Anisotropic strength and deformational behavior of Himalayan schists

Anisotropy, which is characteristic of metamorphic rocks such as schists, is due to a process of metamorphic differentiation. Preferred orientation of minerals like mica and chlorite in response to tectonic stresses makes schistose rocks foliated. As a result their engineering properties vary with the direction of loading.The influence of transverse anisotropy on strength and deformational responses of four schistose rocks obtained from the foundation of two underground powerhouse sites in the Himalayas has been critically examined. Specimens at different orientation (β) of the foliations varying from 0° to 90° with respect to the axial stress (σ₁) in the unconfined state and also in the confined states up to 100 MPa of confining pressure were tested to evaluate the applicability of the non-linear strength criterion for the prediction of triaxial compressive strength and modulus. Based on the analysis of large experimental results it has been possible to predict strength and modulus with minimum pre-evaluation experimental data, i.e. only with three uniaxial compressive strength tests at 0°, 30° and 90° and two triaxial compression tests conducted at convenient confining pressures at β=90°orientation. Predicted non-linear stress–strain curves, using predicted values of strength and modulus have been found to match well with the experimental stress–strain curves even at higher confining pressures.     

Investigation of unsaturated frozen soil behavior: Phase transformation state,post-peak strength,and dilatancy

In this study, a database of triaxial compressive tests on unsaturated frozen soils is compiled to investigate the mechanical behavior that has not been considered in previous studies. The results for the stress-strain volume changes are presented first. Then, the physical mechanisms that might control the deformation of unsaturated frozen soils, namely, volumetric compression and frictional sliding, are used to interpret the changes in volume and deviator stress during the tests. The relationship between the compression rate due to the mean stress and the dilation rate due to shearing determines the changes in sample volume and shear stress with an increasing axial strain. The test results indicate that confining pressure and temperature significantly affect the phase transformation state, the post-peak strength, and the maximum dilation ratio. A higher post-peak strength ratio is observed in tests performed at higher confining pressure or under a higher temperature. As the confining pressure is increased, the shear stress at the phase transformation state initially increases and then decreases or stabilizes. However, the maximum dilation ratio decreases considerably and tends to reach zero after a certain confining pressure is reached. Both the deviator stress at the phase transformation state and the maximum dilation ratio are higher under a lower temperature given constant confining pressure. Ice cementation and pressure melting are attributed to the specific features of frozen soils compared to those of unfrozen soils. This paper provides new insights into the mechanical behavior of frozen soils.     

Mechanical behavior of lightweight soil reinforced with waste fishing net

Lightweight soil is cement-treated and consists of dredged clayey soil, cement, and air-foam. Reinforced lightweight soil (RLS) contains waste fishing net to increase its shear strength. This paper investigates the strength characteristics and stress–strain behavior of reinforced and unreinforced lightweight soils. Test specimens were prepared with varying admixtures of cement content (8%, 12%, 16%, and 20% by the weight of untreated soil), initial water content (125%, 156%, 187%, 217%, and 250%), air-foam content (1%, 2%, 3%, 4%, and 5%), and waste fishing net (0%, 0.25%, 0.5%, 0.75%, and 1%). Then several series of unconfined compression tests and one-dimensional compression tests were conducted. The experiments with lightweight soil indicated that the unconfined compressive strength increased with an increase in cement content, but decreased with increasing water content and air-foam content. The stress–strain relationship and the unconfined compressive strength were influenced by the percentage of waste fishing net. In addition, the strength of RLS generally increased after adding waste fishing net due to the bond strength and the friction at the interface between waste fishing net and soil mixtures, but the amount of increase in compressive strength was not directly proportional to the percentage of waste fishing net. The results of testing indicated that the maximum increase in compressive strength was obtained for a waste fishing net content of about 0.25%. The bulk unit weight of lightweight soil was strongly dependent on the air-foam content. The compression characteristics of lightweight soil, including the yield stress and compression index, did not depend greatly on whether the samples were cured underwater or in air.     

Applicability of Mohr–Coulomb and Hoek–Brown strength criteria to the dynamic strength of brittle rock

A series of dynamic uniaxial and triaxial compression, uniaxial tension and unconfined shear tests were conducted on the Bukit Timah granite of Singapore. The results are analyzed in this paper in order to examine the validity and applicability of the Mohr–Coulomb and the Hoek–Brown criteria to the rock material strength properties subjected to dynamic loads. The study indicates that rock material strength under dynamic loads can be approximately described by the Mohr–Coulomb criterion, at low confining pressure range. The change of strength is primarily due to the variation of cohesion with loading rate. The rock material strength under dynamic loads is better described by the Hoek–Brown criterion. Assessment of the Hoek–Brown criterion shows that the uniaxial compressive strength increases with increasing loading rate, and the parameter m appears unaffected by the loading rate.     

Fibre-reinforced cemented soils compressive and tensile strength assessment as a function of filament length

This study aims to develop a dosage methodology based on tensile and compressive strength for artificially cemented fibre reinforced soils considering filament length. The controlling parameters evaluated were the fibre length (l), the cement content (C), the porosity (η) and the porosity/cement ratio (η/C_iv). A number of unconfined compression and split tensile tests were carried out in the present work. The results show that fibre insertion in the cemented soil, for the whole range of cement content studied, and the increase of reinforcement length improve unconfined compressive and split tensile strengths. It was shown that the porosity/cement ratio, in which volumetric cementitious material content is adjusted by an exponent (0.28 for all the fibre-reinforced and non-reinforced cemented soil mixtures) to end in unique correlations for each mixture, is a good parameter in the evaluation of the unconfined compressive and split tensile strength of the fibre-reinforced and non-reinforced cemented soil studied. Analysis of variance (ANOVA) performed on the results of a factorial experiment considering the effect of adjusted cement content, fibre length and porosity showed that all of these factors are significant in affecting the measured changes in split tensile and unconfined compressive strength values. Finally, unique dosage relationships could be achieved linking the unconfined compressive strength (q_u) and the split tensile strength (q_t) of the sandy soil studied with porosity/cement ratio (η/C_iv) and fibre length (l).     

Stiffness of Frozen Soils Subjected to K0 Consolidation Before Freezing

The stiffness characteristic of artificially frozen soils subjected to K₀ consolidation at small strains are investigated at different initial confining pressure and frozen temperature by secant Young's modulus representing the soil stiffness. The influence of freezing on stiffness is investigated by comparing the behavior of frozen and unfrozen soils with similar conditions. It is observed that the frozen soils secant Young's modulus may decay with increasing strain after keeping a stage of constant value, the threshold yield strain was found to be approximately 0.05% for all tested conditions. On the other hand, the stiffness of studied frozen soils and its degradation was heavily response to the initial confining pressure amplitude and temperature fluctuation.     

冻土二元介质模型探讨——以-6℃冻结粉土为例

二元介质模型已成功用于模拟未冻结岩土材料,比如岩石、均质或各向异性结构性土、超固结黏土、堆石料以及黄土。类似地,为了探讨冻土的应力应变关系,此处引入二元介质模型来模拟其应力应变关系。基于岩土破损力学理论框架和二元介质模型概念,将饱和冻结粉土抽象成具有强胶结特性的胶结元（冻土骨架）和无胶结特性的摩擦元（融土骨架）,胶结元在一定围压下会产生压碎和压融现象,随围压的增大逐步破损并向摩擦元转化,二者共同承担外荷载。在-6℃和0.3~15.0 MPa围压下对冻结粉土进行了一系列低温三轴压缩试验,结果表明：随变形的增大,应力应变曲线均呈三阶段变化,分别是线弹性阶段、弹塑性阶段和应变软化阶段;强度随围压的增大呈先增大后减小的趋势,极限强度对应下的围压称为临界围压,且临界围压下的软化现象最不明显。通过细观角度运用二元介质模型概念探讨了冻土变形破损机理,在非均质材料均匀化理论基础上建立了冻土二元介质模型,讨论了破损率函数演化规律。理论与试验结果对比表明,所建立的模型可以较好地模拟冻土的应变硬化和软化现象。     

纳米黏土改良黄土力学性能试验研究

在陕西安塞地区黄土中掺入两种不同纳米黏土材料,设计不同掺量、含水率、养护龄期等多因素影响下的正交试验,并对改良后黄土进行三轴试验,分析凹凸棒土和纳米蒙脱土改善黄土无侧限抗压强度指标以及抗剪强度指标的变化特征,并采用SEM对两种纳米黏土改良黄土进行改善机理微观分析。结果表明:掺量为1%的凹凸棒土、掺量为2%的纳米蒙脱土对黄土抗压强度影响最为显著,在一定含水率下可使素黄土的抗压强度分别提高21%和42.3%; 两种纳米黏土改良黄土应力-应变曲线受试样干密度、围压及掺量的影响,干密度为1.35 g?cm^-3、围压为50 kPa的改良黄土有应变软化现象,干密度达到1.65 g?cm^-3时,4种不同围压下改良黄土应力-应变曲线均有应变硬化的趋势; 两种纳米黏土对改良黄土的抗剪强度指标影响效果显著,其中1%掺量的凹凸棒土、2%掺量的纳米蒙脱土可以使改良黄土的黏聚力分别提升61.02%和81.70%,内摩擦角分别提升27.86%和21.31%。     

陕北地区黄土隧道围岩工程特性试验研究

对延安-延川(陕晋界)高速公路黄土隧道原状黄土进行了物理力学性质试验,结果表明:除液性指数外,黄土围岩物性参数的变异系数均小于力学参数;可将9个物性指标根据其相关系数和因子分析结果分为3组,并根据它们与土的强度指标的关系,选取天然含水量、干密度和塑性指数作为黄土隧道围岩分级指标;深埋饱和原状Q₂黄土的应力-应变曲线在低围压下为应变软化型,高围压下表现为应变硬化型;无侧限抗压强度试验为小应变脆性破坏,应力-应变关系曲线有两种形式:一种是土样直接发生脆性破坏,丧失承载能力;另一种是先发生局部脆性破坏,仍保留有一定的承载能力。     

Unconfined compressive strength of fly ash reinforced with jute geotextiles

A series of unconfined compressive strength (UCS) tests have been conducted on unreinforced fly ash as well as fly ash reinforced with jute geotextiles. The effects of different governing parameters viz., degree of saturation, size of samples, number of jute geotextile layers and age of sample on UCS have been studied. From the test results it is found that the values of UCS are maximum at degree of saturation of 70–75%. The effect of sample size on the values of UCS for unreinforced fly ash is insignificant, whereas with increase in diameter of sample, values of UCS increase in case of reinforced fly ash. With increase in number of jute geotextile layers for reinforced fly ash samples, values of UCS increase and maximum enhancement is found to be around 525% with 4 layers of reinforcement. A non-linear power model has been developed to estimate unconfined compressive strength (UCS_R) of fly ash reinforced with jute geotextiles in terms of unconfined compressive strength (UCS_UR) of unreinforced fly ash and number of layers of reinforcement (N).     

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.     

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.     

Strength of massive to moderately jointed hard rock masses

The Hoek-Brown(HB) failure criterion and the geological strength index(GSI) were developed for the estimation of rock mass strength in jointed and blocky ground where rock mass failure is dominated by sliding along open joints and rotation of rock blocks. In massive, veined and moderately jointed rock in which rock blocks cannot form without failure of intact rock, the approach to obtain HB parameters must be modified. Typical situations when these modifications are required include the design of pillars,excavation and cavern stability, strainburst potential assessment, and tunnel support in deep underground conditions(around s1/s ci 0.15, where s1 is the major principal compressive stress and s ciis the unconfined compressive strength of the homogeneous rock) in hard brittle rocks with GSI ! 65. In this article, the strength of massive to moderately jointed hard rock masses is investigated, and an approach is presented to estimate the rock mass strength envelope using laboratory data from uniaxial and triaxial compressive strength tests without reliance on the HB-GSI equations. The data from tests on specimens obtained from massive to moderately jointed heterogeneous(veined) rock masses are used to obtain the rock and rock mass strengths at confining stress ranges that are relevant for deep tunnelling and mining;and a methodology is presented for this purpose from laboratory data alone. By directly obtaining the equivalent HB rock mass strength envelope for massive to moderately jointed rock from laboratory tests,the HB-GSI rock mass strength estimation approach is complemented for conditions where the GSIequations are not applicable. Guidance is also provided on how to apply the proposed approach when laboratory test data are not or not yet available.     

冻结砂土三轴试验中颗粒破碎研究

压力作用下颗粒发生破碎是引起砂土力学特性变化的重要因素之一,冻结砂土也是如此。对冻结砂土进行了不同温度和围压下的三轴剪切试验,并筛分得到三轴试验前后的颗粒大小分布曲线。通过引入Hardin定义的颗粒破碎率Br,分析了围压与颗粒破碎的关系及颗粒破碎对冻土抗剪强度的影响。结果表明:在温度为-0.5℃,-1℃,-2℃,-5℃和围压为0.5,2,5,10 MPa的条件下,三轴剪切过程中会产生较为可观的颗粒破碎;颗粒破碎率Br随围压增大,到达一定围压后Br不再随着围压的增大发生明显变化,即存在一个颗粒不再发生明显破碎的临界围压σr。结合前人研究发现,-5℃下一般工程关心的围压范围内压融对冻土力学特性没有显著影响,而颗粒破碎起控制性作用。分析表明:-5℃条件下在不同的围压范围颗粒破碎对抗剪强度具有不同的影响。试验所采用的围压范围内,随着围压的增大,颗粒破碎率增大使得冻土的抗剪强度降低;破碎率达到极限以后,由于破碎的颗粒重排列又导致抗剪强度有所提高。     

冻融循环对青藏粉砂土力学性质影响的试验研究

 为研究冻融循环作用对青藏粉砂土力学性质的影响,对不同冻融循环次数、冻结温度、围压下的粉砂进行常规静三轴剪切试验,研究冻融循环后土体的应力–应变关系曲线、静强度、弹性模量、抗剪强度参数的变化规律。试验结果表明：粉砂土的力学性质受冻融循环次数的影响较大,且在经历7～9次冻融循环后达到最小值。冻结温度对粉砂力学性质的影响较弱,且无一定规律。基于显著性分析理论,研究冻融循环次数、冻结温度、围压以及各因素间的交互作用对粉砂力学性质影响的显著性大小。分析结果表明：围压和冻融循环次数对力学性质影响的显著性较强,而冻结温度的影响较弱。同时,围压与冻融循环次数、冻结温度与冻融循环次数之间的交互作用对于粉砂的力学性质的影响均比较显著。     

长时高压K_0固结再冻结黏土的卸围压强度特性

深厚表土层中立井井筒建设普遍采用冻结法,而深部冻土的原位力学特性是影响冻结壁力学特性及其安全稳定性的关键;现有的浅部冻土的试验方法,由于忽略了深、浅部土体固结、应力环境及形成工况的差异,已难以可靠地获得深部土的力学参数。基于"长时高压K_0固结—冻结—恒轴压卸围压"试验模式,通过三轴试验,研究了深部土重塑人工冻结黏土的强度与变形特征,以及固结时间、固结应力的影响规律。结果表明:卸围压路径下冻土试样呈现为黏–弹塑性破坏,固结时间为1~7 d时,其卸围压强度随固结时间的延长增长显著,而单位降温引起的强度增长速率受固结时间的影响不明显;随着固结时间延长至28 d,其卸围压强度受固结时间的影响不明显,但单位降温引起的强度增长速率增加显著;单位降温引起的冻土卸围压强度增长速率不受固结应力影响。     

高应力下岩石非线性强度特性的试验验证

 深埋工程岩体开挖后围岩的强度特性表现出明显的非线性特征。基于室内岩石三轴加载及卸荷力学试验成果,对高应力下岩石的非线性强度特性予以验证,并开展高应力下应力路径对强度参数影响规律研究。采用已有的二次抛物线型、双曲线型、幂函数型等型式的包络线来研究强度特征的非线性,结果表明,幂函数型Mohr准则能够作为在高应力加载和卸荷应力路径下的岩石破坏的强度判据。在低围压下(＜10 MPa),三轴卸围压破坏强度要小于常规三轴强度;而在高围压下,前者略高于后者。内摩擦角的正切值与等效法向应力的函数关系表明岩样的实际内摩擦角并不是一个不变值,具有幂函数关系的非线性特征,在低应力下卸载破坏内摩擦角要比常规三轴压缩剪切内摩擦角略大,在高应力下则相反;根据Mohr准则中内摩擦角与理论破裂角之间的关系,随着应力增加它们的破裂角均呈非线性衰减并趋向π/4。     

Stress-Strain Relationships and Nonlinear Mohr Strength Criteria of Frozen Sandy Clay

A series of triaxial compressive tests were performed on frozen sandy clay at -4 and -6°C under confining pressures from 0 to 18 MPa. The experimental results indicate that the stress-strain curves show strain softening and hardening phenomena when the confining pressures are below and above 3.0 MPa, respectively. Since the generally hyperbolic model can not describe the strain hardening behavior very well and the Duncan-Chang model can not ideally describe the strain softening behavior of the frozen sandy clay, an improved Duncan-Chang model is proposed. This model can describe not only the strain softening behavior but also the strain hardening behavior of the frozen sandy clay, and the calculated results are rather coincident with the corresponding experimental data. In addition, it is also suitable for frozen silty clay with a high precision. Due to pressure melting, the shear strength of the frozen sandy clay changes nonlinearly with increasing confining pressures. In order to solve the problem that the linear Mohr-Coulomb criteria can not exactly reflect the shear strength of the frozen sandy clay, a nonlinear Mohr criteria of the frozen sandy clay is presented. The calculated results illustrate that it has higher precision and can describe the shear strength of frozen sandy soils more accurately than the linear Mohr-Coulomb criteria does.     

Strength Characteristics of Frozen Saline Soils

Methods for determination of the strength characteristics of frozen saline soils (FSS) are examined. The shear parameters C _lt and _lt for typical soils of the Yamal Peninsula are determined over a broad range of negative temperatures (from – sign;1 to – sign;8°C) and salinities (from 0 to 1.5%). Proposed tabular data on strength characteristics of FSS make it possible to use them in calculations to determine limiting loads on bed soils.     

The Role of Nature of Particles on the Behaviour of Rockfill Materials

The nature of the particles of seven alluvial rockfill materials and three quarried rockfill materials are expressed in terms of uncompacted void content and unconfined compressive strength of each of the rockfill materials. Drained triaxial tests are conducted on modeled rockfill materials. The behaviour of the rockfill materials is related to the uncompacted void content and unconfined compressive strength of the rockfill materials. A prediction procedure is proposed to determine the angle of shearing resistance of the rockfill materials using uncompacted void content and unconfined compressive strength of these materials.