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.     

Proposal of a Simple Method for Assessing the Susceptibility of Naturally Deposited Clay Grounds to Large Long-Term Settlement due to Embankment Loading

A simple method that utilizes the results of laboratory tests has been proposed for determining the susceptibility of soft clay grounds to large residual consolidation settlement due to embankment loading. It was found that there is a possibility of large long-term settlement if the sensitivity and compression index ratios of the clay material that constitutes the ground are equal to or more than 8.0 and 1.5, respectively. The compression index ratio is defined in this paper as the ratio (Cc/Ccr) of the steepest gradient of the compression curve of an undisturbed sample to that of the remolded sample. Through the SYS Cam-clay model, an elasto-plastic constitutive model that describes the actions of the soil skeleton structure, it was found that clays with large sensitivity and compression index ratios are characterized by initially highly structured soils and that decay/upgradation of the structure can easily occur due to plastic deformation. In addition, by following Schmertmann's graphic method for in-situ compression curve (1953), this paper proposes a method of deducing the in-situ initial conditions from the results of laboratory consolidation tests on undisturbed samples. These investigations revealed not only that large delayed settlement is facilitated in clays, which have higher degrees of structure and faster rates of structural decay, but also that the Δe method and other simple methods of predicting settlement may underestimate the amount of settlement.     

Long-term consolidation behavior interpreted with isotache concept for worldwide clays

The consolidation characteristics interpreted with the isotache concept have been studied by many researchers, including the authors. The aim of most of these studies has been to calculate secondary consolidation with high accuracy in order to evaluate the long-term settlement of large-scale structures. In a previous study, the long-term consolidation characteristics of Osaka Bay clay, collected from the construction site of the Kansai International Airport, were examined, and a simplified method based on the isotache concept, using a compression curve and the relationship between the consolidation yield stress (preconsolidation pressure) and the strain rate, was proposed. The former and the latter were obtained from constant rate of strain consolidation (CRS) tests and long-term consolidation (LT) tests, respectively. The latter is expressed by an equation with three isotache parameters. In addition, it is noteworthy that the isotache parameters can be commonly determined for the Osaka Bay clays retrieved from various depths up to 300 m below the seabed. In the present study, the proposed method was applied to worldwide clays with various characteristics using the common values for the isotache parameters determined for the Osaka Bay clays. It was found that the long-term consolidation behavior of those worldwide clays can be well characterized by the proposed method, along with the common values for the isotache parameters.     

考虑两种不同性质超固结的土体本构模型

结构对土体的变形特性有强烈影响,土体本构模型应该反映土体的结构性。通过调研分析将土体变形表现出的超固结划分为结构超固结与应力超固结这2种不同性质的超固结,从而使结构超固结与应力超固结统一起来。土体结构超固结采用结构超固结参数来表示。模型对修正剑桥模型进行扩展,模型的屈服面形状与修正剑桥模型一致,而屈服面的大小由结构超固结和应力超固结ps之和决定,而屈服面的变化由体积硬化与结构损伤共同来描述,模型采用相关联流动法则;从而提出了一个可考虑结构性的粘土本构模型。模型概念明确,参数简单并有物理意义。初步验证表明,模型可以较好的反映结构性土体的变形特性。     

Ageing Effects on Consolidation Properties—Based on the Site Investigation of Osaka Pleistocene Clays—

Soil samples were recovered from the Osaka basin, where Holocene as well as Pleistocene marine clays are thickly deposited. It is found from X-ray diffraction analysis that the component of clay minerals for these day layers is almost identical through all depths. 'This fact suggests that the origin of these sediments has not been drastically changed through at least more than 1,000,000 years. Using data obtained by oedometer tests, this paper examines ageing effects on consolidation properties. It is found that void ratio for the Pleistocene clays keeps large even under high in situ effective overburden pressure (p_vo′). Salinity of pore fluid decreases with depth and becomes nearly zero at depths deeper than 70 m. However, the change in salinity does not give any influences on the index properties, suggesting that the relatively large void ratio for the Pleistocene clays is caused by other reasons than leaching. It is also found that the Pleistocene clays reveal no particular differences in consolidation properties, but fundamentally similar to those of Holocene clays'. There is no distinguished difference in the overconsolidation ratio (OCR), non-linearity of the e-log p' relation after yield consolidation pressure including the maximum compression index (C_cmax) and the hydraulic conductivity (k). The only prominent difference in the Osaka Pleistocene clays, compared to the Holocene clay, is relatively small void ratio due to the large p_vo′.     

Mechanisms of quasi-preconsolidation stress development in clays: A rheological model

This paper deals with the development of a mechanistic model for the ageing consolidation behavior of clays with the focus on aspects related to the development of quasi-preconsolidation pressure. The initial use of such pressure in design met with criticism, but field and laboratory evidence, which highlights its significance, continues to accumulate. A nonlinear rheological model is used to numerically simulate the consolidation process of clay in laboratory tests and to identify the basic mechanical parameters that contribute to the development of the quasi-preconsolidation phenomenon. Methods to identify the parameters of the model from oedometer tests are described. It is shown that while the variation in soil modulus can be characterized by a linear form in the virgin compression region, it is nonlinear in the recompression region and is best characterized by a hyperbolic function. Changes to the modulus in the recompression region, due to ageing, is shown to be the dominant cause of the development of the quasi-p_c phenomenon. Observed results as well as numerical simulations demonstrate that specimens that had aged longer show increased quasi-p_c values. While the variation in soil modulus controls the EOP curve of clays, the observed time effects, such as the “vanishing p_c” phenomenon, are controlled primarily by changes in soil viscosity. However, this has no bearing on the development of the quasi-p_c phenomenon.     

关于粘土压密类型的鉴别问题

首先,本文依据正常固结条件下粘土承受的压力、密度和时间的对应关系,结构性对压密状态和力学性质的影响,形成真正先期固结压力和结构屈服压力的不同机理,提出粘土压密类型的鉴别方法。把粘土的压密类型区分为一般的欠压密、正常压密和超压密,以及有结构强度的欠压密、正常压密和超压密六种。其次,讨论粘土的压密状态和结构强度对静止侧压力系数K_o的影响。反过来,应用K_o的变化特性来鉴别粘土的压密类型,并指出这种方法的优点和存在的问题。最后,提出区分荷载引起超压密和次固结引起超压密的方法。     

Development of a New Cone Penetrometer and its Application to Great Depths of Pleistocene Clays

A new piezocone was developed and applied to Osaka Pleistocene clay layers as deep as 250 m. Its geometry is the same as that of the reference procedure defined by the ISSMFE and standard of JGS: i.e., the projected cross sectional area is 10 cm²; the angle of the cone is 60°; the pore water pressure is measured at the shoulder. This piezocone does not measure the skin friction. Due to great depth at the investigated site, the capacity of the point resistance (q_t) and the pore water pressure is as large as 30 MPa and 20 MPa, respectively. The test was carried out, using a borehole drilled prior to the penetration, because of stiff sand or gravel layers and large skin friction between the rod and the ground. The q_t measured by the cone penetration test (CPT) was correlated to the yield consolidation pressure (p_y) measured by the Constant Rate of Strain (CRS) oedometer test for the soil sample recovered near the point of the CPT investigation. The cone factor for the p_y value (N_pc) was defined by (q_t-p_vo)/p_y, where p_vo is the in situ total overburden pressure. The range of observed N_pc value is relatively narrow and between 2.5 and 2.8, which is in the middle of the range of N_pc factors measured in Holocene clays in the various areas in the world as well as Japan. The overconsolidation ratio (OCR) was also derived by CPT. Variation of the OCR estimated by the CPT is nearly equivalent to that measured by the CRS oedometer. It may be concluded from this investigation that the consolidation properties, especially the p_y value, derived from samples recovered from great depths, are quite reliable as design parameters.     

An Approach to the Interpretation of the Mechanical Behaviour of Intensely Fissured Clays

The paper discusses the intrinsic properties, the geological history, the natural structure and the mechanical behaviour of intensely fissured clays from Santa Croce di Magliano, a site located within the East-side border of the Apennine chain (southern Italy). The clays were originally deposited elsewhere and subsequently moved and largely deformed during the Apennine orogenesis, so that their structure appears severely disturbed and fissured. The applicability of a continuum-based approach to describe the mechanical behaviour of a Representative Element Volume (REV, hereafter) of the natural, fissured clay is experimentally investigated by analysing the clay specimen deformation under plane strain compression by means of False Relief Stereophotogrammetry. The results of such analysis show that, despite the intense network of pre-existing fissures, the patterns of deformation - including the eventual development of shear bands within the REV of the natural clay - are similar to those observed for unfissured clays. Triaxial tests were carried out on both natural and reconstituted clay samples and the results were compared with those recognised in the literature to be typical of unfissured clays. Based on such comparisons, the study provides a few essential elements that are useful to define a general framework for the mechanical behaviour of intensely fissured clays.     

A Stress Path Method for Settlement Computation of Normally Consolidated Clays

A stress path method for computing the settlement of normally consolidated clays is presented. The proposed method introduces the characterization of the stress-strain behavior that eliminates unnecessary experimental cost due to uncertainty in the design stress path. For arbitrary control of the effective stress during consolidation, a technique of back-pressure equalization, which enforces the immediate drained condition on the soil specimen without change of the effective stress state, was used. To account for nonlinear anisotropic response in computing the consolidation strains, the iteration procedure was designed. For the proposed method, the stress path experiments on the normally consolidated kaolin were conducted. An example illustrating the feasibility of proposed method in precisely calculating the settlement is shown for a circular footing on the clay stratum.     

Microstructure and Chemical Properties of Cement Treated Soft Bangladesh Clays

This paper examines the relationship between the microstructure and chemical properties of soft Bangladesh clays due to cementation. The microstructure was investigated using X-ray diffraction, scanning electron microscopy, pH measurement, specific surface area and soil chemical tests. The results indicate that a multitude of changes occurred in the properties and behavior of cement-treated clays that can be explained by the interaction between four underlying microstructural mechanisms. That is, it is suggested that the hydrated lime is formed by hydration, which causes the flocculation of the little clay particles, by the preferential attack of the calcium ions on kaolinite rather than on illite and monmorillinite in the pozzolanic reaction, by surface deposition and shallow infilling by cementitious products such as calcium silicate hydrate and calcium alumino silicate hydrate (CSH and CASH) on clay clusters, and finally, by the presence of water trapped within the clay clusters. The chemical properties of the cement-treated clays were found to depend on the plasticity of soil.     

软土固结中波速与固结度的关系及其工程应用

本文首先应用室内共振柱研究了浙江沿海软粘土在固结过程中剪切波速与固结度的关系,由试验结果发现波速增值△υ_s＇与△p＇/p＇（△p＇为围压增量,p＇为初始有效固结压力）之比与固结度有一线性关系,这为应用现场波速检测软土地基加固效果提供了实验依据。本文然后通过镇海电厂堆煤场砂井预压工程,将实测孔隙水压力消散得到的固结度与现场跨孔试验测得的波速计算的固结度进行比较,显示了该方法有良好的工程应用前景。     

结构性粘土的弹塑性损伤模型

本文从损伤力学观点出发,建议了一个可以考虑粘土结构破损过程的损伤力学模型,推导了弹塑性损伤矩阵,并制定了相应的计算方法和编制了比奥固结理论的有限元计算程序。模型的计算参数共有9个,可以方便地通过原状土的有侧限和无侧限压缩试验测定。文中从湛江粘土的试验资料出发整理了计算参数,并就10m厚粘土地基上承受均布条形荷载的算例进行了计算,结果定性地与国内外原观资料一致。     

Consolidation enhanced membrane behavior of a geosynthetic clay liner

Semipermeable membrane behavior in clays refers to the ability of clays to restrict the migration of solutes. Thus, membrane behavior represents a potential benefit to the containment function of clay barriers used for hydraulic containment applications. In this regard, the potential influence of consolidation effective stress, σ′, on the membrane behavior of a geosynthetic clay liner (GCL) containing sodium bentonite was evaluated in the laboratory by establishing differences in salt (KCl) concentrations ranging from 3.9 to 47 mM across specimens of the GCL in a flexible-wall cell under closed-system boundary conditions. The membrane behavior exhibited by the GCL was enhanced via consolidation such that an increase in σ′ from 34.5 kPa (5 psi) to 241 kPa (35 psi) correlated with an increase in membrane efficiency from 0.015 (1.5%) to 0.784 (78.4%), respectively. The membrane efficiencies measured in this study at σ′ of 172 kPa (25 psi) and 241 kPa (35 psi) were similar to those previously reported for the same GCL using a rigid-wall cell but at unknown states of stress. The practical significance of the results is illustrated in the form of an analysis showing a reduction in liquid flux across the GCL with increasing membrane efficiency.     

超固结土三轴排水不排水试验数值分析

在临界状态土力学的框架内,采用Hashiguchi提出的下负荷面本构模型,建议了一个简化的塑性硬化规律,使之能较合理地描述超固结粘土在正常屈服面和下负荷面之间的变化规律.数值模拟结果表明,修正后的硬化规律能够较好地描述超固结粘土的许多力学特性如应力应变关系、应变软化以及应力剪胀性等.数值预测结果与室内三轴排水试验结果相...     

贵阳红黏土的应力-应变软化模型及参数研究

通过对不同含水量和不同围压下的贵阳红黏土进行三轴固结不排水试验,系统地研究了贵阳红黏土的软化特性及孔隙水压力的变化规律。试验结果表明:贵阳红黏土破坏形态具有明显的剪切带;应力-应变曲线出现了带有驼峰的应变软化现象,固结过程中孔隙水压力与时间呈指数函数关系,剪切过程中孔隙水压力与轴向应变呈双曲线型。研究认为,土体的结构屈服应力和固结不排水试验中产生的孔隙水压力是造成贵阳红黏土软化的主要因素。根据沈珠江、张尔齐等人的软化模型,对贵阳红黏土进行拟合,求解模型参数及置信区间,为贵阳红黏土进一步的数学模型建立提供理论基础。     

Formulation of Gmax from Reconstituted Clayey Soils and its Application to Gmax Measured in the Field

The elastic shear modulus of natural sedimentary clay ground, G_max, is estimated based on laboratory tests for fifteen different reconstituted clays. Two types of tests were performed, i.e., Bender Element and Cyclic Triaxial tests. The proposed formulation is not based on void ratio, e, but consists of only three parameters: w_L (liquid limit), p′ (the current mean effective stress) and p′_max (the maximum mean consolidation pressure). To apply it to the field, this equation is modified for using σ′_v0 (the in situ effective overburden pressure) and OCR, instead of p′ and p′_max. Since existing formulae for G_max are mostly based on e, they are not able to apply to both reconstituted soil and field, without considering the correction factor for structure. This is because e in the field is much larger than that for reconstituted soil even though their consolidation pressures and OCR are the same for these clays. The applicability of the proposed formula was examined by using investigated results from the in-situ seismic surveys performed at eleven worldwide sites. It is well demonstrated that the proposed equation in this paper is capable of predicting G_max of natural sedimentary clay deposits with higher accuracy than the existing empirical formulae using a function of e.     

Clays in radioactive waste disposal

Clays and argillites are considered in some countries as possible host rocks for nuclear waste disposal at great depth.The use of compacted swelling clays as engineered barriers is also considered within the framework of the multi-barrier concept.In relation to these concepts,various research programs have been conducted to assess the thermo-hydro-mechanical properties of radioactive waste disposal at great depth.After introducing the concepts of waste isolation developed in Belgium,France and Switzerland,the paper describes the retention and transfer properties of engineered barriers made up of compacted swelling clays in relation to microstructure features.Some features of the thermo-mechanical behaviors of three possible geological barriers,namely Boom clay（Belgium）,Callovo-Oxfordian clay（France） and Opalinus clay（Switzerland）,are then described,including the retention and transfer properties,volume change behavior,shear strength and thermal aspects.     

Increase in Undrained Shear Strength of Clay with Respect to Rate of Consolidation

Estimating increase in undrained shear strength s_u of clay is an important purpose of consolidation analysis as well as settlement prediction when multi stage loading for construction of earth structures on soft clay is concerned. The present paper investigates the increase in s_u with regard to rate of consolidation. Since values of s_u are usually evaluated as a function of effective vertical stress σˊ_v, rate effect on shear strength increment ratio in normally consolidated state s_un/σˊ_v is first discussed considering the fundamental concept expressed by the equation of s_un/σˊ_v=s_uf/σˊ_y, in which s_uf is in-situ undrained shear strength and σˊ_y is consolidation yielding stress. The paper also describes two case histories where actual increases in s_u were observed in soft clay deposits. The first site is located offshore Osaka-bay where a large-scale seawall was constructed, and soft clay in the site was improved by sand drains. The second is a reclaimed land in Yanai City where a sand fill and a subsequent test embankment were conducted on a soft clay deposit without improvement by vertical drains. It is found from the study that: (1) rate effect on s_un/σ′_v cancels rate effect on σˊ_y, and the equation mentioned above is valid regardless of the rate of consolidation, and (2) the in-situ values of s_un/σˊ_v observed in the two sites vary from 0.27 to 0.37 as consolidation progresses, and they are well related to rate of consolidation. According to the experience at the two construction sites, values of s_un/σˊ_v with regard to rate of consolidation are proposed for design use at field/construction sites.     

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.