规律性较弱,说明孔隙比和液限孔隙比不是决定渗透各向异性比的关键参数;用微观参数表述的渗透各向异性模型有较强的规律性,说明从微观角度探究渗透各向异性模型不仅可行,而且更能揭示混合黏土渗透各向异性的本质,且微观参数模型中,用能反映较大孔隙变化的孔径指数 ～ 表述的渗透各向异性模型相关性最好。     

Radial consolidation with variable compressibility and permeability following pile installation

The excess pore pressure generated by pile installation gradually dissipates following installation, which mainly dominates the increase of pile capacity with time. The dissipation of the excess pore pressure following pile installation has been widely investigated by experimental and theoretical studies. However, in most research, the consolidation coefficients were assumed to be constant for simplification, which may lead to errors. In the present study, the dissipation of the excess pore pressure is analyzed by radial consolidation theory with a variable consolidation coefficient based on the linear responses of e − ln(p^′) and e − log(k). The governing equation of the radial consolidation considering variable compressibility and permeability are solved by the variable separation method. Bessel functions are used to solve the differential functions for the time independent part. The excess pore pressure immediately after pile installation is obtained from cavity expansion theory and is used as the initial condition of the consolidation process. Moreover, the influences of variable compressibility and permeability on the variation of consolidation coefficient and excess pore pressure dissipation are analyzed.     

Frequency-dependent dynamic behavior of a poroviscoelastic soil layer under cyclic loading

In this study, a linear poroviscoelastic model based on the Biot theory is proposed to analyze the dynamic response of partially saturated soil. Both the flow-dependent and flow-independent poroviscoelastic behaviors are described in the proposed model. The compressibilities of both the constituents, including the skeleton material and porefluid, are considered, and the effective skeleton stress is determined using the linear viscoelasticity law based on the generalized spring-dashpot model. The soil surface is subjected to two types of harmonic loading, namely, vertical compressive loading and lateral shear loading. The influences of the relaxation time, saturation degree, and soil permeability on the surface and interior dynamic responses of the soil layer are investigated. It is revealed that the amplification factor, effective stress, and pore pressure decrease as the viscous damping increases, indicating that neglecting the viscoelastic property of solid skeleton could overestimate the induced dynamic responses. Furthermore, in comparison with the viscous damping of the solid skeleton, the viscous coupling involving the viscous resisting forces between the solid skeleton and pore fluid has limited effect on the dynamic behavior of the soil layer.     

A constitutive model for unsaturated soils with consideration of inter-particle bonding

The paper presents a physically-based constitutive model for unsaturated soils that considers the bonding effect of water menisci at inter-particle contacts. A bonding factor has been used to represent the magnitude of the equivalent bonding stress, defined as the bonding force per unit cross-sectional area. The average skeleton stress is employed to represent the effect of average fluid pressures within soil pores. Based on an empirical relationship between the bonding factor ζ and the ratio e/e_s (where e and e_s are void ratios at unsaturated and saturated states, respectively, at the same average skeleton stress), we propose an elasto-plastic constitutive model for isotropic stress states, and then extend this model to triaxial stress states within the framework of critical state soil mechanics. Because only one yield surface is needed in the proposed model, a relatively small number of parameters are required. Comparisons between experimental data and model results show that, in most cases, the proposed model can reasonably capture the important features of unsaturated soil behavior.     

软土场地地铁车站结构地震反应特性振动台模型试验

作为地铁车站结构地震破坏机制大型振动台系列模型试验之一,开展了近、远场强地震动作用下软弱粉质黏土场地框架式地铁车站结构体系的大型振动台模型试验。测试分析了模型地基的加速度、孔压、地表震陷和模型结构的加速度、应变、水平位移反应等。结果表明：地震波在模型地基的传播过程中呈现出自下而上低频增大、高频减小的现象;强地震动作用下模型地基的基频明显降低,呈现出明显的低频聚集（放大）、高频滤波效应;模型地基的孔压比增长较小,在不同特性地震动作用下模型地基孔压比的发展过程存在较大的差异,并显示出显著的空间效应;近、远场地震动作用下模型地下结构的加速度反应存在明显差异,模型地下结构对软土地基地震动加速度反应的影响具有显著的空间效应;模型地下结构相对变形小、未出现明显上浮现象,地震动频谱特性对其侧墙的变形模式和大小存在显著的影响;模型结构中柱为地震损伤最严重部位;模型结构整体损伤情况较轻、处于非破坏状态。     

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.     

Fractures, convection and underpressure: hydrogeology on the southern margin of the Piceance basin, west-central Colorado, USA

Grand Mesa is an erosional remnant on the southern margin of the Piceance basin (Colorado, USA) that appears to host topographically driven groundwater flow in low permeability strata via a pervasive network of vertical extensional fractures. The vertical fractures cut more than 1 km of clay-rich lithology ranging in age from Upper Cretaceous through Eocene, and likely formed from horizontal dilation, cooling, and erosional unloading associated with 2.8 km of regional uplift and 1.5 km of incision by the Gunnison and Colorado rivers. The vertical fractures create anisotropy in which vertical permeability exceeds horizontal permeability. This enhances vertical flow and depth of penetration of groundwater, favors local flow regimes over regional flow, and results in groundwater temperatures that are elevated by up to 10°C over mean surface temperatures at the location of springs. The uplift and cooling that formed the fractures may also have produced domains of abnormally underpressured pore fluids and natural gas within blocks of low permeability bedrock bounded by the fractures. Pore pressures inside these blocks may be in disequilibrium with the groundwater flow system due to ongoing stress release, and the long time scales required for pressure equilibration in the low permeability strata.     

Nonlinear consolidation of vertical drains with coupled radial-vertical flow considering time and depth dependent vacuum pressure

The system of vacuum pressure combined with vertical drains to accelerate soil consolidation is one of the most effective ground improvement methods. The consolidation theories of soft soil improved by vertical drains including void ratio–dependent compressibility and permeability have been widely applied in practice to predict the consolidation behavior. In this paper, analytical solutions of the consolidation of vertical drains are derived incorporating the loss and propagating stage of vacuum pressure. In addition, special solutions are obtained for the cases of instantaneous surcharge loading and staged surcharge loading, based on the general solution. The solution is verified by ignoring the propagating stage of vacuum pressure formation and comparing it with an existing solution. The effects of vacuum pressure loss and propagating stage combined with other parameters are investigated through the ratio between excess pore water pressure and surcharge loading.     

On the Use of Skempton’s Compression Index Equation

In this study, an alternate approach to establish the e-log p relationships for clayey soils within a vertical pressure range of 10–1,000 kPa is discussed. Skempton’s compression index equation correlating the liquid limit, w _L, and the compression index, C _c, and the reported equation correlating the void ratio at liquid limit, e _L, and the void ratio at a vertical pressure of 100 kPa, e ₁₀₀, by Burland (1990), were used to establish the e-log p relationships for several reconstituted normally consolidated clayey soils. Consolidation test results of 13 clayey soils covering a sufficiently wide range of liquid limit were selected from the literature. Also, consolidation tests were carried out on two highly expansive soils in this study. A comparison of the experimental consolidation test results with the calculated e-log p relationships in the current study indicated that in general, the agreements between the calculated relationships and the experimental results are good. The agreements were found to be slightly better for soils with liquid limits less than about 70%. A comparison of the calculated e-log p relationships in the current study with that determined following methods suggested by Nagaraj and Srinivasa Murthy (1983) and Burland (1990) showed that all the three methods yielded very similar results for soils with liquid limit less than 70%. For soils with liquid limits greater than 70%, the difference between the e-log p relationships calculated in this study and that following Burland (1990)’s method was insignificant, whereas Nagaraj and Srinivasa Murthy (1983)’s method slightly over-predicted the void ratios at larger vertical stresses.     

Analytical solution for two-dimensional dynamic consolidation in frequency domain

An analytical solution to the two-dimensional wave propagation in fluid-saturated half-space subjected to a strip load with vertical harmonic oscillation at the surface is presented. The basic equations have been derived on the basis of Biot's linear theory of poro-elasticity and then solved using Fourier complex transform for the horizontal direction. The importance of a number of soil characteristics including compressibility, degree of saturation and soil permeability has been examined. It is shown that the effect of pore fluid is dominant only for fully saturated soils with incompressible solid grains and low permeability. For partially saturated, compressible or very permeable soils, the stresses would be mainly transferred to solid part and there will be considerable reduction in pore pressure amplitude.     

锌离子污染对膨润土一维压缩特性影响试验研究

为研究锌离子对膨润土重塑土压缩特性的影响,对人工制备的受不同浓度Zn2+污染的高含水率膨润土开展一维压缩、自由膨胀率、物理化学性质和扫描电镜试验。试验结果表明,在竖向压力低于某临界值（200 kPa）时,相同压力下随着Zn2+浓度增大,膨润土的孔隙比和压缩指数均降低,但当Zn2+浓度高于某临界值（0.02 mol/L）时,继续增加污染浓度膨润土的孔隙比基本保持不变,压缩指数略有降低。当竖向压力超过该临界值（200 kPa）后,膨润土的孔隙比基本不受孔隙液物理化学性质影响,仅由竖向压力决定。此外,随着孔隙液Zn2+浓度增大,膨润土的孔隙液pH值和土样比表面积减小,颗粒直径增大,膨润土中蜂窝状结构遭到破坏,黏土矿物片逐渐团聚成大直径颗粒,小孔隙消失,生成一些大的团粒间孔隙。     

可液化场地碎石桩复合地基地震动力响应分析

采用砂土液化大变形弹塑性本构模型分析可液化砂土,采用模量随应力与应变变化的等效非线性模型增量形式分析碎石桩,应用FLAC3D有限差分软件对地震动力作用下可液化场地碎石桩复合地基进行三维动力响应分析。模拟分析了在地震作用下碎石桩刚度效应和排水效应对加固处理可液化场地的抗液化效果,从初始小变形到液化后大变形的变形发展,超静孔压累积与消散,及桩与土的变形与应力分配变化等。结果表明,所用模型与方法可合理描述可液化场地碎石桩复合地基在地震作用下场地的动力响应特性和抗液化效果;在地震作用下可液化场地中桩周土体与碎石桩体的竖向应力与水平向剪切应力向碎石桩体集中,竖向有效应力比可降至约1/6～1/3;桩周土体与桩体为非协调变形,剪应变比可达7～10;碎石桩抗液化影响范围约为2.5～3倍桩径,对超过3.5倍桩径范围影响较小;碎石桩与砂土渗透系数比大于100时对降低砂土中超静孔隙水压影响明显;碎石桩对场地的加密效应可显著降低超静孔隙水压力,而碎石桩刚度则对超静孔隙水压力变动影响较小,但有助于减低地面加速度响应峰值。     

Novel procedure for the estimation of swelling pressures of compacted bentonites based on diffuse double layer theory

Bentonite clays are proven to be attractive as buffer and backfill material in high-level nuclear waste repositories around the world. A quick estimation of swelling pressures of the compacted bentonites for different clay–water–electrolyte interactions is essential in the design of buffer and backfill materials. The theoretical studies on the swelling behavior of bentonites are based on diffuse double layer (DDL) theory. To establish theoretical relationship between void ratio and swelling pressure (e versus P), evaluation of elliptic integral and inverse analysis are unavoidable. In this paper, a novel procedure is presented to establish theoretical relationship of e versus P based on the Gouy–Chapman method. The proposed procedure establishes a unique relationship between electric potentials of interacting and non-interacting diffuse clay–water–electrolyte systems. A procedure is, thus, proposed to deduce the relation between swelling pressures and void ratio from the established relation between electric potentials. This approach is simple and alleviates the need for elliptic integral evaluation and also the inverse analysis. Further, application of the proposed approach to estimate swelling pressures of four compacted bentonites, for example, MX 80, Febex, Montigel and Kunigel V1, at different dry densities, shows that the method is very simple and predicts solutions with very good accuracy. Moreover, the proposed procedure provides continuous distributions of e versus P and thus it is computationally efficient when compared with the existing techniques.     

低渗岩石非线性渗流机理讨论

回顾了单相液体在低渗透饱和岩石的非线性渗流研究某些进展。从岩石孔隙结构、岩石与流体之间的相互作用和流体性质方面介绍了对低速非线性渗流产生机理的不同观点,探讨了孔隙大小、孔喉对启动压力的影响,讨论了界面张力、边界层、耦合渗流、非牛顿流体对非线性渗流形成的作用机制,并对进一步开展低渗透岩石渗流研究提出了建议。     

非连续变形分析方法中的阻尼问题研究

定量研究了非连续变形分析(DDA)方法中的黏性阻尼与数值阻尼。首先,基于Newmark直接积分法,推导了块体系统的运动方程。其次,通过动力学中的黏性阻尼理论建立了DDA中动力系数、时间步长与黏性阻尼比的关系式,探讨了DDA中的常加速度积分方案的数值阻尼分区及阻尼比计算方法,进而得到两种阻尼共同作用时的阻尼比表达式,并分析了频域内阻尼比的分布情况。最后,以谐振激励下的块体振动为例,通过对比不同阻尼作用下块体位移的DDA计算值与理论解,验证了本文提出的阻尼比计算公式的正确性。研究表明：黏性阻尼对低频的衰减作用明显,数值阻尼则可以很快地消除高频干扰,而二者共同作用下可降低阻尼的频率相关性。该研究成果为DDA的振动、波动等动力计算的阻尼取值提供了理论依据。