水泥土强度预测室内试验研究

强度是水泥土设计的重要指标,水泥土强度预测可以为工程设计提供理论依据。目前国内外学者均是利用室内试验研究水泥土强度特性,导入强度评价参数,建立水泥土强度预测公式,但是强度评价参数不同。本文利用一系列室内试验结果,提出综合参数和水泥土强度预测公式,并对比分析目前较常用的水泥土强度预测公式。研究结果表明:水泥土强度随水泥掺入比和养护龄期的增加而增加,随含水比的增加先升高后降低;水泥土密度不随养护龄期的改变而改变。灰水比与水泥土初始密度的比值是描述水泥土强度的一个合理参数,提出的水泥土强度预测公式预测结果与试验结果基本吻合。     

原土环境下水泥土强度衰减过程室内试验研究

在水土耦合的室内原土环境中,通过微型贯入、扫描电镜(SEM)、能谱(EDS)分析、X射线衍射(XRD)、离子含量及pH值测试等多种试验手段,研究滨海相软土场地形成的水泥土强度的分布规律及其衰减过程,并阐明水泥土劣化层和未劣化层的发展规律。结果表明:水泥土劣化深度随养护时间的增长和水泥掺入比的减小不断增大,至360 d时最大劣化深度达到11.9 mm,明显大于同龄期海水环境中养护的水泥土的劣化深度;与未劣化层相比,劣化层的孔径增大,孔隙增多,水泥水化产物减少;经原土养护相同时间,水泥土中pH值及主要离子含量分布规律与室内海水环境中的水泥土相似,其中pH值和Ca~(2+)含量随着试样深度的增大而增大,而Mg~(2+)、SO_4~(2-)、Cl~-含量随试样深度的增大而减小;水泥土中Ca~(2+)含量沿试样深度方向的分布规律与强度变化规律相似。在原土条件下,水泥土中Ca的溶出更加显著,导致后期水泥土强度衰减加剧。原土中水泥土强度衰减过程与海水中相同。     

场地环境变化引起的水泥土劣化深度及预测

长期处于腐蚀场地中的水泥土等加固体会发生强度降低、渗透性增大的劣化现象,劣化的发生严重影响加固体的使用寿命。研究水泥土劣化随时间的演化规律及对劣化深度的预测,对于腐蚀地基中水泥土桩的长期承载力预测具有一定的理论和实际工程应用价值。利用室内试验模拟研究了场地环境变化引起的水泥土劣化问题。试验结果表明:水泥土劣化深度随浸泡时间的增加而增大,浸泡前期劣化速度快,后期劣化速度降低;水泥土初始强度越低劣化速度越快,当水泥土超过28 d强度后,劣化速度增加变慢,且趋于稳定。本文基于试验结果,提出了根据28 d劣化深度推测长期劣化深度的预测式,根据试验数据回归得到的待定参数A约在0.2～0.8之间,与腐蚀场地形成的水泥土劣化问题的0.5～0.7不同。     

海相软土场地形成的水泥土劣化深度预测

基于日照岚山港滨海相软土场地形成的水泥土的微型贯入试验,采用BP神经网络建立了水泥土劣化深度的网络模型来预测场地形成的水泥土的劣化深度。在模型建立过程中,将养护时间、水泥掺入比、养护条件、水泥种类、水泥强度等级、含水量及搅拌条件与水泥土劣化深度密切相关的7个参数引入到输入层,用Visual Basic语言编制了以水泥掺入比和养护时间为主要输入因素的计算程序,在样本训练和学习过程中,程序可对比显示实测和计算曲线。结果表明:水泥土劣化深度的预测结果与实测值较为吻合。说明运用BP神经网络模型预测水泥土劣化深度的方法是切实可行的。     

不同粘粒含量粉质土的动力强度特性研究

通过室内实验研究了黄河三角洲地区粘粒质量分数为3%～15%的饱和粉土动力强度特性.使用动三轴施加轴向动力,测得试样在动荷载作用下达到破坏时所需的振动次数及破坏过程中孔隙水压力的变化情况.试验结果显示每一组试样在不同循环应力作用下饱和粉土的破坏振次随剪应力比的增加而减小.通过对比分析不同粘粒含量粉土动强度特征曲线,得出粉土动强度对粘粒含量有很大的依赖性,在一定粘粒含量范围内,粉土动强度随粘粒含量增加先减小后增大.     

软黏土中成桩工艺对HSCM桩抗压承载性能影响的模型试验研究

螺旋桩芯劲性复合桩（helix stiffened cement mixing pile,简称HSCM桩）是一种新型复合桩,其成桩工艺会对桩身及其承载性能有较大影响。为验证HSCM桩在软黏土中同步旋进注浆工艺的可行性,并研究其成桩参数对抗压承载性能的影响,设计了2组缩尺模型试验,包括不同叶片数量与钻进速度的HSCM桩与对比螺旋桩。通过在高岭土制备的软黏土中成桩,并进行抗压承载性能及桩身几何尺寸测试,分析HSCM桩的成桩参数与水泥土桩身间的关系。试验结果表明：同步旋进注浆工艺能够在螺旋桩周围形成倒圆台状的水泥土桩身,水泥土桩身的平均黏结直径约为叶片直径的1.17～1.35倍;适当增加叶片数量能够使水泥与土体充分拌和,提高水泥土桩身的完整性与连续性,以改善HSCM桩的成桩质量;钻进速度大幅提高会导致注浆量不足,减小水泥土桩身的黏结直径与刚度;试验条件下HSCM桩的抗压极限承载力是螺旋桩的3.83～3.93倍,桩径扩大提高了侧摩阻力,注浆工艺加固并提高了土体强度,弥补了叶片在旋进过程中扰动土体造成强度降低的问题。     

循环荷载下粉土液化流动特性拖球试验研究

基于流体力学中的Stokes黏滞阻力理论,以振动台试验为基础,开发了一套测量液化过程中粉土流变特性的拖球试验装置。在铺有粉土海床的模型箱内埋设光滑小球,通过测量小球水平运动过程中所受阻力值的大小,计算粉土液化的表观动力黏度,分析粉土液化过程中的表观动力黏度与超孔压比之间的关系,以及液化后表观黏度与应变率的变化规律。试验结果表明,振动台试验下,孔隙压力表现为迅速上升,粉土迅速达到液化状态;振动过程对海床固结影响较大;粉土海床在未达到完全液化状态时（ru<1）,表观黏度随超孔压比增大而减小,在液化状态下（ru=1）,剪应力随应变率增大而减小,粉土呈现出剪切稀化的特点,为典型的非牛顿流体特征。     

海相软土场地水泥土劣化机理室内试验研究

将水泥土和周围土体作为研究对象,利用室内化学分析试验得到了离子浓度的时空分布规律,并从腐蚀离子干预水化反应进程和分解水化产物两个过程揭示了海相软土场地水泥土劣化机理。Ca^2+由水泥土向土体中扩散,Mg^2+、SO42^-及Cl^-从土体向水泥土扩散;随着水化反应的进行,Ca^2+不断生成,水泥土中足够多的Ca^2+是保证水化反应进行并维持水化产物稳定的必要条件,Ca^2+不断向土体扩散是水泥土劣化的原因之一;水泥土内部的SO42^-及Cl^-在浓度较低(分别低于9和15 g/L)时有利于水泥土强度的提高,浓度较高时则导致水泥土发生胀裂;水泥土中Mg^2+的存在会阻碍水化产物的生成并分解水化产物,但浓度较低(低于3 g/L)时,影响不明显;土体中Mg^2+、SO42^-及Cl^-浓度高于水泥土中的浓度,在水泥土表层与水化产物反应生成胶结性差及膨胀性高的物质,促使水泥土劣化。     

波浪作用下粉土中的孔压响应及其在粉土海床稳定性评价中的应用

粉土在波浪等动荷载作用下极易发生液化破坏,而孔隙水压力在粉土动力学行为中扮演了一个很重要的角色,其发展变化会直接影响到土体的稳定性。因此,通过室内波浪作用下的粉土孔压响应模型试验探讨了孔压与波浪之间的响应情况,发现波浪能量的影响沿土层深度递减,水深条件相同时,响应的孔压随波高的增大而增加,当波浪作用足够长时间后粉土发生液化破坏,此时粉土内累积的孔压小于上覆土体的自重应力。根据结果提出了1种评价粉土海床稳定性的方法。     

低围压下埕北海域重塑粉土振动孔压模型试验研究

开展了低围压条件下固结不排水振动三轴实验,对埕北海域重塑粉土振动孔压发展模型进行研究。低围压条件下粉土孔压随振次的发展曲线呈现两种形态,具体呈现何种形态与粉土轴向动应力和临界循环应力有关。对孔压数据进行了归一化处理,发现低围压条件下粉土孔压模型可以用指数函数进行拟合,且黏土含量并不影响孔压模型形式,只会影响a、b两个实验参数。孔压影响因素分析表明,少量黏粒含量的加入可以使粉土的孔压发展速度增大;振动频率对粉土孔压发展的影响也存在一个临界值,约0.2 Hz,当振动频率小于该值时,粉土孔压增长速度随频率的增加而减缓;当振动频率大于该值时,粉土孔压增长的速度随频率的增加而增大。     

Laboratory study on cement admixed marine silt bricks

For the purpose of efficient utilization of sediments dredged from harbor, a new method was proposed in this study. Marine silt bricks were made by mixing sediments with cement and gypsum, placing it in a cubic mold with 240 mm in length, 115 mm in width, and 53 mm in height, and curing for certain days. To investigate the effects of cement and initial water content of soil on the mechanical behavior of marine silt bricks, unconfined compressive and flexural strength tests were carried out. Given the same curing time and cement content, the higher the initial water content, the lower the compressive and flexural strength. After 60 days of curing, the compressive strength of marine silt bricks with cement content = 20% and water content = LL (liquid limit) reached approximately 5 MPa. The flexural strength was relatively low. The flexural strength of marine silt bricks with 20% cement and water content = LL was around 1.5 MPa. The compressive and flexural strength decreased with the increase of water/cement ratio. As for the curing time, longer curing time had a positive impact on the compressive strength. The ratio of flexural to compressive strength varied slightly in the range of 0.4–0.5.     

Experimental Investigation of Unconfined Compression Strength and Stiffness of Cement Treated Salt-Rich Clay

Soft clay with high sodium chloride salt concentration is a problem encountered by geotechnical and highway engineers. Chemical stabilization using cement is an attractive method to improve the engineering properties of soft soil. However, very limited studies have been conducted to reveal the effect of salt concentration on the engineering properties of cement-stabilized soil and the reported results in literature are not consistent. The impact of sodium chloride salt on the strength and stiffness properties of cement-stabilized Lianyungang marine clay is studied in this study. The clay with various sodium chloride salt concentrations was prepared artificially and stabilized by various contents of Ordinary Portland cement. A series of unconfined compressive strength (UCS) tests of cement stabilized clay specimen after 7, 14, and 28 days curing periods were carried out. The results indicate that a high sodium chloride salt concentration has a detrimental effect on the UCS and stiffness of cement-stabilized clay. The detrimental effect of salt concentration on the strength and stiffness of cement-stabilized clay directly relates to cement content. Soils mixed with high cement content are more resistant to the negative effect of salts than soils mixed with low cement content. The ratio of modulus of elasticity to UCS of cement treated soil does not have an obvious relationship with salt concentration. The findings of this study present a rational basis for the understanding of the impact of salt on the engineering properties of cement-treated soil.     

Primary yielding locus of cement-stabilized marine clay and its applications

ABSTRACT

Strength and stiffness properties of materials are widely studied and used in civil engineering practice. However, most studies are based on unconfined conditions, which are different from real status of soil. This study investigated the primary yielding and yield locus for cement-stabilized marine clay. In this study, two types of cement-stabilized soils were studied through isotropic compression, triaxial drained shearing, unconfined compression, and bender element testing. Specimens with 20–50% of cement content and 7–90 days of curing period were used for the tests. Stress–strain behavior and primary yielding were evaluated, followed by construction of the primary yield locus. The characteristics of the primary yield locus and its development with curing time then were studied. The results showed that the properties of the primary yield locus were dependent on the type of stabilized soil, but were independent of the cement content and curing period. Thus, the approach provides a way to estimate the primary yield stress and drained stress path before primary yielding for cement-stabilized soil under confined condition. An empirical function was used to fit the primary yield locus. The primary isotropic yield stress was correlated to unconfined compressive strength or maximum shear modulus. Three indirect methods were proposed to predict the primary yield stress for cement-stabilized marine clay. The results showed that the primary yield stress can be estimated with reasonable accuracy.     

Strength Characteristics of the Cement-Stabilized Surface Layer in Dredged and Reclaimed Marine Clay, Korea

A series of tests in both laboratory and field were performed to investigate the engineering and mechanical properties, especially flexural strength, of cement-stabilized soils. The strength of cement-stabilized soils mainly depends on water-to-cement ratio and curing temperature. The higher curing temperature and the longer curing time, the higher strength in cement-stabilized soils generates. The high ratio of water-to-cement results in lower strength. The compressive strength observed in the field is similar to the strength in the laboratory. Field tests on a cement-stabilized soil layer indicate that the strength is significantly affected by the thickness of the improved layer, which is directly related to the moment of inertia. In addition, the failure shape observed in a cement-stabilized layer in the field looks likes a bending failure type, because the flexural tensile strength, rather than the compressive strength, mainly dominates the failure of cement-stabilized layer. The flexural tensile strength is closely related to the moment of inertia. Therefore, the flexural tensile strength should be considered for determining the thickness and strength in improvement of soft clay.     

Strength Characteristics of the Cement-Stabilized Surface Layer in Dredged and Reclaimed Marine Clay,Korea

ABSTRACT

A series of tests in both laboratory and field were performed to investigate the engineering and mechanical properties, especially flexural strength, of cement-stabilized soils. The strength of cement-stabilized soils mainly depends on water-to-cement ratio and curing temperature. The higher curing temperature and the longer curing time, the higher strength in cement-stabilized soils generates. The high ratio of water-to-cement results in lower strength. The compressive strength observed in the field is similar to the strength in the laboratory. Field tests on a cement-stabilized soil layer indicate that the strength is significantly affected by the thickness of the improved layer, which is directly related to the moment of inertia. In addition, the failure shape observed in a cement-stabilized layer in the field looks likes a bending failure type, because the flexural tensile strength, rather than the compressive strength, mainly dominates the failure of cement-stabilized layer. The flexural tensile strength is closely related to the moment of inertia. Therefore, the flexural tensile strength should be considered for determining the thickness and strength in improvement of soft clay.     

Strength evaluation of marine clay stabilized by cementitious binder

Abstract

Evaluation of the strength of cement-treated clay with a broad range of mix ratios and curing periods was conducted using unconfined compression tests (UCTs). The influence of cement content, total water content, and curing period on the unconfined compressive strength of cemented clay are investigated. It is found that, at constant total water content, higher cement content results in higher unconfined compressive strength, while the total water content has an opposite effect. A power function can be used to correlate the unconfined compressive strength with the cement content or the total water content. For a fixed mix ratio, the unconfined compressive strength of cement-stabilized clay increases with the curing period, the effect of which can be characterized by a semi-log formula. Also, a strength-prediction model that considers both mix ratios and curing periods for cement-admixed marine clay is developed and validated; the model can capture the effect of clay type by considering the plastic index of untreated soils. It is also proved that the proposed framework for strength development is also applicable for other cement types.     

Stress-Strain Behavior of Light-Weighted Soils

Unconfined and triaxial compression tests were carried out to examine the behavior of light-weighted soils (LWS) consisting of expanded polystyrene (EPS), dredged soils, and cement with respect to initial water content. The stress-strain behavior of LWS are analyzed with varying initial water content and silt contents of dredged soils, cement ratio, and confined stress. As initial water contents increase, the compressibility index increases and the preconsolidation pressure was vice versa. As initial water contents increase, the slope of stress-strain curve in elastic zone increases and strain rate at failure decreases and the strain rate at failure was not changed by the being of foams. As initial water contents increase, a compressive strength of LWS decreases. The decrement ratio of compressive strength of LWS with foams increases as cement content increases and initial water contents decreases. The compressive strength increases as silt contents increases.     

On the compression behavior of remolded cement-admixed soft clay

Although extensive research has been performed on the mechanical properties of cement-stabilized clays, quite a few attempts have been made on the compression behavior of remolded cement-admixed clays. The results from oedometer tests have been discussed to investigate the compressibility of remolded cement-admixed clays, taking into consideration cement amount and curing time. The findings show that the difference in shape and position of compression curves is attributed to cement amount and curing time. Most compression index (C_c) values of remolded cement-admixed clays are greater than those of untreated clay due to the presence of remolded yield stress σ′_yr that is closely related to initial water content and clay fabric. Based on the obtained test data, the relationships of C_c vs. e₀, C_c vs. w₀, C_c vs. e₁, C_c vs. e_yr, and σ′_yr vs. e_yr are preliminarily discussed and quantitatively established. Especially, an important divergence of void index I_v at effective stress σ′_v less than remolded yield stress σ′_yr can be observed at different cement amounts and curing durations. Being independent on cement amount, curing time, and initial state of soil, an excellent convergence occurs at stress σ′_v greater than yield stress σ′_yr. The normalized compression curves of I_v vs. σ′_v at σ′_v?>?σ′_y can be expressed by a unique line that agrees well with intrinsic compression line (ICL) and extended ICL.     

Stabilization Mechanism and Effect Evaluation of Stabilized Silt with Lignin Based on Laboratory Data

The potential of a lignin-based by-product to stabilize silt was evaluated. The physical and mechanical properties of silt in its natural state, as well as when treated with varying proportions of lignin, were analyzed. The parameters tested include the particle size distribution, Atterberg limits, compaction characteristics, unconfined compressed strength, pH value, and electrical resistivity. To understand the stabilization mechanism of lignin-treated silt at a microscopic level, scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and Fourier transform infrared resonance (FTIR) spectroscopy were also carried out on lignin and representative samples after 28 days of curing. The results indicate that the engineering properties of silt are improved by the addition of lignin. Particle size distribution is changed and plastic index is reduced from 8.8 to 7.7. After improvement, the maximum dry density increases and the optimum moisture content decreases, while the change of dry density with moisture content is enhanced. The treated silt has greater strength performance than the natural silt in terms of unconfined compressed strength and all of the samples have a pH value lower than 10. Curing time and moisture content have a significant impact on unconfined compressed strength but almost no effect on pH. Micro-chemical analysis reveals that the improvement of performance exhibited by lignin-treated silt may be mainly attributed to the cation exchange and the formation of more stable soil structure by lignin cementing. The stabilization mechanism of lignin-treated silt was proposed according to the results of chemical analysis. It is shown that lignin-based stabilizers have potential to improve the engineering properties of silt.     

杭州湾浅层粉土动力特性研究

选用杭州湾广泛分布的浅层粉土为研究对象,进行了一系列动三轴加载试验,研究了结构性、加载频率及动应力对杭州湾海底浅层粉土动应变、临界循环应力比和动强度的影响。试验结果表明,同一加载条件下,重塑样比原状样在更小振次内达到破坏,原状样动强度大于重塑样;结构性是影响土样动强度的重要因素,结构性对土样强度的影响随着动应力的增大而增强,随着频率的增大而减弱;加载频率的变化也对试样动力特性有较大影响,随着频率的增大,试样达到破坏所需振次加大,试样临界循环应力比也随之增大。