Influence of Non-Plastic Fines on the Response of a Silty Sand to Cyclic Loading

Data from cyclic loading tests on sand-fine mixtures made of Ahmedabad sand and quarry dust are presented. Tests were performed at constant void ratio, constant relative density and constant sand skeleton void ratio, for a variety of fines contents. Instead of looking at the direct effect of fines content on the cyclic response of the sand-fine mixtures, a novel approach of analysing the cyclic test data is proposed, by normalising these data with respect to state. The important difference with other similar work on plain sand is that the reference line for normalising must take account of the fines content. Simple definitions of equivalent void ratio, which take account of fines content, have been used. The results presented indicate that state plays an important role in defining the cyclic resistance ratio and pore water pressure generation during cyclic loading.     

The Effect of Fines on Critical State and Liquefaction Resistance Characteristics of Non-Plastic Silty Sands

Monotonic and cyclic triaxial tests were carried out on sand-silt mixtures for the investigation of the effect of fines content on their critical state and liquefaction resistance characteristics. Both the undrained and the drained monotonic tests produce a unique critical state line for each tested mixture, which moves downwards with increasing fines content up to a threshold value of 35% and then upwards. At a given void ratio and mean effective stress, the liquefaction resistance ratio decreases with increasing fines content up to the same threshold value of 35%, and increases thereafter with further increasing fines content. However, at a given intergranular void ratio, defined as the ratio of the volume of fines plus voids to that of sand particles, liquefaction resistance ratio increases with increasing fines content up to the threshold value. The threshold fines content value, which is an important parameter in determining the transition from the sand dominated to the silt dominated behaviour of sand-silt mixtures, is related to their particle packing. An expression is proposed for the estimation of the threshold fines content as a function of the mean diameter ratio, d₅₀/D₅₀, and the void ratio. The results, presented herein, also show that for each tested mixture the liquefaction resistance ratio is related to the state parameter and that this relation is influenced by the effective stress level and fines content. The results on the sand-silt mixtures are supported by similar results on natural silty sands.     

Effects of Clay Content on Liquefaction Characteristics of Gap-Graded Clayey Sands

A series of undrained, cyclic simple shear tests were performed on reconstituted specimens with various clay contents to study the effects of clay content on liquefaction characteristics of clayey sands based on a framework of an idealized binary packing model and intergrain state parameters. From observed liquefaction characteristics, clayey sands with different clay contents can be grouped as sand-like or clay-like soils depending on the clay content and the transitional fines content of the sand-clay mixture. A simple equation is derived and verified to correlate the transitional fines content with the void ratios of the clean sand and the pure clay consisting of the mixture. In addition, a new relationship for clay content correction is proposed based on the linear relationship between the cyclic resistance ratio and the clay content at the same intergranular void ratio. The cyclic resistance ratio of sand-like clayey sands can be divided into two components: (1) the resistance of the sand skeleton at the specific intergranular void ratio, and (2) the increment of cyclic resistance from clayey fines. The rate of increment for cyclic resistance varies with the properties of contained clay particles. Data from three independent studies have shown the proposed procedure is promising.     

Maximum and Minimum Void Ratio Characteristics of Sands

Characteristics of the maximum and minimum void ratios of sands and their possible use for material characterization have been investigated in this study. Data of over 300 natural sandy soils including clean sands, sands with fines and sands containing small amount of clay-size particles have been used to examine the influence of fines, grain-size composition and particle shape on e_max, e_min and void ratio range (e_max - e_min). A set of empirical correlations are presented which clearly demonstrate the link between these void ratios and material properties of sands. The key advantage of (e_max - e_min) over-conventional material parameters such as Fc and D₅0 is that (e_max - e_min) is indicative of the overall grain-size composition and particle characteristics of a given sand and that it shows off the combined influence of relevant material factors. The void ratio range provides a general basis for comparative evaluation of material properties over the entire range of cohesionless soils.Important issues related to the laboratory procedures used for determination of e_max and e_min as well as their applicability to fines-containing sands are also addressed. Three distinct linear correlations were found to exist between e_max and e_min for clean sands, sands with 5-15% fines and sands with 15-30% fines respectively, thus illustrating that the standard JGS procedures for minimum and maximum densities of sands can provide reasonably consistent e_max and e_min values for sands with fines content of up to 30%. The importance of the grain-size distribution and presence of gaps in the grading of composite soils or mixtures of sands with fines produced in the laboratory is also discussed.     

粉粒含量对砂土强度特性的影响

对含无塑性粉粒的砂土进行了三轴固结不排水试验,结果显示:粉粒含量通过颗粒组成和结构对粉砂强度和变形产生重要影响。在粉粒含量为6%,9%和12%时,松散试样在100 kPa围压下出现了静态液化现象,而粉粒含量增至15%时该现象消失,且随着围压的增大该现象也消失;在同一粉粒含量下,粉砂土残余内摩擦角高于峰值内摩擦角,但残余偏应力值很小,这是由于孔压增长很大,抵消了内摩擦角由?p′提高到?r′的有利影响;稳态线也受粉粒含量的影响,表现在随着粉粒含量的增加,稳态线逐渐向下移动;各粉细砂强度和变形对围压的敏感性强于纯净砂;粉粒含量对孔隙比的影响是含量超出一定值后才变得显著的,这说明粉粒加入到砂骨架中时,并未全部充填至砂粒孔隙中,而是以砂粒间接触点或面上为主,这样的接触结构导致了粉砂具有高体缩性,由此使得粉砂样表现出典型的应变软化特征。     

Experimental study of the effect of fines content on dynamic compaction grouting in completely decomposed granite of Hong Kong

This paper addresses the effects of fines contents on the mechanical behavior of CDG (completely decomposed granite) during dynamic compaction grouting. The first set of tests involved adding different amounts of kaolin clay into CDG to study the effects of fines contents on the compaction characteristics, permeability, consolidation behavior and shear strength of soils. It is found that, as the fines content increases, the dry density decreases, however the optimum water content increases. In addition, the permeability decreases due to reduction of void ratio, and the coefficient of consolidation increases with higher fines content. Furthermore, with increasing fines contents, the shear stress tends to be constant after it reaches the peak strength. However, when reaching the peak value, the shear stress reduces with further adding of fines content. In the second set of tests, the dynamic compaction grouting tests are carried out to study the effect of fines content on the compaction efficiency of Hong Kong CDG. It is found that, the compaction efficiency increases with increasing fines content, and reaches the peak compaction efficiency when the fines content reaches 6%, and then the compaction efficiency drops abruptly from a peak value of 0.55–0.25 when the fines content increases from 6% to 8%. However, when the fines content increases from 8% to 41%, the compaction efficiency decreases very slowly.     

Correlating Cyclic Strength with Fines Contents Through State Parameters

Attempts have been made in the past few decades to establish an equivalent void ratio indicative of the effective density state of sands that contain fines. The concept of an equivalent void ratio is most useful when the grain size distribution of the sand/silt mixture is similar to that of binary packing. Due to its grain characteristics and the wide range of compressibility when the fines content changes, correlating the cyclic strength to fines content via the equivalent void ratio for Mai Liao Sand (MLS) is not a straightforward procedure. Earlier studies have demonstrated the effectiveness of relating cyclic strength to state parameters for sands. Using a series of cyclic triaxial test results as a database, the authors demonstrated the effectiveness of relating cyclic strength to fines contents (f_c) through a state parameter for MLS with an f_c ranging from 0 to 30% that takes advantage of the equivalent void ratio. In addition to the density and stress states, the state parameter based correlations indirectly reflect the effects of soil grain characteristics. The drawback of requiring a separate critical state line when f_c changes can be circumvented by using a unified critical state line based on the equivalent void ratio.     

A new laboratory apparatus for studying dynamic compaction grouting into granular soils

As granular soils may be compressible or have inadequate strength, compaction is particularly useful when soils are subjected to dynamic loading or cyclic loading. A new laboratory apparatus for investigating dynamic compaction has been designed and fabricated. The basic principle of this new technique is to introduce vibrations during the expansion process in static compaction grouting. In these tests, the injection pressure, the excess pore water pressure, and the change in void ratio of the specimens are measured. The main focus is to investigate the development of the injection pressure, the void ratio, and the excess pore water pressure due to dynamic compaction and the subsequent consolidation of the soils. In addition, the relative density of the soils is used to evaluate the dynamic compaction efficiency. Scaled laboratory experiments are conducted to study the effect of this dynamic compaction frequency on compaction efficiency. The experimental results show that the change in void ratio in the dynamic compaction tests is about four times greater than that in the static compaction tests. Dynamic compaction frequency plays an important role in soil densification due to dynamic compaction.     

重塑膨胀土的电阻率特性测试研究

通过对重塑膨胀土的电阻率进行测试研究,提出了一种饱和/非饱和土的假想电阻率模型。研究表明:膨胀土的电阻率主要与土骨架、孔隙水的电阻率以及膨胀土的含水率、饱和土度和孔隙比等有关。研究提出了用土的水体积比表征膨胀土的电阻率特性,研究建立了电阻率–水体积比与吸力–水体积比(水分特征曲线)之间的关系,以探索研究电阻率的应用。     

电渗作用下软土细观孔隙结构

软土细观孔隙结构变化是其宏观变形的根本原因。为了探索电渗中软土细观孔隙结构变化以及与宏观变形之间的关联机制,采用杭州软土开展电渗试验,监测了电渗中土体孔隙结构分布特征和含水量,从定性和定量两个方面对试验结果进行分析。研究发现,电渗过程中,粘土颗粒重新排列形成面面接触的片堆结构,土体孔隙比降低,孔隙空间形态变光滑,结构复杂性减弱。通过含水量计算所得孔隙比较实测孔隙比小,说明电渗排水量大于土体收缩量,这是因为,电渗本质为离子带动水分子的迁移,不能直接引起土骨架压缩。实际工程中,电渗法应与堆载、真空预压等联合使用。     

水土压力分算与合算的统一算法

 以土中孔隙水特征为基础,假定土体中的黏性土颗粒吸附的结合水会抵消一部分土中孔隙,最终把土的孔隙比、界限含水量、颗粒分析等物理参数引入土水压力计算,提出一个可以采用土层物理指标计算的水压力比率 ,给出通过土体渗透系数试验测定 值的方法,通过该参数可以把有效应力强度指标与总应力强度指标统一在一个强度公式中。同时,给出一个可以统一水土分算和合算的算法,实现水土压力分算和合算结果之间的过渡,为解决水土压力分算和合算结果的跳跃提供一个新思路。根据理论分析,渗透系数–孔隙比实测曲线应该沿e轴正向平移,土体中黏粒成分含量越高,平移量越大,相关文献的试验数据可证实这点;含有黏粒成分的砂性土采用水土分算偏于保守,而孔隙比较大的黏性土采用水土合算偏于不安全;围护结构上的水土压力计算不但与土体分类有关,还与孔隙比直接相关。     

高吸力下高庙子钙基膨润土的土水-力学特性

高庙子钙基膨润土具有明显的湿胀干缩特性,吸力变化会引起持水状态和孔隙比的变化。用蒸汽平衡法测得高吸力段（3～287 MPa）高庙子钙基膨润土的土水特性和变形特性。分析比较了吸力路径、试样初始孔隙比、应力加载历史以及土体结构对土水特性、变形及吸力应力的影响。研究表明,脱湿和吸湿过程中孔隙比的差异会引起体积含水率和饱和度的滞回。饱和度与吸力间关系受试样初始孔隙比和孔隙结构影响较大,与应力加载历史无直接关系。由吸力引起的吸力应力随吸力的增大而非线性增大,并在较高吸力时趋向于一定值。吸力应力受吸力路径、初始孔隙比以及孔隙结构的影响。     

Effects of void ratio and grain size distribution on water retention properties of compacted infilled joint soils

The effects of the initial void ratio and the grain size distribution (GSD) on the water retention properties of a compacted infilled joint soils from Beihetan (China) were investigated. Three initial void ratios (0.3, 0.4, and 0.5) and three GSDs were selected based on the in situ soil states. A total of nine drying water retention curves (WRCs) was established with the filter paper method. The microstructure of the specimens was also studied to better understand the water retention properties. It was found that the denser samples underwent smaller volume changes, and that the volumetric strain increased with the increasing clay size fraction. The void ratio had a significant effect on the WRCs in terms of the degree of saturation; however, the WRCs were independent of the void ratio in terms of the gravimetric water content. In terms of the degree of saturation, the WRCs were seen to shift upwards with the increase in clay size fraction, indicating an increase in the water retention capacity. The results from mercury intrusion porosimeter (MIP) tests revealed that the difference in the inter-aggregate pores is the main reason for the different shapes of the WRCs. Moreover, the infilled joint soils with lower void ratios and coarser particles were found to be more suitable for MIP-based evaluations of water retention properties.     

Strength and durability of concrete incorporating crushed limestone sand

In recent years, there is a growing interest in the use of crushed sand obtained from limestone quarries in some countries where river sand is not widely available. The fines content is usually high in crushed sand which can adversely affect concrete properties. The influence of fines in crushed sand on physical and mechanical properties of concrete has been widely investigated. The results proved that up to 15% of fines content in crushed sand could be used without adversely affecting concrete strength. However, little work has been done so far on the effect of fines in crushed sand on the durability of concrete. This paper examines the influence of limestone fines in crushed sand on concrete properties. Properties include strength, water, gas and chloride-ion permeability and capillary water absorption. Four different cement types were used while maintaining a constant water/cement ratio. The results show that concrete containing 15% of limestone fines as replacement of crushed sand reduces the water permeability and increases the gas and chloride-ion permeability.     

外海料源回填造地土壤静动态压密沉陷特性之探讨

采用云林离岛基础工业区水力抽砂回填外海料源区土壤为试验土样，以海水作为孔隙水介质，在实验室利用简连贵（１９９３）发展之水力抽砂法模拟现地水力抽砂回填方式准备试体，以获取回填造地土壤之颗粒堆积特性。另配合传统的湿捣法，以重量控制方式，配比不同相对密度与细料含量试体，进行一系列Ｒｏｗｅ氏压密试验，同时针对回填土壤之堆积特性、细料含量、相对密度及孔隙介质等影响因素深入探讨，藉以评估回填土壤之压密沉陷特性。利用本文所建立之关系式可完整评估回填造地土壤受静载重与动态荷重后的总沉陷量，提供填海造地高程设计与压密沉陷稳定分析之依据。     

基于流动性的饱和砂砾土液化机理

砂砾土的地震液化至今仍存较大的争议,相应的液化机理解释主要沿用传统的砂土液化分析思路和方法。利用动态圆柱扭剪仪开展了100 mm直径、3组典型级配（含砾量分别为37%,45%和60%）的饱和砂砾土试样循环动三轴实验。基于实验得到的应力-应变率关系曲线,定义了反应饱和砂砾土流动性的平均流动系数和流动性水平。实验发现,初始动应力比对不同含砾量下的平均流动系数-孔压比关系曲线无影响;相对密度越大、含砾量越大,饱和砂砾土的流动性水平越低;有效固结压力对饱和砂砾土平均流动系数-孔压比关系曲线的影响与含砾量相关。推测饱和砂砾土在循环荷载下的流动性由其粗粒接触状态和数量决定;粗粒间的接触在高孔压状态下不能顺利解除是饱和砂砾土与饱和细粒土抗液化性能的本质区别。提出的基于流动性的饱和砂砾土液化机理较好地解释了以上现象。     

Effect of Lateral Earth Pressure Coefficient on Pressure Controlled Compaction Grouting in Triaxial Condition

The design, fabrication, and assembly of a new laboratory apparatus for the investigation of the behavior of compaction grouting in triaxial condition are presented in this paper. Using this laboratory apparatus, pressure-controlled compaction grouting tests were carried out in specimens of completely decomposed granite (CDG) in Hong Kong. Precisely controlled injection water into a specially designed latex balloon in the specimen was to simulate a compaction grouting process. In these tests, the effective confining pressure, lateral earth pressure coefficient (K), excess pore water pressure, back pressure, void ratio change, and vertical deformation of the specimen were measured. The main focus was to investigate the development of injection pressure, void ratio, and excess pore water pressure due to compaction grouting and subsequent consolidation of soils. In addition, both the compaction efficiency and the average strength enhancement ratio are defined to evaluate the effect of compaction grouting.     

由于应力释放引起的深水土样扰动性研究(英文)

从深水海底取得的土样要经受很大的应力释放。模拟孔隙水中溶解有不同含量气体的深水黏土取样过程 ,对Lierstranda黏土进行了一系列三轴和高压固结试验。试验结果表明 ,自重应力下的孔隙比变化与孔隙水中的气体饱和度 η 成线性增长关系。其它测得的土性参数 ,如不排水强度、峰值应力下的应变、剪胀系数和先期固结压力等均随着 η 有规律地变化 ,显示土样的扰动性也随着 η 的增加而增加。据此 ,本文提出了对可能含有气体的深水土样的最佳贮藏与处置方案。     

基于等效骨架孔隙比指标的饱和砂类土抗液化强度评价

为探讨各类饱和砂类土的物理特性对其抗液化强度CRR的影响,将具有不同细粒含量FC的福建、南京和南通砂类土分为3组:(1)相同的相对密度Dr;(2)相同的孔隙比e;(3)相同的骨架孔隙比esk。对具有不同物理特性的三类饱和砂类土进行了一系列均等固结不排水循环三轴试验,结果表明:e或Dr相同的饱和砂类土的CRR随FC的增加而降低;但esk相同的砂类土的CRR随FC的增加而增大。结合文献中十类砂类土的CRR试验数据,分析表明:各类饱和砂类土的CRR随其等效骨架孔隙比*ske的增大而单调地降低,且两者呈现较好的幂函数关系。这表明能综合反映砂类土颗粒组成,密实状态和粗细粒接触状态的*ske是表征饱和砂类土CRR的一个有效物理特性指标;并首次发现饱和砂类土的CRR–*ske关系曲线的最佳拟合参数A和B值由砂粒和细粒的物理特性共同决定。据此,饱和砂类土的CRR可仅由其物理特性指标较好地预测。     

PHC管桩锤击施工效应分析

PHC管桩施工效应对地基土性质和桩体受力特性有很大的影响。为了查明PHC管桩施工效应,以PHC管桩加固某电厂古河道地基为依托,进行了PHC管桩沉桩效应的测试和分析。通过观测沉桩的锤击数和土塞效应,分析了PHC管桩沉桩规律性;利用静力触探、标准贯入试验、孔隙水压力和侧向位移观测等现场测试手段,研究了PHC管桩对桩周土体的影响。研究发现:沉桩锤击数与地基土性质有很大相关性,土芯长度与总桩长之比为22%～35%,且桩壁越薄其土塞效应越明显,在桩体承载力计算时忽略土塞效应是很不经济的;沉桩过程产生很大的挤土效应,不仅产生超孔隙水压力,而且随孔压消散,地基土得到密实、消除或降低液化性,其侧摩阻力可提高80%以上,有利于桩体强度的发挥。