The Influence of the Dielectric Constant and Electrolyte Concentration of the Pore Fluids on the Undrained Shear Strength of Smectite

It is common knowledge that the engineering properties of clays are greatly influenced by the type of pore fluids. However, the impact of the pore fluids on the geotechnical properties of the samples is even more dependent on the type of mineral and interlayer ions. Completely different behaviours could be observed with identical pore fluids but different clay minerals and vice versa. The liquid limit, plastic limit and undrained shear strength were determined for two types of smectites with different interlayer cations, namely Ca and Na-smectite. The pore fluids were varied by using different dielectric constants (ε) and electrolyte concentrations (n₀). The results show that the two kinds of soils respond in a similar way but with a different magnitude depending on the ion occupation.     

Undrained shear strength of clays as modified by pH variations

The undrained shear strength of clays is an important geotechnical parameter used during construction processes. Several laboratory tests were performed on kaolinite and smectite mixed with pore fluids with different pH values. Vane shear tests were carried out and it was found that the undrained shear resistance for clays increased considerably if the pore fluid had a high or a low pH. A possible explanation could be the dissolution of Al³⁺ which acts as a coagulant, increasing the internal shear resistance. Geochemical computations, Al measurements and ζ-potential experiments were performed to confirm this theory. The research suggests varying the pH may make a useful contribution to soil improvement techniques.     

温州煤场软土结构性试验研究

以温州发电厂 2 # 、 3# 煤场工程为背景 ,通过现场静探、十字板试验和室内试验 ,分析煤场软土的工程特性、软基十字板强度及其排水板施工扰动影响 ;从各项试验的结果出发 ,探寻温州软土的结构性在试验中力学性状上的体现 ,指出三轴试验中结构性土样破坏的新规律 ,最后提出确定结构屈服应力、原位压缩曲线的新方法     

Stabilization of clay soils using lime and effect of pH variations on shear strength parameters

This paper presents the results of geotechnical and mineralogical investigations on lime treated clay soils from Hamedan City, Iran, and effects of pH variations on their shear strength parameters. Initially, lime was added in different percentages and laboratory experiments were conducted after curing times. The results indicate that these soils can be stabilized satisfactorily with the addition of about 7 % lime. Also, investigation of the relationship between lime-treated geotechnical properties and lime percentage and curing time demonstrates high regression coefficients for the proposed relationships. Several laboratory tests were performed on treated and untreated clay soils with lime mixed with pore fluids with different pH values including 3, 5, 7 and 9. The results of shear strength tests indicated that the undrained shear strength parameters for untreated clays increased considerably if the pore fluid had a high pH (pH = 9) or a low pH (pH = 3). It can also be found that for lime-treated soils, maximum cohesion and friction angle values are achieved at pH = 9.     

The reliability of CPTu and DMT for the mechanical characterisation of soft tailings

Nowadays, the stability and the safe cover and closure of soft tailings has become one of the most challenging topics in geotechnical and environmental engineering. For appropriate analysis and modelling, geotechnical properties of tailings should be well defined. Due to low strength, viscous nature and poor trafficability, as well as due to the specific geometrical properties of fine particles and pore water chemistry in tailings, different test methods and their modification and improvements must be used for valid and reliable characterisation of tailings. This paper analyses and compares the results of different field test methods, piezocone penetration test (CPTu), flat dilatometer test (DMT) and field vane test (FVT), performed on the red gypsum sludge and complementary laboratory tests. CPTu and DMT measurements were evaluated using procedures developed for soils and the obtained mechanical properties were compared with those measured in the laboratory. The significant distinctions between different test methods and different empirical correlations were analysed taking into account differences in index properties of soils and the red gypsum sludge. Based on analysis, some modifications of conventional empirical correlations were proposed for field tests results interpretation and sludge characterisation. The obtained data also show the advantages and limitations of some conventional methods and prove the need for further development in that field of geotechnical investigation.

     

孔隙液体对玻璃砂透明土强度特性影响研究

玻璃砂透明土由一种玻璃砂(或称熔融石英砂)和与之折射率相一致的孔隙液体制配而成的新型人工合成透明土,为岩土工程可视化模型试验方法提供技术支撑。开展混合油、溴化钙溶液、蔗糖溶液以及纯净水等不同孔隙液体所制配成的透明土的三轴固结排水(CD)试验,分析该新型透明土材料的应力–应变关系曲线、体积变形等力学特性以及推导其邓肯–张模型参数。试验结果表明,不同孔隙液体所制配成的透明土试样力学特性规律相似、数值上有所差异;由蔗糖溶液制配成的透明土强度最大、由混合油制配成的透明土强度最小、由溴化钙溶液制配成的透明土强度值与水样熔融石英砂的强度相对更接近。     

The study of pore structure and its influence on material properties of cement-based piezoelectric ceramic composites

In order to investigate the pore structure and its influence on the material properties of cement-based piezoelectric ceramic composites, specimens with different ceramic/cement ratio of composite are prepared. The pore structure of composite is obtained by using mercury intrusion porosimetry (MIP). The variety of pore structure is attributed to the chemical reaction at the surface of PZT particles and cement grains, which is different with that of cement paste. And the porosity of composite reduces with the increasing of ceramic/cement ratio. Moreover, the influence of pore structure on the mechanical and dielectric properties of composite are discussed in detail. A monotonic decreasing of material properties of composite with total intruded pore volume is found.     

黄河三角洲细粒土微振液化分析

在黄河口潮坪选择典型研究区,利用现场原位对其循环荷载试验,通过实时孔压监测、静力触探试验、十字板剪切试验及原状样土工试验对比,研究了在循环荷载作用下黄河三角洲地区饱和细粒土的动力响应过程、土体比贯入阻力、峰值强度、残余强度和灵敏度的变化以及孔隙水压力和液化发生层的变化对黄河口粉质土液化过程的影响。研究结果表明:循环荷载作用使土体对能量吸收特性的变化、孔压的响应与振动导致的土体结构性变化有密切的关系;循环荷载作用导致黄河三角洲地区粉质土液化主要发生在硬壳层;随着荷载循环次数增加,液化发生层的深度向深部扩展;循环荷载作用使土体的物性指标变化趋势因深度的不同而具明显的差异性。     

基于CPTu数据的黏性土不排水抗剪强度#br# 神经网络预测

利用人工神经网络模型,建立基于孔压静力触探（CPTu）现场测试数据的黏性土不排水抗剪强度的预测方法。为建立和验证人工神经网络模型,在3个场地开展CPTu和十字板剪切现场测试,共取得33个测孔的CPTu试验数据和相对应的不排水抗剪强度实测值。通过对比分析不同输入向量、不同网络隐层数、不同神经元数及不同改进算法对人工神经网络模型性能的影响,确定人工神经网络模型的具体形式。通过对训练组数据开展机器学习,所建立的人工神经网络模型能够有效地基于CPTu获得的端阻力和孔隙水压力现场测试数据对黏土不排水抗剪强度进行预测,预测结果与十字板剪切试验实测结果非常接近。与传统用于估算不排水强度的经验关系相比,采用人工神经网络模型预测结果与实测结果相关性显著提高、误差明显降低。     

Fines content for optimum stability within coastal dune sands within Northern KwaZulu-Natal,South Africa

This paper describes the results of geotechnical and geochemical laboratory tests, carried out on dune sands from northern KwaZulu-Natal, to which increasing percentages of fines (<63 µm) were added. The geotechnical tests detail the effects of the fines content on a number of properties of the dune sands, such as the permeability, maximum dry density and shear strength. The peak shear strength and dry density of the dune sands occurs when 20–30% fines were added, whilst the permeability was seen to decrease continuously with increasing fines. Compaction of the samples was seen to significantly increase the shear strength of the dune sands together with a corresponding decrease in the permeability. The results from these tests enabled slope stability analysis to be carried out. Typical fines used in the geotechnical tests were analyzed by XRD, and the clays were isolated and their morphology studied using a transmission electron microscope (TEM). Fines identified include kaolinite, quartz, montmorillonite, calcite, feldspars, iron-oxides and illite. The fines content of the dunes sands has been shown to profoundly effect the geotechnical properties of the dune sands.     

Consolidation of soft clay foundation improved by geosynthetic-reinforced granular columns: Numerical evaluation

Soft clays are problematic soils as they present high compressibility and low shear strength. There are several methods for improving in situ conditions of soft clays. Based on the geotechnical problem's geometry and characteristics, the in situ conditions may require reinforcement to restrain instability and construction settlements. Granular columns reinforced by geosynthetic material are widely used to reduce settlements of embankments on soft clays. They also accelerate the consolidation rate by reducing the drainage path's length and increasing the foundation soil's bearing capacity. In this study, the performance of encased and layered granular columns in soft clay is investigated and discussed. The numerical results show the significance of geosynthetic stiffness and the column length on the embankment settlements. Furthermore, the results show that granular columns may play an important role in dissipating the excess pore water pressures and accelerating the consolidation settlements of embankments on soft clays.     

含水砂层的剪切流变力学特性试验分析

通过十字板剪切流变仪试验,得到了剪切阻力、含水量、孔隙水压力与应变速率的变化曲线。试验表明,含水砂层的流变曲线呈现稳态流变阶段和加速流变阶段。稳态流变阶段又可分为弹性变形、初始流变和非加速流变过程。初始流变过程含水量和孔隙水压力上升,后保持稳定;经过较长时间的流变后,进入非加速流变阶段,剪切阻力呈现波动下降趋势;达到屈服抗剪强度时,进入加速流变阶段,应变速率迅速增大。通过对加速流变过程曲线进行拟合,得出加载剪应力、屈服抗剪强度和应变速率的关系。分析表明,干燥细砂的抗剪强度主要来自颗粒间的摩擦力,当含水量增加时,颗粒被水分子包围,削弱了颗粒间的作用力,其流变特性就发生了明显的变化;反映这一变化过程的主要参数是屈服抗剪强度,其与加载剪应力和含水量有关。     

上海软土小应变三轴试验及本构模拟

软土地区新建地下工程为了减少对周边的影响,周边土体的变形通常要求控制在小应变范围内(0.001%~0.1%)。近年来随着环境要求的不断提高,软土在小应变范围内的力学特性日益受到关注。目前国内在三轴试验中测量软土小应变力学特性的研究相对较少,也缺乏相应的试验数据。利用安装有LVDT局部位移传感器的三轴仪,对上海(2)~(6)层软土进行了K0固结不排水剪切试验,成功获得了上海软土从0.001%小应变到20%大应变范围内的剪切模量变化规律。通过分别用初始剪切模量和有效平均主应力来对剪切模量进行归一化处理,揭示了上海软土的非线性特征、土体剪切模量的衰减规律等。考虑土体应力状态、孔隙比和超固结比的经验公式能够合理地描述上海软土层初始剪切模量;经典的骨干曲线模型能较好地拟合各土层的剪切模量衰减规律。     

高压旋喷扩底桩承载机理的现场试验研究

为了研究高压旋喷扩底桩和高压旋喷桩作用机理及加固效果的异同,结合某粮食储备库平房仓车间粮食堆场地基处理工程,详细介绍了工程处理中现场地面沉降、土体变形、桩周土体水平位移、孔隙水压力、桩土应力比、十字板剪切、静力触探、载荷板等试验的现场测试方法与成果。试验结果表明：高压旋喷扩底桩对桩周土体的加固效果比高压旋喷桩明显,尤其是高压旋喷扩底桩的承载能力明显大于高压旋喷桩,采用高压旋喷扩底桩的复合地基承载力满足工程设计要求。     

Effects of gas hydrates dissociation on clays and submarine slope stability

Gas hydrate dissociation is often considered as a precursor or triggering factor for submarine slope failures occurring in relatively deep waters where the bulk of the gas hydrate is found in fine-grained sediments. However, there are actually relatively few studies that focus on the effect of gas hydrate dissociation on the behavior of clays, and very few on what physically happens to clay during and after the dissociation process and how gas hydrate dissociation affects the geotechnical properties of clays. In this paper, we illustrate the effects of hydrate dissociation in clays from laboratory strength tests (direct simple shear) combined with visualization including very-high-resolution 3D imaging (computed tomography), using R11 as the hydrate forming fluid in both laponite and Onsøy clay. The test results reveal that the hydrate dissociation creates bubbles in the surrounding clay matrix and around pipe/well models. In addition, we use CO₂-saturated water as the pore fluid in soft clay, and test results show that cracks may develop, allowing gas migration to take place after reducing back pressure in an oedometer cell. Direct simple shear tests show that the undrained shear strength decreases by up to ～15% due to this process. The test results were then implemented in a 2D finite element model to assess the influence of hydrate dissociation on submarine slope stability. We chose a slope segment west of Svalvard—an area where methane gas bubbles escape from the seabed. The gas bubbling in this area is likely due to climate-controlled hydrate-dissociation (warming of bottom water masses). In the finite-element model, we include the change of methane hydrate stability zone (MHSZ) with time as well as the hydrate-dissociation-induced failure zone, which may be a potential leakage pathway. The numerical study indicates that the hydrate dissociation caused by bottom water warming is unlikely to be the main cause generating a leakage pathway or failure plane. However, the hydrate dissociation causing the reduction in shear strength facilitates a potentially unstable condition. The results imply that the hydrate dissociation may contribute to slope failure as a secondary driver, but are unlikely the main driving force. The aim of this study was to improve our understanding of the physical processes of gas expansion, migration and effect of hydrate dissociation through visualization and a finite element model. In addition, this study discussed methods to detect gas hydrate through a case study, and it was found possible to predict average gas hydrate saturation at sites where the sulfate-methane transition depth is known.     

Effective Stress Change and Post-Earthquake Settlement Properties of Granular Materials Subjected to Multi-Directional Cyclic Simple Shear

In order to investigate the effect of cyclic shear direction on the properties of saturated granular materials, such as effective vertical stress reduction and post-earthquake settlement, several series of multi-directional cyclic simple shear tests under constant volume conditions are performed on Toyoura sand and granulated blast furnace slag (GBFS), as an alternative material. The GBFS has particular properties such as light weight, high shear strength and high permeability, and it is considered to be one of the most promising materials in geotechnical engineering. From the test results, it is clarified that the shear strain amplitude has a significant effect on the changes in the effective stress of granular materials. However, at higher levels of shear strain amplitude, the cyclic shear direction has little influence on the effective stress reduction. It is found that the vertical strain, after the cyclic shearing of the GBFS samples, was lower than that of the Toyoura sand under the same test conditions. Finally, to evaluate the changes in effective stress under uni-directional and multi-directional cyclic simple shear conditions, an estimation method is represented by a function of cumulative shear strain G* and resultant shear strain Γ. The validity of this proposed model is confirmed by comparing the experimental and the calculated data obtained under multi-directional cyclic simple shear conditions.     

Geo-mechanical behavior of clay soils stabilized at ambient temperature with fly-ash geopolymer-incorporated granulated slag

Geopolymer is a cementitious material that can replace ordinary Portland cement in several geotechnical engineering applications, such as soil stabilization, with the advantages of much lower harmful emissions and energy consumption. This paper presents a rigorous evaluation of the geo-mechanical behavior of different types of clay soils treated with geopolymer, including the influence of soil characteristics and mineralogy. Two natural clay soils in addition to a commercially available kaolin clay were used for this investigation. Laboratory experiments were performed including unconfined compressive strength (UCS) and consolidated undrained (CU) triaxial compression tests under different confining pressures. The UCS and triaxial tests indicated that the addition of geopolymer considerably increased the yield stress and initial stiffness of all examined clays. With the increase of geopolymer content, the stress–strain behavior of treated clays was found to develop progressively from ductile response into a post-peak brittle fashion. The CU tests also demonstrated that the addition of geopolymer changed the initial characteristics of remolded clays from quasi-over-consolidated to heavily over-consolidated, rendering high yield surface and more effective shear strength parameters (i.e., cohesion and friction angle). Moreover, although the overall qualitative stress–strain and stress path responses of the clays were similar, significant quantitative differences were observed, particularly in terms of the attainable yield strength, stiffness, and shear strength. These differences can be attributed mainly to the heterogeneity associated with the soil mineralogy and the corresponding differences in the interaction between the clay/non-clay minerals and geopolymer.     

主应力轴旋转下(b=0.5)原状软黏土孔压特性

采用浙江大学空心圆柱扭剪仪(ZJU-HCA),对杭州原状软黏土进行固结不排水主应力轴旋转试验,探讨剪应力变化、初始剪应力水平高低及主应力轴正向和逆向旋转对黏土孔隙水压力(简称孔压)发展的影响。试验中保持平均主应力不变,中主应力系数b=0.5。试验结果表明:定向剪切产生的孔压主要受剪应力控制;用双重屈服面理论得到孔压系数,结合孔压线性拟合结果发现,若初始剪应力低于峰值剪应力,体积屈服面函数不受主应力轴旋转影响;若初始剪应力接近峰值剪应力,主应力轴开始旋转时的孔压上升速率相对较高,但后期孔压反而会下降;纯主应力轴旋转过程中的孔压上升速率主要受初始剪应力大小控制。主应力轴正向旋转较逆向旋转产生的孔压小;主应力轴逆向旋转相对正向旋转对整个剪切过程的孔压发展有较大影响。     

Effects of fine aggregate content on the mechanical properties of the compacted decomposed granitic soils

Decomposed granitic soil is widely used as the subgrade material in Korea. The mechanical behavior of the soil can differ depending on the fine aggregate content of the soil. Due to particle crushing during loading and compaction, the soil may behave as either cohesive or cohesionless. In order to evaluate the effects of fine aggregate content on the mechanical properties of the compacted decomposed granite soils, the direct shear test, the triaxial shear test as well as some fundamental property tests were made by using samples with the five different fine aggregate contents of 10, 20, 30, 40, and 50% by weight. Results of the triaxial tests showed that as the percentage of the fine aggregates decreased, the cohesion decreased as well as the internal friction angle increased. In the direct shear test, the soils containing lower amount of fine aggregates exhibited that the shear strength was considerably reduced after it reached the peak value. Comparably, soils with the high fine aggregates showed that the shear strength was quite constant after the peak strength. The critical state friction parameter showed that the parameter decreased with an increase of the fine aggregate content. Results of the triaxial and direct shear tests indicated also that the fine aggregate content of 30% could be one indicator for soil materials behaving either as sand or as clay materials. Thus fine aggregate content, less than 30% by weight, could be more obtainable for the subgrade materials when leading also to the increased stability of the roads. With the lower fine aggregate content the soils then also show similar and better characteristics of sands not clays. Further studies are recommended on understanding of the mechanical behavior of the decomposed granitic soils.     

Properties of Cement-Treated Soils During Initial Curing Stages

The engineering properties of cement-treated soils manufactured by the so-called “Pipe Mixing Method” and “Super GeoMaterial (SGM) Method” were studied. In these methods, clayey soils with high water contents are mixed with cement and used as fill material. Since the cement-mixed soils are transported through a pipeline, whose length at times exceeds 2 km, the properties of the treated soil during the initial stages of the hardening process are important. Bender element, vane shear and fall cone tests were performed to obtain such engineering properties as the shear modulus and the shear strength. The study revealed the following: 1) The minimum shear wave velocity of treated soils is detectable at around 2.8 m/s, corresponding to a shear modulus of about 12 kPa. 2) A linear correlation between the shear modulus and the shear strength exists even in the very early stages of curing, approximately G=300 s, where G and s are the shear modulus and the shear strength, respectively. This relation is similar to that for natural clays. 3) The “setting time” observed for concrete is also apparent in cement-treated soil materials. 4) Fall cone tests comprise a useful and simple technique for measuring very low levels of shear strength.