Stress-Deformation Behavior Under Anisotropic Drained Triaxial Consolidation of Cement-Treated Soft Bangkok Clay

In this study the compression behavior of high water content cement-treated soft Bangkok clay is further investigated by conducting a constant stress ratio (CSR) test at various stress ratios (η). The test utilized cement-treated clay specimens with cement contents (A_w) of 10% and 15%, each of which was in combination with 100% and 130% total clay water contents. The test results confirmed that the ratio of after-curing void ratio to cement content (e_ot/A_w) can effectively characterize the compression behavior of cement-treated clay. The specimens with higher values of e_ot/A_w yielded higher volumetric and shear strains at the same stress ratio. While those with lower values of e_ot/A_w resulted in lower shear strains, with consequent higher values of strain increment ratios (dɛ_v/dɛ_s) both before and after transitional yield points. Significantly, the e_ot/A_w ratio has described the relationship of the compression yield loci of cement-treated clay at various stress ratios and mixing conditions.     

基于扰动状态概念理论的固化淤泥一维压缩模型

采用低碳、环保的镁质水泥对淤泥进行固化处理,研究了不同龄期下固化淤泥一维压缩特性。采用新型环刀制样方法,开展了镁质水泥固化淤泥的一维压缩实验研究。结果表明:固化淤泥的压缩曲线与结构性土变化趋势类似,固化淤泥的ln(1+e)–lgσ′v曲线存在一个明显的拐点(结构屈服应力),且随着龄期的增长,固化淤泥的结构屈服应力逐渐增大。基于扰动状态概念理论,提出了能反应固化淤泥强化与弱化并存的一维压缩模型,试验验证了该模型可以较好预测任意龄期下固化淤泥的一维压缩特性。     

结构性黄土压缩屈服特性及湿压模型

湿陷性黄土具有很强的结构性,在增湿及侧限压缩应力作用下,其变形特性表现出与一般黏性土不同的特性。针对这一问题,通过对3种不同初始结构性黄土的侧限压缩试验,对黄土在增湿及压缩应力条件下黄土的宏观力学特性及其结构变化特征进行研究。研究结果表明:1)在增湿及侧限压缩应力作用下,黄土的e-lgp曲线分成三段,即平缓状态段、非线性段和线性段;2)原状黄土的压缩曲线与重塑土压缩曲线的变化给出了同一压力下黄土架空排列结构破损后孔隙比的可能变化量;3)原状黄土与饱和黄土压缩曲线的差别揭示了同一压力下黄土颗粒间胶结丧失后结构性黄土孔隙比的可能变化量;4)结构屈服应力与初始含水率之间满足幂函数关系,压缩指数与初始含水率之间呈指数函数关系。     

Modeling compression behavior of cement-treated zinc-contaminated clayey soils

In this paper, the compression behavior of cement-treated soil with various cement contents and zinc concentrations is presented and modeled by the destructuring framework and the concept of the Intrinsic Compression Line (ICL). The void ratio of a cement-treated sample with Zn contamination is the sum of the void ratio sustained by the intrinsic soil fabric (destructured void ratio) and the additional void ratio due to cementation. The compression index at the pre-yield state, C_s, increases as the Zn concentration increases or as the cement content decreases. At the post-yield state, the additional void ratio is inversely proportional to the effective vertical stress. The rate of reduction in the additional void ratio is controlled by the destructuring index, b. The values for b and yield stress are mainly dependent upon the degree of cementation, which is controlled by the cement content and the Zn concentration. Based on a critical analysis of the test data, a practical (simple and rational) method for assessing the compressibility of cement-treated soil with various Zn concentrations is suggested. The proposed predictive method is useful not only for quickly determining compression curves, with acceptable errors, but also for examining the results of tests on cement-treated zinc-contaminated soil.     

Macroscopic and microscopic investigation of the structural characteristics of artificially structured marine clay

Artificial structural soil is the best medium for simulating the structural characteristics of undisturbed soil, which is a unique property of undisturbed soil. To figure out the macroscopic and microscopic structural properties of artificially structured soil, this study proposed a method of disposing artificial structural soil to simulate in situ marine clay. In addition, a series of one-dimensional compression tests considering three influencing factors—the initial void ratio, interparticle cementation strength (determined by cement content) and pore size distribution (determined by initial salt content)—were conducted to explore the macroscopic structural properties of artificially structured marine clay. Then, SEM and MIP tests were performed to investigate the microstructure properties of artificially structured marine clay. With respect to the macro-scale experiment, the results show as the initial void ratio increases, the yield stress decreases while the compressibility increases in postyield stage. The pore size distribution mainly affects deformation, which increases with the quantity of relatively large pores. In terms of the micro-scale test, we found that the general microstructure is composed of aggregates of small clay platelets, some of which are either attached to the aggregates or linked to the surrounding aggregates with cemented bridges crossing the interaggregate pores. The most and least common pore space types feature entrance pore diameters of 1 ～ 17 μm and larger than 17 μm, respectively. An increase in cement content causes increases in intra-aggregate pore space and interparticle cementation strength. Initial salt particle content exerts an influence predominantly in the pores with entrance diameters of 0.3 ～ 1 μm and 1 ～ 17 μm.     

One-Dimensional Compression Behaviour of Uniformly Graded Sand Related to Single Particle Crushing Strength

In order to compare one-dimensional compression behaviour with the crushing strength characteristics of single particles, one-dimensional compression and single particle crushing tests were carried out on various granular materials with uniform gradings. The average characteristic tensile stress acting on a particle in a sample was calculated using a simplified approach. The one-dimensional compression yield stress was related to the particle size and this was also related to the single particle crushing strength. The decrease of the vertical yield stress with increasing initial void ratio can be explained by the increase in the particle characteristic stress as the void ratio increased and a corresponding decrease in co-ordination number. The single particle strengths were compared with the characteristic tensile stress for a particle embedded in the soil matrix. The logarithm of the characteristic tensile stress at maximum compression index for a particle embedded in a granular matrix could be related to the logarithm of the single particle strength multiplied by a proportional factor. The introduction of a factor was necessary because the calculated average characteristic tensile stress did not take into account the non-uniform distribution of inter-particle stresses. The ratio of the single particle strengths to the average characteristic tensile stress for a particle embedded in the soil matrix was considered to be an indicator of the non-uniform distribution. This ratio decreased and hence uniformity of inter-particle stress distribution increased with angularity and surface roughness.     

Geotechnical characterization of a clay–cement mix

Soft clay deposits are highly plastic, normally consolidated fine grained soils characterized by their low inherent shear strength. The mixing of soft clays with cement as a chemical stabilizer has become a well-known stabilization technique. The resulting strength of the clay–cement mix is controlled by different factors, but mainly the water to cement ratio, the cement content, and the curing conditions. It is crucial to develop a clear understanding of the changes in engineering behavior of the clay–cement mix that result from changes in controlling factors. A phase diagram was established to define the initial conditions of the mass–volume relationships of air, cement, clay, and water of a typical clay–cement mix. This phase diagram was then used to determine the total dry density, void ratio, and specific gravity of the clay–cement mix as a function of the cement content and water to cement ratio. The main objective of this work was to develop generalized trends for the geotechnical properties of clay–cement mixes. These trends were evaluated based on unconfined compressive strength as well as consistency tests carried out on soft clay samples before and after mixing with cement and at different curing times. A reduction in the plasticity index (PI) of 16 % and an increase in the unconfined shear strength of more than 200 kPa were obtained from the addition of 15 % cement. The reduction in the PI of the clay–cement mix was found to be an efficient tool to represent the improvement in the strength of the clay after mixing with cement.     

Yielding of cement-treated marine clay

This paper presents the findings of an experimental study on the primary yielding and post-yield behavior of cement-treated Singapore marine clay. The study was conducted using unconfined compression tests and triaxial tests. The results show that all the primary yield loci for the cement-treated marine clay have a consistent shape regardless of the mix ratio, curing stress or curing period. Three relationships are proposed for determining the size of the primary yield locus. The first two involve the direct determination of the isotropic primary yield stress, whereas the third makes use of the unconfined compressive strength. The first two relations are valid only for 7-day specimens. The third appears to have slightly larger scatter, but it is also applicable over a wider range of curing period and curing stress. Post-yield, over-consolidated samples were obtained by compressing specimens isotropically under effective stress levels higher than their isotropic primary yield stress and then allowing them to swell back to a lower effective confining stress prior to shearing. The normalized yield loci of these pre-yielded samples show a “collapse” from steep arches to more-rounded ellipses, while the yield loci expand with isotropic pre-compression pressure.     

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.     

Assessment of strength development in blended cement admixed Bangkok clay

Fly ash and biomass ash have been widely accepted as waste materials substituting Portland cement. In this paper, the role of these two ashes on the strength development of cement admixed low-swelling Bangkok clay is investigated via unconfined compressive (UC) test and thermal gravity (TG) analysis. Fly ash and biomass ash are dispersing materials, increasing the reactive surface of the cement grains. The pozzolanic reaction does not play any significant role on the strength development with time since the amount of Ca(OH)₂ is insufficient to react with the ashes. The contribution of the dispersing effect to the strength development is regarded akin as an addition of cement. Based on this premise, the clay–water/cement ratio hypothesis for blended cement admixed clay is proposed for analyzing and assessing the strength development. Even with the difference in water content, cement content and ash content, the blended cement admixed clay samples having the same clay–water/cement ratio, w_c/C possess practically the same stress–strain response and strength. The relationship among strength, clay–water/cement ratio, and curing time for the blended cement admixed Bangkok clay is finally developed and verified. It is useful to assess the strength at any curing time wherein water content, cement content, and ash content vary over a wide range by using the test result of a single laboratory trial. For the economic mix design (the most effective dispersing effect), an addition of 25% ash is recommended. It can save on the input of cement up to 15.8%.     

Influence of applying overburden stress during curing on the unconfined compressive strength of cement-stabilized clay

In the case of cement-stabilized soils cured under the influence of overburden stress, it is necessary to consider two different timelines of cementation and consolidation, along which the structures of stabilized soils evolve. These two timelines are interrelated and ought not to be considered separately, especially in the early stage of curing when significant structural changes occur. In this study, the strength and deformation characteristics of cement-stabilized clay subjected to overburden stress during curing was investigated using an unconfined compression test apparatus. For this purpose, three types of specimens were prepared by stabilizing very soft clay with different amounts of cement. Overburden stress was applied for three different time durations during curing. In the series of unconfined compression tests conducted on the specimens with no overburden stress applied during curing, the axial stress initially increases rapidly, then was maintained before finally dropping. In the series of tests with overburden stress applied during curing, the axial stress continues to increase gradually before rapidly declining. The values of unconfined compressive strength are larger for the specimens subjected to overburden stress during curing. The values of the unconfined compressive strength also increase linearly as the duration of applied overburden stress increases. The increase in strength apparently occurs due to increasing dry density accompanied by a decrease in the water content due to the drainage of pore water during the application of overburden stress.     

饱和高庙子膨润土的渗透特性

在核废料深层地质处置工程中,膨润土的渗透特性是需要考虑的重要因素之一。对浸水饱和的高庙子钙基膨润土压实试样进行压缩试验,利用时间平方根法测定压缩试验中每级荷载下的固结系数,然后用太沙基一维固结理论计算土的饱和渗透系数。试验研究表明:在弹塑性阶段,计算所得饱和渗透系数随竖向应力的增大而减小,在双对数坐标下,计算渗透系数与竖向应力呈线性关系;孔隙比对渗透性起着主导作用,在半对数坐标下,计算渗透系数随孔隙比的减小而线性减小,由该线性关系得到的某一孔隙比下的计算渗透系数与公开发表的相同孔隙比的高庙子钙基膨润土的渗透系数接近。研究结果及相关文献证明应用固结理论方法间接推算膨润土饱和渗透系数的方法是可行的,该方法适用于低渗透性黏土的饱和渗透系数的量测。     

红粘土的水—土作用力学效应试验研究

对红粘土试样进行物化分析试验和常规室内试验,结果表明该土样结构为铁质胶结,胶结物主要为针铁矿,且土样含水量高、孔隙比大、塑性高。对不同含水量的试样进行直剪强度试验,在三种垂直压力作用下,红粘土的抗剪强度都随含水量的升高而减小。对经过不同pH值的Al（NO3）3溶液和Fe（NO3）3溶液浸泡后的试样进行直剪试验,结果显示...     

改善高塑性有机质红黏土强度试验研究

桂林石灰岩地区其基岩面处的红黏土常处于高塑性状态,含有机质时其强度低。通过对高塑性有机质红黏土的加固试验,研究其在有无外加高效减水剂的情况下,水泥掺量、养护时间及水灰比对其无侧限强度的影响。试验表明:高塑性有机质红黏土在水泥加固条件下,其无侧限抗压强度会随着水泥掺量及养护时间的增加而增加,随着水灰比的提高而减小;高效减水剂能明显提高和改善高塑性有机质红黏土的强度力学性质。该试验研究可为红黏土地区的水泥土加固优化设计及施工提供科学依据,以满足实际工程的需要。     

江苏海相软土压缩特性试验研究

对原状江苏海相软土样进行一维压缩和蠕变试验研究。试验数据表明,江苏海相软土的初始压缩指数Cc和次压缩指数Cα与天然含水率和孔隙比具有一定相关性,尽管初始压缩指数Cc和次压缩指数Cα不是常数,Cc和Cα与垂向压力大小有关,Cc和Cα在统计意义上又有相关性,且Cα/Cc接近常数,基于Cα/Cc比值可预测次固结变形,理论预测结果与实测结果基本一致。     

固化污泥压缩特性研究

固化技术是污泥进行填埋处置常用的处理技术之一,固化污泥的压缩参数是进行填埋场库容计算和稳定性评价的重要指标。选用水泥作为固化材料,开展了不同材料添加量和不同养护时间条件下固化污泥的压缩特性研究,并从固化污泥水分形态和转化角度对压缩性变化机理进行了探讨。结果表明,即使水泥添加量达到30%,固化污泥的压缩指数也高达0.71,是一种高压缩性的土。水泥的添加量在10%~20%时,固化污泥压缩指数降低显著,但是超过20%以后,压缩指数变化趋于稳定。其原因主要在于污泥中的水分具有较高的结合势能,过多的水泥并不能获得更多的自由水发生水化反应。固化污泥的压缩指数随着养护时间的增加而降低,当达到14 d时趋于稳定,但60 d以后压缩指数又出现降低趋势,其原因主要是微生物逐步分解污泥结合水中的碳氢化合物,有机质含量下降,导致压缩指数降低。     

侧限条件下增湿时湿陷性黄土的变形及持水特性

 用改装的非饱和土直剪仪在无应力及净竖向应力作用下对湿陷性黄土进行分级增湿试验,测量增湿过程中变形及吸力的变化,分析增湿前孔隙比(无应力时)及净竖向应力对增湿时湿陷性黄土的变形及持水特性的影响,探讨无应力与净竖向应力作用时增湿持水特性之间的关系,提出力水耦合作用下孔隙比与吸力关系的表达式及以饱和度与吸力关系表征的持水特性模型。研究结果表明：力水耦合作用下,孔隙比与归一化吸力关系可用对数函数来描述,可以用孔隙比的变化来反映净竖向应力对持水特性的影响。无应力及净竖向应力作用时,只要孔隙比相同,则饱和度与吸力间关系相同。吸力小于阈值时,增湿前孔隙比和力水耦合作用所致孔隙比的变化对含水率与吸力间关系皆有较大影响;大于阈值时,几乎没有影响,且可用同一幂函数描述,据此提出考虑孔隙比变化的修正V-G模型,该模型可以预测无应力及净竖向应力作用时湿陷性黄土的增湿持水特性,预测结果与试验结果吻合较好。     

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.     

非饱和土一维压缩试验及变形规律探讨

为研究非饱和土在侧限条件下的变形规律,进行了控制初始干密度和基质吸力的一维压缩试验,提出切线压缩系数和切线体积压缩系数的概念,发现切线压缩系数和切线体积压缩系数均随净竖向压力的增大呈指数衰减规律,建立了描述孔隙比–净竖向压力关系和竖向应变–净竖向压力关系的经验模型,结合国内外文献中的试验数据对模型的合理性及其对不同类别非饱和土的适用性进行了验证。提出的模型可用于计算任意压力间隔内非饱和土的平均压缩性指标,计算非饱和土由于浸水产生的一维湿化变形,还可用于描述非饱和土随基质吸力增大的一维收缩变形规律。     

常含水率三轴条件下非饱和原状黄土的吸力和力学特性

为了模拟实际工程在不排水条件下土体的快速破坏,对不同初始吸力非饱和原状黄土进行了常含水率等向压缩和三轴剪切试验,研究了常含水率下非饱和原状黄土的变形特性、临界状态、屈服及吸力变化特性。研究结果表明: 在等向压缩和三轴剪切应力条件下,非饱和黄土的变形和吸力变化特性不同;不同初始吸力下,偏应力与净平均应力临界状态线为平行直线,偏应力与有效平均应力临界状态线可以归一为一条直线,且可近似用饱和土临界状态线表示,半对数坐标系上孔隙比与净平均应力及有效平均应力临界状态线皆近似为平行直线,直线斜率比饱和土的大,比屈服后等向压缩曲线的小;对于不同初始吸力,相同有效平均应力下非饱和原状黄土与饱和黄土的临界状态孔隙比之比与气体饱和度关系可以归一;屈服净平均应力及屈服偏应力皆随着初始吸力的增大而增大;q-p平面上初始及后继屈服曲线为对称于K₀线的椭圆,吸力及应力硬化对屈服曲线的影响是等向的。