Estimation of In-Situ Undrained Strength of Soft Soil Deposits by use of Unconfined Compression Test with Suction Measurement

Unconfined compression tests have been widely used in Japan for the purpose of determining undrained strengths of clay samples, but the unconfined compressive strengths are usually scattered even if specimens tested seem to have been subjected to the same stress history. This is due to the characteristic of the test itself, which is performed under unstable confining stress given by capillary pressure of the sample itself. In order to make clear the cause of scattering of measured strengths, three series of tests simulating the process from sampling to unconfined and triaxial compression tests on saturated soil samples are performed with three remolded and three undisturbed clays together with an undisturbed peat, and the influence of stress release and mechanical disturbance of the test specimen on the undrained strength is examined. Based on the test results, a practical method for estimating the in-situ undrained strength of soft soils from the results of a routine unconfined compression (UC) test together with suction measurement is proposed. From comparisons of the undrained strength profiles estimated by the proposed method with the strength profiles obtained by in-situ sounding test, it is found that the strength obtained by UC test can be reasonably corrected to estimate the in-situ undrained strength of soft soil deposits.     

天津滨海软土不同应力路径下抗剪强度对比试验研究

现有基坑支护设计参数试验条件与实际不符。选取天津滨海新区海相软土为研究对象,进行三轴固结不排水试验、三轴不固结不排水试验和减p路径试验并对比分析了几种试验条件下土的强度参数。得到:滨海软土在不同应力路径下的强度与初期固结方式有关。减p路径对总应力状态下抗剪强度有影响,但对有效应力状态下抗剪强度基本无影响。在滨海软土基坑支护设计中,选用CU试验参数进行计算,设计偏于安全。     

灵敏性粉土的压缩及触变特性研究

山西汾河一级阶地的饱和粉土大多具有灵敏性,使得其强度和变形性质在扰动前后差别很大。为了研究灵敏性粉土的压缩及触变特性,对探井取样的原状土和重塑土样进行了一维固结压缩试验和无侧限抗压强度试验。试验结果表明,原状土加载到结构屈服应力处,沉降量突增,固结系数突增,主固结比突降,次固结对沉降的贡献开始上升;原状土与重塑土的差异是原状土的结构强度引起的,在高压固结压力下两者压缩曲线约在0.58e₀处重合;灵敏土无侧限抗压强度试验的应力应变曲线表现出类似超固结土的应变软化特性,其触变后恢复具有时效性,弹性模量受扰动影响较为敏感,可考虑作为扰动指标的量度。     

Effect of initial water content on undrained shear strength of K0 consolidated clay

This study investigates the effect of initial water content on the pore pressure response and undrained shear behavior of K₀ consolidated reconstituted clay. A series of K₀ consolidated undrained triaxial compression tests were conducted on reconstituted Lianyungang clay. Results were compared to those obtained by isotropic consolidated undrained triaxial tests. The testing results showed that the K₀ consolidated undrained strength envelope of reconstituted soil content is a straight line passing through the origin regardless of the initial water content. The initial water content would affect the undrained strength of K₀ consolidated clay as decreased normalized undrained shear strength was observed with clay at higher initial water content. The slope C of normalized pore pressure and stress ratio is affected by the consolidation method, where C is found to be a soil constant for K₀ consolidated clay and the value would be higher with clay under K₀ consolidation. The pore pressure increases with increasing initial water content at a certain axial strain under given consolidation pressure, and the difference in excess pore pressure increases with the increasing consolidation pressure. Pore pressure coefficient at failure (A_f) increases as the initial water content increases, where a trendline can be well fitted between the pore coefficient at failure and the ratio of initial water content to the liquid limit of clay. The undrained strength indexes, i.e., effective cohesion and effective internal friction angle have decreasing tendency with increasing initial water content; however, changes in the total strength indexes of soil in this study are insignificant with varying initial water content.     

天然沉积结构性黏土的不排水强度性状

通过对福州天然沉积黏土原状样及不同初始含水率重塑样进行三轴固结不排水剪切试验,探讨土结构性对天然沉积黏土强度性状的影响规律及作用机理。研究结果表明：结构性对天然沉积土固结不排水抗剪强度的作用程度受固结压力大小影响可以分为3个阶段：固结压力小于屈服压力阶段,不排水抗剪强度主要由土结构性控制,与应力水平基本无关;土结构逐渐屈服阶段,固结压力大于屈服压力,不排水抗剪强度随固结有效应力的增大逐渐趋于重塑样的强度线;土结构性影响消失阶段,原状样的不排水抗剪强度随固结有效应力的增长规律与重塑样相同。     

Evaluation of shear strength of clayey soils by using their liquidity index

 It is well known that overconsolidated clays have higher shear strength than unconsolidated clays. The liquidity index makes possible an evaluation of the consolidation degree of clays. However, there is no empirical information about this relation. In this study, clayey soil samples have been collected from various locations and tested. The tests include the determination of liquidity index and shear strength. Obtained parameters were correlated and regression equations were established among liquidity index and undrained shear strength, presenting high coefficients of correlation (R=–0.93). So, an equation [c_u=e^{(0.026–1.21 IL)}] that makes possible a rough evaluation of the shear strength of clayey soils by using their liquidity index value is an improvement. Received: 10 December 1999 · Accepted: 4 April 2000     

SIMULATION OF SHEAR AND ONE-DIMENSIONAL COMPRESSION BEHAVIOR OF NATURALLY DEPOSITED CLAYS BY SUPER/SUBLOADING YIELD SURFACE CAM-CLAY MODEL

Naturally deposited clays exhibit complicated mechanical behavior that differs from that of remolded clays. For example, clay in a normally consolidated state commonly exhibits softening in undrained shear tests or “rewinding” in a heavily overconsolidated state. The Super/subloading Yield Surface Cam-clay model (Asaoka et al., 1998, 2000, 2002) was proposed in an attempt to clarify the complicated mechanical behavior in naturally deposited clays. In this constitutive model, the concepts of “structure,” overconsolidation, anisotropy, and their evolution laws, are introduced into the modified Cam-clay model. In the present study, undrained triaxial compression tests and oedometer tests were carried out on two types of naturally deposited undisturbed clay, Pleistocene clay and Holocene clay, and the behavior was then simulated using the Super/subloading Yield Surface Cam-clay model. The findings of the present study are as follows:

• 1)For the two types of undisturbed clay, the Super/subloading Yield Surface Cam-clay model can simulate undrained triaxial compression behavior ranging from the normally consolidated state to the overconsolidated state, corresponding to various isotropic pressures using a single set of material constants.
• 2)In addition, the model can simulate one-dimensional compression behavior using the same material constants as those used for the simulation of the undrained triaxial compression behavior.
• 3)Through the simulation, the mechanical behavior of Pleistocene clay and Holocene clay, which have different loading histories and have undergone different aging effects, can be described by the different evolution parameters, as well as the elasto-plastic parameters.

     

基于原位十字板强度的沿海地区软黏土超固结比分析

超固结比OCR是评价软黏土变形和强度特性的重要参数。基于天津、上海、连云港、深圳、中山和珠海这6个沿海地区软黏土的原位十字板强度,系统地分析这些地区软黏土的固结状态,并探讨其形成机制。研究结果表明:(1)基于原位十字板强度得到的OCR要明显大于室内一维压缩试验的结果,室内压缩试验会显著低估沿海软黏土的OCR;(2)6个地区的OCR随深度的变化规律较为一致,OCR随深度增加,逐渐减小至某一稳定值,浅层均表现出一定的超固结现象,而深层的固结状态则表现出一定的复杂性;(3)现有模型能够解释我国沿海软黏土超固结的成因以及变化规律,浅部的超固结现象源于以风化胶结为主的"表层作用",而深部的超固结则源于次压缩。考虑到软黏土突出的取土扰动问题,这种基于原位十字板强度的软黏土OCR评价及成因分析方法值得在我国沿海地区推广。     

饱和黏土不排水抗剪强度各向异性的热力学本构模型研究

基于饱和土体TTM（Tsinghua thermodynamic soil model）热力学本构模型分析研究了饱和黏土的不排水抗剪强度各向异性问题。模型及试验研究表明：非等向或固结历史是引起土体强度各向异性的重要原因,固结应力比越小,不排水强度各向异性越大。不排水加载过程中主应力轴的方向对土体不排水抗剪强度和变形有着重要影响。一般而言,当主应力轴方向从0°旋转到90°时,土体的不排水抗剪强度逐渐下降,峰值应变却逐渐增大。此外,非等向固结会导致土体主应力与主应变的非共轴性。利用TTM理论模型,对Kaolin Clay 和Boston Blue Clay的不同试验结果进行了模拟验证和预测。结果表明,TTM理论模型具有反映和预测应力引起的饱和黏土强度各向异性和应变软化等特性的能力,同时也具备精准描述主应力轴旋转等复杂路径下饱和黏土的强度和变形特征的能力。     

Peak and residual strength characteristics of cement-treated soil cured under different consolidation conditions

The shear strength of cement-treated soil can be changed by both cementation and consolidation during the early stages of hardening because of cement hydration. Based on the results of triaxial and unconfined compression tests, this paper describes the effects of isotropic and one-dimensional consolidation stress, applied during the curing period, on the undrained peak and residual shear strengths of cement-treated soil. The sample used was a mixture of fine-grained sand and ordinary Portland cement. A consolidated undrained triaxial compression test (ICU) was conducted on the specimens immediately after the cement treatment. Each test was conducted under different consolidation pressures, curing times and delayed loading times. The following conclusions were developed from the results and discussions: (1) the undrained peak shear strength of cement-treated soil, cured under different consolidation conditions, increases with an increase in either the consolidation pressure or the curing time, whereas it gradually decreases with an increase in the delayed loading time. (2) The rate of undrained strength increase resulting from consolidation differs significantly between isotropic and one-dimensional consolidations. (3) For a curing time of between one and seven days, the rate of strength increase by isotropic consolidation exceeds that by one-dimensional consolidation. The simultaneous volumetric change of cement-treated soil during consolidation depends on the stress conditions of the specimen, that is, the difference between isotropic and one-dimensional consolidations. (4) When the test is not conducted under nearly in-situ conditions, the undrained shear strength may be underestimated, depending on the time interval between the cement treatment and the start of consolidation. (5) The shear strength in the residual state is influenced by the consolidation pressure during curing. (6) As the consolidation pressure during curing increases, the specimens exhibit a higher residual strength.     

不同固结路径温州黏土固有不排水强度性状试验研究

为研究初始条件和应力状态对重塑黏土固有不排水强度性状的影响,采用应力路径三轴仪对室内制备的温州黏土进行一系列不同固结路径下的三轴固结不排水剪切试验。通过室内试验研究探讨温州黏土不同固结路径下不同平均有效应力固结后三轴不排水剪切应力应变关系和不排水强度特性;引入不同平均有效应力p’下的孔隙指数I_v与不排水强度比R^*_su=S_u/ p’,分析不同固结路径下不排水强度S_u的变化规律,并与Chandler提出的固有强度线IS_uL进行比较分析。结果表明：相同平均有效应力p’下,不同固结路径下不排水剪切强度S_u和不排水强度比R^*_su=S_u/ p’随固结路径的变化而变化;当不排水强度比S_u/ p’=0.33时,孔隙指数I_v与固结不排水强度S_u之间的关系与Chandler的固有强度线IS_uL一致。     

Effect of Specimen Size on Unconfined Compressive Strength Properties of Natural Deposits

In order to use the advantages of unconfined compression tests, a new testing procedure using S (or Small size) specimens (15 mm in diameter and 35 mm in height) is proposed and a new portable unconfined compression test apparatus with suction measurement is outlined. The effect of specimen size on unconfined compressive strength properties of natural deposits is discussed from laboratory tests. The standard deviations of the ratios of q_u and E₅₀ values of the S specimens to O (or Ordinary size) specimens (35 mm d and 80 mm h) were in the range of 0.09 to 0.16. The 10% variation from the mean value reflects the homogeneity of soils since the coefficient of variations of the undrained shear strength for the undisturbed and reconstituted soils were 8% to 17% (Matsuo and Shogaki, 1988). In an engineering sense, there was no difference in shear strength and deformation characteristics between the S and O specimens for soils having plasticity indexes ranging from 10 to 370 and unconfined compressive strengths of 18 kPa to 1000 kPa, that were taken from 26 different sites in the United Kingdom, Korea and Japan. These soils consisted of Holocene and Pleistocene clays plus diatomaceous mudstone and highly organic soils.     

Basic parameters governing the behaviour of cement-treated clays

Although extensive research has been conducted on the mechanical behaviour of Portland cement-treated soft clays, there has been less emphasis on the correlation of the observed behaviour with clay mineralogy. In this study, experimental results from the authors have been combined with the data found in the literature to investigate the effect of parameters such as curing time, cement content, moisture content, liquidity index, and mineralogy on the mechanical properties of cement-treated clays. The findings show that undrained shear strength and sensitivity of cemented clays still continue to increase after relatively long curing times; expressions are proposed to predict the strength and sensitivity with time. This parametric study also indicates the relative importance of the activity of the soil, as well as the water–cement ratio, to the mechanical properties of cementitious admixtures. Two new empirical parameters are introduced herein. Based on the results of unconfined compression, undrained triaxial, and oedometer tests on cement-enhanced clays, expressions that use these parameters to predict undrained shear strength, yield stress, and the slope of the compression line are proposed. The observed variations in the mechanical behaviour with respect to mineralogy and the important effect of curing time are explained in terms of the pozzolanic reactions. The possible limitations of applying Abrams׳ law to cement–admixed clays are also discussed.     

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...     

Comparison of Mechanical Behavior of Two Overconsolidated Clays: Yamashita and Louiseville Clays

A site investigation program was carried out at the Yamashita site, Yokohama, Japan, by means of laboratory as well as field tests. It was found from the investigation that the clayey layer is overconsolidated with an OCR of about 2.0. It is evident from geological consideration that this overconsolidation may not have been caused by stress change, but was probably due to structure effects such as cementation or delayed consolidation. The physical and mechanical properties of the Yamashita clay is compared with those of Louiseville clay, a Champlain Sea Clay widely distributed in Quebec, eastern Canada, and a well known cemented clay. Louiseville clay was retrieved by Japanese sampler and transported to the Port and Harbour Research Institute (PHRI) for laboratory investigation. Comparison of the two clays shows that strength properties such as the stress-strain curve or stress path for both clays are very similar to each other. However, the pattern of the e-log p curve measured by Constant Rate of Strain oedometer, particularly beyond the yield consolidation pressure, is remarkably different between the two clays.     

RELATIVE DENSITY EFFECTS ON UNDRAINED SAND BEHAVIOR AT HIGH PRESSURES

A systematic investigation of the undrained behavior of sand at high pressures was performed to study the effects of initial relative density on the stress-strain, pore pressure, and strength behavior. Experiments were conducted on Cambria sand with initial relative densities of 30% and 60%, and the results are compared with previously published data for an initial relative density of 90%. Triaxial compression and extension tests were performed on cylindrical specimens with isotropic consolidation pressures in the range from 8 to 60MPa. Particle crushing played an important role in the observed behavior. Stress-strain, pore pressure, and strength behavior were found to be very similar and almost independent of initial relative density at high pressures, because the isotropic compression curves tend to merge once particle breakage becomes important at these high pressures. Effective stress friction angles for undrained compression and extension tests varied systematically in the range from 32° to 35° with slightly higher values for the compression tests. Interpretation of all results from the experimental study in terms of total stresses shows that sand at high pressures behaves similar to a normally consolidated clay.     

基于扰动状态概念硬化参量的结构性黏土边界面模型

在临界状态理论和边界面模型的框架内,引入结构性扰动对弹性变形、结构性屈服面大小、胶结吸力和屈服面各向异性的影响,建立一个基于扰动状态概念硬化参量的结构性黏土边界面模型。首先,通过定义3个结构扰动度R_c,R_b和R_a来定量反映塑性变形对结构性黏土的结构性屈服面、胶结吸力和屈服面各向异性的扰动程度,并对弹性特性进行各向异性和结构性影响的修正。然后,通过对天然沉积上海黏土和Vallericca硬黏土的三轴不排水剪切试验和一维固结压缩试验的模拟验证模型的预测能力。最后,通过模型与有限差分法相结合,以简单的平面应变试验模拟为例,展示模型能够描述结构扰动度的变化及其在土体中分布的模拟能力。     

Triaxial extension and tension tests on lime-cement-improved clay

This paper presents the results of a series of undrained and drained isotropic consolidated triaxial extension, tension and compression laboratory tests on lime-cement-improved very soft clay. The main objective of these tests was to investigate the material strength and stiffness properties for stress conditions similar to those expected on the passive side of excavations where a retaining structure is supported by Deep Mixing columns. The different stress paths to failure were obtained by varying the directions of the major and minor principal stresses in a conventional triaxial test cell. The undrained tests conducted at low consolidation stresses, corresponding to depths of approximately 0–10 m below the ground surface, revealed significant differences in undrained strength depending on the directions of the major and minor principal stresses, indicating anisotropic material behavior. Based on the undrained triaxial test results, the relationship among the undrained strength, the effective consolidation stress and the over-consolidation ratio (OCR) is presented for different stress paths to failure.The experimental data from the drained tests show that a failure surface comprised of a shear failure function based on the Mohr-Coulomb failure criterion and a tensile failure function based on the tensile strength and the confining stress can be applied for lime-cement-stabilized clay.     

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′.