Microstructures within and outside the smear zones for soft clay improvement using PVD only,Vacuum-PVD,Thermo-PVD and Thermo-Vacuum-PVD

Previous investigators have found increasing rates odf consolidation with increasing degrees of soft clay improvement using PVD only, Vacuum-PVD, Thermo-PVD and Thermo-Vacuum-PVD). This paper utilized scanning electron microscope (SEM) to evaluate and compare the microstructures of the clay specimens obtained from within the smear zone of both undisturbed and reconstituted samples in small consolidometer as well from within and outside the smear zone of reconstituted samples in large consolidometer. Before improvement, both reconstituted and undisturbed specimens showed anisotropic microstructures. In reconstituted specimen, face to face preferred orientations are revealed parallel to the horizontal plane. Meanwhile, in the undisturbed specimen, some degree of random edge to face mixed with face to face orientations were displayed. After improvement, the microstructures of Thermo-Vacuum-PVD revealed high levels of edge to face orientation in the vertically dominated smear zone, followed by Thermo-PVD, Vacuum-PVD, and PVD only. Moreover, the microstructures of specimens in the horizontally dominated outside of smear zone exhibited mainly face to face orientation and progressively mixed with edge to face orientation. The increasing intensities of edge to face microstructures were successfully correlated with increasing flow parameters and measured shear strengths corresponding to the increasing levels of improvement.     

PVD improvement combined with surcharge and vacuum preloading including simulations

This paper presents the study of PVD improved reconstituted specimen with and without vacuum preloading on large-scale consolidometer in the laboratory tests. Subsequently, the results of the laboratory tests were analyzed and simulated by 2D (axisymmetric) finite element method (FEM) to back-analyze and confirm the related design parameters which were used further in subsequent numerical experiments. The laboratory test results indicated that the increased hydraulic conductivity in the smear zone of PVD with vacuum preloading (Vacuum-PVD) resulted in the increase in the coefficient of horizontal consolidation (Ch) by 16% as well as the decrease in the ratio between the horizontal hydraulic conductivity of the undisturbed zone (Kh) to the horizontal hydraulic conductivity in the smear zone (Ks) or (Kh/Ks) of about 10%. The Vacuum-PVD and PVD only have the same settlement magnitudes with similar equivalent loads.     

结构性海积软土强度与变形时效特性试验研究

以天津地区海积软土为研究对象,在不同固结压力、不同固结时间作用下开展分别加载的一维固结试验,对固结后的原状样与相应重塑样进行微型十字板剪切测试,分析海积软土强度和变形特性受固结压力、固结时间及结构性等因素的影响。试验结果表明:天津海积软土在短时间固结作用下,强度和变形时效特性表现明显,且受结构性影响显著;在长时间固结作用下,土的强度和变形随固结压力和固结时间均呈衰减型增长,结构性对其影响程度随时间增长在减小;长期强度与土的初始强度、固结压力和时间有关,随土的初始强度和固结压力的增大而增大,而强度随时间增长达到最大值后又随之降低。     

Innovative thermal technique for enhancing the performance of prefabricated vertical drain during the preloading process

This paper examines the effect of raising the temperature of soft Bangkok clay, up to 90 °C, on the performance of the prefabricated vertical drain (PVD) during the preloading process. The effect of temperature on the engineering behavior of soft Bangkok clay was first investigated using a modified triaxial test apparatus and flexible wall permeameter which can handle temperatures up to 100 °C. The results of the triaxial tests on clay specimens demonstrate that raising the soil temperature increases its shear strength, under drained heating condition, as well as its hydraulic conductivity. In addition, large oedometer tests were performed to investigate the performance of PVD at elevated temperatures. The response of the soil sample with PVD for the thermal consolidation path which involved increasing the soil temperature at constant vertical effective stress condition and the thermo-mechanical path which involved increasing simultaneously both the soil temperature and the vertical effective stress were investigated. The consequent results indicated that the thermo-mechanical path shows promising results regarding the consolidation rate. For both reconstituted and undisturbed specimens, higher consolidation rate was observed for the soil specimen with PVD loaded under elevated temperature. This behavior can be attributed to the increase in the soil hydraulic conductivity as the soil temperature increases. Therefore, raising the soil temperature during the preloading period can enhance the performance of the PVD, particularly, by reducing the drainage retardation effects due to the smear zone around PVD.     

动力固结后饱和软土三轴剪切性状的试验研究

通过淤泥质饱和软粘土的三轴固结不排水剪切试验,研究了海相沉积原状土、重塑土以及超固结土在动力固结后的应力–应变性状表现。原状软土经过动力固结后在剪切过程中表现出应变软化,重塑软土在低围压下具有一定的应变硬化现象,而在高围压下则为应变软化型。3种状态下的软土的应力–应变关系曲线按照双曲线形式对围压均具有较好的归一性。应力路径分析表明,原状土经过动力固结后在剪切过程中表现出一定的超固结土的性状,而经过动力排水固结后,重塑土则具有明显的超固结土特性。     

Characterization of a Cohesive Soil Bed using a Cone Pressuremeter

A new cone pressuremeter has been developed at the University of Engineering and Technology, Lahore-Pakistan. The new device is called the Akbar Pressuremeter (APMT). This paper is based on the APMT testing of a cohesive soil deposit comprised of low plastic lean clay (CL) to sandy silty clay (CL-ML). The APMT testing, using a full-displacement technique, standard penetration tests and undisturbed samples, was carried out at two locations. The soil strength and type were determined using the undisturbed samples. The applied pressure-cavity strain curves of the APMT tests performed at different levels were analysed to determine soil strength and stiffness. The undrained shear strength of the undisturbed samples was determined in the laboratory by unconfined compression tests. This paper provides a comparison of the parameters interpreted from the pressuremeter and those determined from other field and laboratory methods.     

Residual Strength Characteristics of Naturally and Artificially Cemented Clays in Reversal Direct Box Shear Test

The naturally cemented clay preserving chemical bonds that was gradually disintegrated by weathering is a soil exhibiting a progressive failure such as a landslide. The residual strength of soil possessing cementation properties given by diagenesis has not yet been investigated. The objective of this study is to clarify the residual strength characteristics of naturally and artificially cemented clays using an improved reversal direct box shear test apparatus. Based on the test results of reconstituted Kaolin clay, undisturbed, remolded and reconstituted samples of three natural clays, this paper describes the influence of normal stress, shear displacement rate, consolidation and shear histories on the residual strength of cemented clay. Especially, to simulate the same mechanical behavior as the naturally cemented clays, the cementation was artificially reproduced by adding cementing agents to slurry clay. Consequently, 1) the residual strength of cemented clay is independent of consolidation yield stress and initial void ratio. 2) The residual strength of cemented clay as well as non-cemented clay increases with increasing the shear displacement rate. 3) The residual strength of cemented clay as well as non-cemented clay is not affected by any stress history.     

Permeability Characteristics of High-Quality Undisturbed Gravelly Soils Measured in Laboratory Tests

A series of permeability tests on both high-quality undisturbed samples and reconstituted samples was performed using a large-scale triaxial cell. Based on the test results and discussion, the following conclusions were noted.

• (1)The coefficient of permeability in the horizontal direction is larger than that in the vertical direction. However, its difference is between 10% and 70% and not so large.
• (2)The coefficient of permeability decreases with increasing confining stress. The effect of the confining stress can be understood as the effect of the void ratio. And the small change of the void ratio due to consolidation only leads to a small change of the coefficient of permeability.
• (3)There is no clear correlation between the physical properties and the coefficient of permeability.
• (4)The coefficient of permeability of gravelly soils is almost the same as that of sandy soils, even though the 50% diameter of gravelly soils is about ten to a hundred times that of sandy soils. This result implies that the large size particles of gravelly soils have no significant effect on permeability characteristics of gravelly soils.
• (5)A new definition for determination of D₁₀, D₂₀ and fines content only from those soil particles with a diameter smaller than 2 mm was introduced. A correlation similar to sandy soils can be seen between the coefficient of permeability and D₁₀, D₂₀, and fines content based on the new definitions.
• (6)The effect of the small size particles of the gravelly soils on the coefficient of permeability was found significant based on the test results using samples with a special blend of particle size and density reconstituted from undisturbed samples.
• (7)The coefficient of permeability of multi-layer gravelly soils, in the direction perpendicular to the sedimentation, was significantly affected by the lowest permeability. And the coefficient of permeability of the multi-layer in total agrees well with the theoretical estimation.
• (8)Although the data is limited, there was no significant difference of coefficient of permeability between undisturbed and reconstituted samples. This result accords with that reported for sandy soils by Hatanaka et al. (1997). This result also means that the coefficient of permeability of gravelly soil is not affected by the soil fabric. As a result, the in-situ coefficient of permeability of gravelly soils can be well estimated for practical purposes from the reconstituted samples with the same gradation properties.

     

三亚海相软土次固结特性研究

港口工程建设中经常遇到海相软土地基,海相软土是一种流变性很强的饱和软粘土,次固结变形在总变形中占有一定的比重,次固结系数是用来计算次固结变形量的一个重要参数。现有计算软土次固结变形的常用方法是不考虑荷载对其影响的,大量的试验表明荷载对次固结有较明显的影响。本文以三亚海相软土为例,根据室内试验成果,对其次固结变形特性与荷载的关系进行了分析研究,得出了一些规律性结论,对海相软土地基的设计与工程实践有指导意义。     

恒荷载作用下复合地基非线性分析

从桩周土体径竖向渗流方程出发,采用土体e–logσ和e–logk半对数模型,对于排水桩桩体仅考虑竖向渗流并考虑涂抹区影响,建立恒荷载作用下复合地基非线性固结方程。通过与已有解的对比,证明此解析解的正确性。根据对复合地基非线性固结分析发现:复合地基固结速率随土体非线性参数Cc/Ck和荷载增量σ0/σsi的增大而减小,但相比Cc/Ck,荷载增量σ0/σsi对固结速率影响很小;随着桩土模量比增大,复合地基固结速率加快,且非线性参数Cc/Ck对固结速率的影响程度减小;排水桩复合地基固结速率受kh/kc即桩体渗透性影响很大。     

Unconsolidated clays from the inner shore of the Gemlik Gulf in the Marmara Sea (Bursa-Turkey)

The consolidation properties of clay samples from a marine environment were examined. Index, triaxial and odeometer tests were performed on the undisturbed/disturbed clay samples from 13 boreholes in the Gemlik Gulf (Bursa-Turkey). Laboratory tests indicated that the clays are soft and have a high moisture content and hence are highly compressible. The majority of the clays are unconsolidated due to the accretion rate being faster than the dissipation of pore water.     

Modelling the Effects of Surcharge to Reduce Long Term Settlement of Reclamations over Soft Clays: A Numerical Case Study

Long-term settlement of reclamations constructed over soft soils may be reduced by the use of temporary pre-loading, although there is often uncertainty over how much surcharge to use, and how long it should be maintained. An elastic visco-plastic constitutive model is described and its application in assessing surcharging strategy for reclamation schemes is discussed. The model is incorporated in a one-dimensional finite difference consolidation analysis with vertical and radial drainage of a multi-layer soil profile. The analysis allows the inclusion of vertical drain resistance, a zone of peripheral smear around the drain, permeabilities that vary with void ratio, and creep both during and after primary consolidation. Drawing on data from the Bothkennar soft clay research site, the model is used to predict settlements beneath a hypothetical test fill, including the creep settlement behaviour when a surcharge is removed after different time periods. The behaviour during primary consolidation is back-analysed using Asaoka’s and the hyperbolic observational methods, and both are found to under-predict the magnitude of the settlement.     

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。     

软土变形时效特性的试验研究

软土变形的重要特征为具有时效性,这将导致软土工程的工后沉降。根据实际工程需要,采用原状土样和扰动土样对软土变形机理进行了一系列室内试验,包括软土的应力–应变特性、固结效应、次固结特性、蠕变特性等。通过试验成果系统分析可得:(1)初始固结度对应力–应变关系的影响;(2)次固结系数与固结压力的关系以及应力历史对次固结系数的影响;(3)次固结系数与压缩指数的关系;(4)排水蠕变和不排水蠕变的变形特征;(5)蠕变变形的影响因素和降低蠕变效应的技术路径等。研究结果表明:软土变形的时效性产生于固结特性和蠕变特性的耦合效应,土体的变形过程实际是固结和蠕变共同作用的过程,任一时刻两种变形在总变形中所占比例取决于多种因素,其中最重要的是应力水平和排水条件。     

秦皇岛滨海饱和软土的试验研究

为保持饱和软土的天然结构特征，野外采用开挖探井，环刀取土的方法获得了大量Ⅰ级土试样，并在室内进行有关饱和软土的物理力学性质试验。结果表明：秦皇岛滨海饱和软土的天然含水量介于37％～44％，土性属淤泥质粉质粘土。基于浅层平板载荷试验、静力触探试验以及十字板剪切试验，对饱和软土的天然抗剪强度、灵敏度、承载力进行了试验研究。建立了十字板抗剪强度和试验深度、静力触探比贯入阻力的经验公式，为工程应用提供了便利条件。秦皇岛滨海饱和软土的天然不排水抗剪强度平均值为27kPa，灵敏度为2．4～12．5，具有中高灵敏性。其地基强度的使用应综合考虑建筑物的加载速率、地基容许变形等多种因素。     

Performance of Band Shaped Vertical Drain for Soft Hai Phong Clay

Performance of band shaped prefabricated vertical drain (PVD) installed into soft Hai Phong clay with a 110 cm triangle arrangement is reported together with the engineering properties of the clay investigated by field and laboratory tests. Stationary piston sampling was carried out to obtain high quality undisturbed soil samples for laboratory tests and reliable engineering characteristics of the clay. It was assumed for the design of PVD spacing and preloading that the ratio of apparent value of horizontal coefficient of consolidation c_h(ap) to vertical coefficient of consolidation c_v is equal to 1.0. The settlement monitored in the field, which clearly showed that the actual settlement was faster than expected, resulted in the c_h(ap) value 1.5 times as much as c_v determined by the laboratory test.     

黄河口沉积物固结过程电阻率监测研究

在现代黄河口采集沉积物,通过室内土样真空预压装置使沉积物固结,利用高密度电阻率仪监测沉积物固结过程中电阻率的变化;利用孔隙水压力监测、十字板剪切测试与微型贯入测试等测试手段,实时测定固结沉积物的孔压和不排水抗剪切强及贯入强度的变化;并定期取土样进行常规土工试验和室内固结试验。研究结果表明,黄河口沉积物在固结过程中土体电阻率大小同固结程度密切相关,固结过程中影响黄河口饱和粉土电阻率的主要因素为孔隙率;黄河口饱和粉土的电阻率与土体物理力学指标之间存在良好的相关性,通过电阻率测定可以有效估算土体各项固结指标,本研究为海床沉积物固结过程的原位监测提供了一条新途径。     

基于位错理论的饱和土渗流特性CPTU评价研究

孔压静力触探（CPTU）测试数据通常用于土层划分和岩土工程设计参数评价。国际上CPTU测试技术的最新进展之一在于采用测得的锥尖阻力、侧壁摩阻力和孔隙水压力快速、连续地获得饱和土的渗透系数,以避免孔压消散试验费时的缺点。首先简要回顾了基于CPTU测试资料确定饱和土渗透系数的的研究成果,并进行了分析与改进,提出了基于位错理论和圆柱面径向流模型的水平向渗透系数预测新方法。选取典型场地进行了CPTU测试,并在黏性土场地采用薄壁取土器获得无扰动的原状试样,进行室内水平向渗透试验,在无黏性土场地进行现场钻孔抽水试验。以室内渗透试验和现场抽水试验的结果为参考,对3种基于CPTU连续评价饱和土渗透系数的方法进行了对比分析。研究表明,位错理论适用于评价饱和土的原位渗透系数,然而准确性与所采用的渗流模型有关。对国际标准CPTU探头,孔压过滤环位于锥肩（u₂）位置时,采用圆柱面的径向流模型能够获得最为准确的水平向渗透系数预测结果,采用球面流或半球面流模型时趋于低估土层的渗透性。     

Analytical solutions to the axisymmetric consolidation of a multi-layer soil system under surcharge combined with vacuum preloading

Surcharge combined with vacuum preloading is a common technique for accelerating the consolidation process in ground improvement. A unit cell model for the axisymmetric consolidation of a soft soil using a prefabricated vertical drain (PVD) under a surcharge, combined with vacuum preloading, is investigated in this study. Based on this model, analytical solutions for a multi-layer soil system are put forward and the explicit expressions for two-layer and one-layer systems are presented. The accuracy of the proposed solution is verified using an analytical solution available in the literature. In the parametric study, the influencing factors on the consolidation process, such as, the smear zone, the PVD spacing, the hydraulic conductivity in the radial direction, the coefficient of vacuum decrease, are taken into account. The water flow in the radial direction plays an important role in the consolidation process while the impact of the vertical flow mainly develops around the interfaces between two adjacent layers. In addition, the proposed analytical solution is applied in a case history with three different layers and the results are reasonable.     

General solutions for consolidation of multilayered soil with a vertical drain system

A quasi-analytical method is newly introduced to solve the equal-strain consolidation problem of multilayered soil with a vertical drain system. Both vertical and radial drainage conditions are considered, together with the effects of drain resistance and smear. By using the method of Laplace transform with respect to time, a general explicit analytical solution for the consolidation in transformed space is obtained. Numerical inversion of the Laplace transform in the time domain is then applied to obtain the solution for calculating excess pore-water pressure. This solution is explicitly expressed and conveniently coded into a computer program for ease and efficiency of practical use. Its validity and accuracy are verified by comparing the special cases of the proposed solution with a finite-element solution and an available analytical solution. Moreover, the consolidation behavior of a four-layered soil with a vertical drain is investigated. The order of soil layers is shown to have a significant effect on the behavior of consolidation. This highlights that caution should be exercised when weighted average consolidation parameters of multilayered soil are used to analyze the consolidation behavior.