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。     

高温后混凝土三轴压的强度及破坏准则研究

利用大型混凝土静、动三轴试验系统,对经历了200、300、500、600℃高温后的普通混凝土进行了三轴抗压试验,应力比设为0.1:0.25:1、0.1:0.5:0.1:0.75:1和0.1:1:1,测得了混凝土的强度,并根据试验结果,系统地探讨了应力比和温度对处于常规三轴和真三轴受压下混凝土强度的影响.发现随温度的增加,混凝土三个方向极限抗压强度均相应降低,但在相同温度下,混凝土的极限抗压强度比单轴时增大.建立了三轴压混凝土在高温后的破坏准则.     

冻结盐渍土三轴剪切试验过程中的损伤及压融分析

三轴剪切过程中损伤及压融等力学性质对冻结盐渍土的强度及变形影响显著。通过对-15℃德令哈含盐砂土进行一系列的三轴剪切循环试验及CT扫描试验,分析冻结盐渍土在三轴试验过程中的损伤演化以及压融量变化规律。研究发现,在三轴剪切中损伤变量随静水压力的变化而改变,在本文条件下当静水压力在1~12 MPa之间时,损伤变量随围压的增加而增加,当静水压力超过12 MPa时,损伤变量反而呈现减小趋势。同时,定义压融阈值p_s以及压融量达到最大值时对应的静水压力值p_m,研究表明:当静水压力小于p_s时,试样内部几乎不存在压融现象;当静水压力大于p_s且小于p_m时试验内部的压融量可用二次函数描述;当静水压力超过p_m时,试样中的压融量达到最大值λ_m,且压融极值点与损伤变量极值点对应的静水压力值基本一致。     

Strength and failure characteristics of jointed marble under true triaxial compression

The rock structure and three-dimensional stress state play a vital role in the mechanical behaviour of rock masses. Here, a series of true triaxial compression tests (σ₁ > σ₂ > σ₃) are conducted on jointed marble (50 × 50 × 100 mm³) containing a natural stiff joint, taken from the China Jinping Underground Laboratory (CJPL-II) project. The purposes of this study are to investigate the joint effect and estimate the stress dependency of jointed marble. The test results show that jointed marble can fail in four distinct forms, namely, splitting or shearing of intact marble, opening of the joint or sliding along the joint, and these failure modes are influenced by the joint configuration and the minimum and intermediate principal stresses. Generally, jointed marble has more brittle post-peak behaviour than intact marble. The linear Mogi-Coulomb failure criterion can be modified to describe the strength of the jointed marble under true triaxial compression. The jointed marble strength is more sensitive to the minimum principal stress than to the intermediate principal stress. A maximum decline of 25% in strength is observed, which corresponds to a joint dip angle of 60° at σ₂ = 60 MPa and σ₃ = 30 MPa. The link between the experimental results and in situ fracturing at CJPL-II is also demonstrated.

     

Strength and deformation properties of frozen sand under a true triaxial stress condition

In recent years, the mechanical properties of frozen soils under complex stress states have attracted significant attention; however, limited by the test apparatus, true triaxial tests on frozen soils have rarely been conducted. To study the strength and deformation properties of frozen sand under a true triaxial stress state, a novel frozen soil testing system, i.e., a true triaxial apparatus, was developed. The apparatus is mainly composed of a temperature control system, a servo host system, a hydraulic servo loading system, and a digital control system. Several true triaxial tests were conducted at a constant minor principal stress (σ₃) and constant intermediate principal stress ratio (b) to study the effect of intermediate principal stress (σ₂) on the mechanical properties of frozen sand. The test results showed that the stress–strain curve can be mainly divided into three stages, with evidence of strain hardening characteristics. The strength, elastic modulus, and friction angle increased with the increase in b from 0 to 0.6, but decreased when increasing b from 0.6 to 1, whereas the cohesion varied little with the variation in b. The deformation in the direction of σ₂ changed from dilative to compressive and that in the direction of σ₃ remained dilative throughout.     

COx黏土岩三轴压缩蠕变特性及速率阈值试验研究

 黏土岩具有低渗性和自我裂缝修复能力等优点,被用于高放废物地质处置的候选基岩。出于高放废物处置库的长期性及高安全性要求,研究黏土岩的蠕变破坏特征显得极其重要。通过一系列黏土岩的单级三轴压缩蠕变试验,获得Callovo-Oxfordian(COx)黏土岩较为精确的蠕变速率阈值范围。试验结果表明,发生蠕变破坏的蠕变速率阈值与黏土岩的湿度及所处围压等因素有关。总体上,当轴向蠕变速率低于2.5 µε/h,该类黏土岩很难发生蠕变破坏;但若高于58 µε/h,则黏土岩极易发生加速蠕变破坏;而处于两者之间的速率值,目前试验尚无明确结论。该阈值可用于在稳定蠕变阶段判断黏土岩是否会出现加速蠕变破坏。     

恒应变率下的岩石三轴动态变形过程模拟方法

针对岩石动态变形特点,首先引进Kelvin模型研究思路与方法,将岩石变形过程中的动态应力视为静态应力分量与动态应力分量的叠加,分别以黏性元件和非线性元件模拟岩石单元的动态应力分量与静态应力分量,并建立出岩石动态应力的力学分析模型;然后在此研究基础上,利用黏性元件的力学特性和统计损伤理论分别建立出黏性元件和非线性元件变形过程模拟方法,进而得到岩石三轴动态变形过程模拟方法,并提出其模型的参数确定方法;最后通过实例分析与讨论,表明该模型不仅能反映岩石动态变形过程的应变软化特性,而且能较好地反映加载速度或应变率对岩石三轴动态变形过程的影响,具有较强的合理性和可行性。     

考虑加载速率影响的冻结含盐砂土强度准则研究

围压、加载速率等外部条件对冻结盐渍土强度影响显著。对-15℃德令哈含盐砂土进行了一系列不同加载速率、不同围压下的常规三轴剪切试验,依据广义非线性强度理论建立了考虑加载速率影响的冻结含盐砂土强度准则。依据试验结果采用二次函数拟合得到了子午面上的破坏函数,分析了加载速率对冻结含盐砂土强度及内摩擦角的影响。通过修正的Lade-Duncan强度准则给出了?平面上的破坏函数,探讨了加载速率对子午面破坏函数及偏平面形状函数的影响。提出的模型能够反映在加载速率、压融以及冰晶破碎等因素共同影响下的冻结含盐砂土强度的非线性特点。     

Behavior of zeolite-cement grouted sand under triaxial compression test

Permeation grouting with cement agent is one of the most widely used methods in various geotechnical projects,such as increasing bearing capacity,controlling deformation,and reducing permeability of soils.Due to air pollution induced during cement production as well as its high energy consumption,the use of supplementary materials to replace in part cement can be attractive.Natural zeolite(NZ),as an environmentally friendly material,is an alternative to reduce cement consumption.In the present study,a series of consolidated undrained(CU) triaxial tests on loose sandy soil(with relative density D_r=30%)grouted with cementitious materials(zeolite and cement) having cement replacement with zeolite content(Z) of 0%,10%,30%,50%,70% and 90%,and water to cementitious material ratios(W/CM) of 3,5 and 7 has been conducted.The results indicated that the peak deviatoric stress(q_(max)) of the grouted specimens increased with Z up to 50%(Z_(50)) and then decreased.The strength of the grouted specimens reduced with increasing W/CM of the grouts from 3 to 7.In addition,by increasing the stress applied on the grouted specimens from yield stress(q_y) to the maximum stress(q_(max)),due to the bond breakage,the effect of cohesion(c') on the shear strength reduced gradually,while the effect of friction angle(φ')increased.Furthermore,in some grouted specimens,high confining pressure caused breakage of the cemented bonds and reduced their expected strength.     

三轴压缩作用下断续节理砂岩力学特性研究

节理岩体的力学特性直接影响工程岩体的安全。为了研究节理岩体的各向异性力学特性和破坏特征,设计进行了0°,30°,45°,60°,75°和90°等6种角度断续节理砂岩的三轴压缩试验,详细分析了节理倾角对断续节理岩体变形强度特征和破坏模式的影响。研究结果表明：①在加载过程中,随着围压增大,断续节理砂岩应力-应变曲线的屈服阶段逐渐明显,峰值强度和残余强度逐渐提高,破坏时延性特征逐渐明显;②随着节理倾角增大,断续节理砂岩的变形模量、抗压强度、黏聚力和内摩擦角等力学参数均呈现先减小后增大的U型变化趋势;③节理对岩样破坏裂纹的形成与开展具有明显的诱导和控制作用,不同倾角岩样的破裂面均顺节理倾角方向发展,当节理倾角与岩样计算破坏角接近的时候,岩样的破裂面顺节理面开展,变形和强度参数达到极小值;④随着围压增大,不同倾角断续节理岩样的变形和强度参数差别逐渐减小,各向异性特征逐渐减弱;⑤断续节理砂岩的破坏模式可分为张拉破坏、折线型的复合剪张破坏、沿节理面剪切破坏等3种类型,节理倾角的分布决定了断续节理砂岩在加载作用下的变形破坏模式,变形破坏模式的差异决定了断续节理砂岩变形和强度参数的各向异性特征。研究成果可为工程中节理岩体的各向异性特征分析提供较好的参考。     

冻结黄土抗拉强度与应变率和温度的关系

饱和冻结黄土的抗拉强度与多种因素有关，其中应变率和温度是最主要的因素。本文在试验基础上，得到冻结黄土拉伸时的应力-应变关系曲线，进而分析了极限抗拉强度随应变率和温度变化的响应规律，并对上述响应规律给出了相应的拟会公式。     

大理岩三轴压缩破坏的能量特征分析

岩石材料的变形和破坏与能量的变化密切相关。利用刚性伺服系统对大理岩岩样进行了系列三轴压缩试验,基于试验结果,对大理岩在加载过程中各阶段能量变化的具体数值进行了计算和整理,研究了能量变化在加载破坏各阶段分别与围压、应力、应变的内在联系。结果表明,初始围压的增大能够相当程度上提高岩样的破坏应变能。在既定围压下,岩样在弹性变形阶段的能量变化与偏应力和应变均成正线性关系。随着初始围压的增大,岩样所吸收的能量随偏应力变化的增长速率降低,随应变变化的增长速率加快。在三轴压缩过程中,岩样在弹性变形阶段所吸收的能量占总能量的比重较小,绝大部分能量耗散于岩样的屈服变形阶段;并且随着初始围压的增大,屈服变形阶段所吸收的能量占总能量的比重提高。     

混凝土三轴压力学性能研究现状

基于普通混凝土和钢纤维混凝土常温单调比例加载下的三轴试验研究,较为系统地介绍了混凝土三轴压试验的研究内容和试验方法,对比分析了两种混凝土的破坏形态、强度、变形等特性,对混凝土破坏准则和本构关系进行了简要论述,根据混凝土三轴试验中存在的问题,提出进一步研究的方向,为混凝土材料的深入研究提供基础。     

北山花岗岩在三轴压缩条件下的强度参数演化

 采用MTS815岩石力学试验机对北山新场深部花岗岩进行三轴循环加、卸载试验,研究岩石强度参数的演化特征。基于Mohr-Coulomb相关理论推导与分析,探讨岩石发生屈服后的强度变化规律。在分析不同围压条件下岩石全应力–应变曲线的基础上,以塑性剪切应变为塑性参数,建立北山花岗岩黏聚力、内摩擦角和剪胀角随塑性参数变化的数学模型。研究结果表明：(1) 在损伤应力点,岩石塑性剪切应变接近于0,损伤应力可作为北山花岗岩塑性参数的零点,其亦可作为岩石强度参数演化的起点。(2) 在损伤应力点后,岩石黏聚力随塑性参数的增加呈指数函数形式衰减并最终趋近于0;内摩擦角随塑性参数的增加以对数正态函数的形式表现出先增加后减小的趋势,且岩石残余内摩擦角值与起始内摩擦角值接近。(3) 损伤应力后的岩石剪胀行为与峰后剪胀行为相似,剪胀角随着塑性参数和围压的增加而不断减小,且对低围压条件更为敏感。(4) 将建立的模型嵌入到数值模拟工具中,通过模拟岩石三轴压缩试验,可证实模型的准确合理性。     

Effect of coarse aggregate size and cement paste volume on concrete behavior under high triaxial compression loading

When massive concrete structures (high-rise buildings, tunnels, dams, nuclear power plants, bridges, protection structures, …) are subjected to extreme loadings (aircraft shocks, rock falls, near-field detonations, ballistic impacts, …), the material undergoes triaxial compression loading at a high confinement. In order to reproduce high stress levels with well-controlled loading paths, static triaxial tests are carried out on concrete samples by mean of a very high-capacity triaxial press. It is a well-known fact that the cement paste volume and the coarse aggregate size are two important parameters of concrete formulation. This article focuses on identifying the effect of coarse aggregate size and cement paste volume on concrete behavior under high triaxial compression. This article shows that at low confinement, the concrete strength slightly increases as the coarse aggregate size increases. At high confinement, the coarse aggregate size has a slight influence on concrete deviatoric behavior and a significant influence on concrete strain limit-state. The higher the coarse aggregate size, the lower is the mean stress level corresponding to concrete strain limit-state. Furthermore, this article highlights that at low confinement, the concrete strength significantly increases with an increase in cement paste volume. Increasing confinement tends to reduce cement paste volume effect on concrete strength. At high confinement, contrary to what has been observed in unconfined compression, the cement paste volume has little effects on concrete deviatoric behavior. Otherwise, decreasing cement paste volume increases concrete deformation capacity. At very high confinement levels and at very high deviatoric stress levels, the axial tangent stiffness of concrete increases as the coarse aggregate size or the cement paste volume is reduced.     

不同应变率下煤矸石混凝土受压破坏研究

通过静态与动态试验,从弹性模量及临界应变两方面,探讨了不同应变率下煤矸石混凝土的受压破坏规律,得出了一些结论,为类似问题的研究提供了依据。