A new criterion for strength of frozen sand under quick triaxial compression considering effect of confining pressure

The strength of frozen soils has been one of the most extensively studied aspects in frozen soil mechanics. When carrying out deep excavations using freezing methods, high stress states are often encountered. Therefore, the strength of frozen soils under high confining pressures is of major concern. This paper first reviews the present failure criteria for frozen soils, especially with regard to the effect of confining pressure on the strength. It is suggested that the strength consists of two components, cohesion and friction, and can be expressed by the Mohr–Coulomb criterion and in the framework of the Drucker–Prager criterion. However, these two components are both dependent on the stress state. Duncan’s equation for the friction angle is extended to frozen soils. A frozen Lanzhou fine sand is taken as study subject. The frozen sand is compressed at a high strain rate under three different temperatures and under a wide range of confining pressure. Experimental results have confirmed the generally recognized principle that the strength increases with the confining pressure up to certain value. Thereafter, it decreases with continued increase in confining pressure. Experimental data from our own tests and literature are fitted to the new criterion, which shows its validity.     

定向剪切应力路径下冻结黏土变形特性试验

为研究复杂应力路径下冻土的强度与变形特征,采用冻土空心圆柱仪（FHCA-300）对不同负温状态下的饱和冻结黏土开展定向剪切试验,基于不同剪切方向下冻结黏土的轴向和扭剪分量的应力-应变关系,探讨土样的剪切变形特征、各向异性属性以及剪切带的演变规律,并考察温度、大主应力方向角、平均主应力以及中主应力系数等因素对冻结黏土强度的影响。结果表明：平均主应力p值对冻结黏土的应力-应变关系影响显著,尤其是p=4.5 MPa时具有较高的剪切强度,且该值可能为压融临界p值;大主应力方向变化会诱发冻结黏土的各向异性,随着大主应力方向角的增加,冻结黏土剪切强度逐渐降低,并有明显的剪切带产生;中主应力系数的增加使得轴向强度有逐渐降低的趋势,但对剪切强度影响不明显;随着温度的降低,冻结黏土强度逐渐增大,试样发生脆性破坏并出现剪切破裂面,其剪切强度主要取决于冰颗粒和土颗粒的胶结力。     

Anisotropic mechanical properties and brittleness evaluation of layered phylliteEI北大核心

为了研究层状千枚岩的力学特性与各向异性特征,开展了不同层理倾角与不同围压下的千枚岩力学试验。对比分析了千枚岩试样强度、变形、脆性与破坏模式等各向异性特征。结果表明:(1)随着层理倾角β增加,岩样的强度、变形特征曲线形状呈U型;随围压增加,岩样强度及塑性增强,各向异性度逐渐减弱稳定。(2)采用多种强度准则描述岩样强度各向异性,其中Saeidi准则和改进Ramamurthy准则能很好地预测岩样在不同层理倾角下的抗压强度。(3)基于岩样峰前应力-应变曲线与能量特征提出了综合脆性评价指标,在层理倾角β=45°左右时,岩样脆性指标较低,更易发生剪切滑移破坏,得出脆性下降顺序为:沿层理面拉伸劈裂>穿层理面拉伸劈裂>沿层理面剪切>穿层理面剪切。(4)千枚岩的破坏模式与层理倾角和围压有关,单轴条件下,岩样劈裂破坏后易形成复杂裂纹网络;高围压下,岩样破裂后多形成单一的沿层理面或贯穿多层理面的剪切破坏。     

循环荷载作用下软土的孔压模式和强度特征

通过循环三轴剪切试验,研究原状软土的变形和孔压发展规律。建议了一个循环荷载作用下孔隙水压力的发展模式。研究了不排水循环荷载作用下软土的动强度以及作用后饱和软土静强度的衰减特征,讨论了周围固结压力和荷载作用频率对动强度的影响。研究表明,在某一轴向应变条件下,循环次数随循环剪应力比的增加而迅速减小;随循环剪切应力比的增大,饱和软土的静强度有一定衰减。     

超固结黏土单调和耦合循环的剪切特性研究

针对超固结黏土空心试样,利用土工静力-动力液压三轴-扭转多功能剪切仪,在均等固结条件下进行了单调扭剪和三轴-扭转耦合循环剪切试验。试验结果表明：不同超固结比黏土的单调扭剪强度也可由正常固结黏土的单调扭剪强度得到,得到了不同超固结比下饱和黏土的强度及模量的退化规律;随着超固结比的增大,相同破坏循环次数的动应力比和临界循环应力比均线性增大;超固结比对耦合循环剪切的孔隙水压力的发展模式影响显著。参考Yasuhara的测量方法,采用荷载停止后继续采集孔压的方法可以更好地反映黏土在耦合循环荷载下产生的真实孔压和孔压的增长情况。提出的综合应变式同时考虑了剪切变化和正向偏差变形的共同效应,适合作为主应力连续旋转的耦合循环剪切试验的破坏标准。     

冻土的单轴抗压、抗拉强度特性试验研究

在寒区工程建筑物设计中,冻土的抗压、抗拉强度是两个重要的力学指标.在负温条件下,对粉质黏土、黄土和砂土进行单轴抗压和劈裂抗拉试验,研究冻土破坏时的破坏形态、破坏机理、应力-应变曲线和拉应力与径向位移关系曲线的形式,分析单轴抗压强度和劈裂抗拉强度的差异以及这两种强度随土质特性和温度的变化规律.试验结果表明：单轴载荷作用下试样破坏后呈鼓状,且表现为应变软化型塑性破坏特征;劈裂作用下产生沿直径向试样两侧延伸的裂缝,不同土质破坏后裂缝扩展的宽度和深度不同;冻土的抗压强度与抗拉强度均与负温存在很好的线性相关性,随温度的降低而增大;在相同温度条件下,冻土的抗压强度大于其抗拉强度;对于同一种冻土,其抗压强度的温度效应比抗拉强度的温度效应显著.本试验分析结果可为寒区工程的实际应用提供参考.     

中主应力系数影响下的冻结砂土损伤本构模型

人工冻结法是饱水砂层开挖过程中常用的止水和临时支护方法,通过冻土损伤特性研究为冻土力学特性和冻结体稳定性分析奠定基础。为研究冻结砂土的损伤力学特性,在-5℃下进行了不同中主应力系数的冻结砂土三维室内试验。从冻土微元破坏服从Weibull随机分布的特点出发,将Drucker-Prager强度准则作为冻土微元统计分布变量,利用应变等价性假说,建立了三维应力状态下冻结砂土损伤本构模型;在此基础上,讨论模型参数F₀与m和中主应力系数的关系,对模型参数进行合理修正,建立中主应力系数影响下的冻结砂土损伤本构模型,并与试验结果进行对比。分析结果表明:参数F₀和m随着中主应力系数的增大呈现先减小后增大的趋势;参数F₀反映了冻结砂土的强度特性,参数m代表了冻结砂土的延性及脆性特征,考虑中主应力系数影响的冻结砂土损伤本构模型能很好地模拟冻结砂土应力-应变全过程曲线。研究成果为人工冻结法工程设计提供一定的理论依据。移动阅读      

DEM analysis of the onset of flow deformation of sands: linking monotonic and cyclic undrained behaviours

This study explores the link between the monotonic and cyclic undrained behaviour of sands using the discrete element method (DEM). It is shown that DEM can effectively capture the flow deformation of sands sheared under both monotonic and cyclic undrained loading conditions. When subjected to cyclic shearing, flow-type failure is observed for a loose sample, while cyclic mobility is observed for a dense sample. A strong correlation between the monotonic and cyclic loading behaviour that has been revealed experimentally is also confirmed in DEM simulations: (a) flow deformation occurs in the compressive loading direction when the cyclic stress path intersects the monotonic compression stress path prior to the monotonic extension stress path, and vice versa; (b) the onset of flow deformation in q–\(p^{\prime }\) space is located in the zone bounded by the critical state line and the instability line determined from monotonic simulations. Hill’s condition of instability is shown to be effective to describe the onset of flow failure. Micro-mechanical analyses reveal that flow deformation is initiated when the index of redundancy excluding floating particles drops to below 1.0 under both monotonic and cyclic loading conditions. Flow deformation induced by either monotonic or cyclic loading is characterized by an abrupt change of structural fabric which is highly anisotropic. The reason why the dense sample dilated during monotonic loading but showed cyclic mobility (temporary liquefaction) during cyclic loading is attributed to the repeating reversal of loading direction, which leads to the periodic change of microstructure.     

Undrained Cyclic and Monotonic Strength of Sand-Silt Mixtures

In an attempt to correlate the monotonic peak strength and the cyclic strength of sand-silt mixtures over a wide range of parameters and to clarify some of the existing confusing conclusions in the literature regarding the undrained strength response of sand-silt mixtures, a series of stress controlled cyclic and strain controlled monotonic triaxial tests was carried out on sand-silt mixture specimens of 50 mm diameter and 100 mm height with varying silt content. In these experiments, various measures of sample density was adopted through different approaches such as constant gross void ratio approach, constant relative density approach, constant sand skeleton void ratio approach, and constant interfine void ratio approach. Also the effect of relative density and confining pressure on these strengths was studied. It is observed that the limiting fines content and the relative density of a specimen play the key role in deciding the cyclic and monotonic resistance of sand-silt mixtures when studied through any approach. For any silt content with relative density more than 70%, cyclic and monotonic resistances are observed to be independent of silt content. When the undrained cyclic strengths of these specimens are plotted against their respective undrained monotonic peak strengths, it is observed that there exists a definite exponential relationship between the two with an excellent correlation coefficient. An expression is proposed in this regard to help engineers assess the cyclic strength of sand-silt mixtures from monotonic test results.     

T形短肢剪力墙静力性能有限元仿真分析

利用有限元分析软件ANSYS,首先采用其中三维实体单元SOLID65建立了T形短肢剪力墙有限元分析模型,从弹性到混凝土开裂直至破坏的全过程进行了仿真试验分析。分析了影响短肢剪力墙受力的几种因素:混凝土强度等级、配筋率、轴压比、墙肢截面高厚比对短肢剪力墙承载能力、变形能力及延性的影响,剖析了短肢剪力墙破坏过程及其原因。比较真实的反映了短肢剪力墙在轴压力和逐步加载侧向力共同作用下的响应。试验结果表明:增加混凝土等级和轴压比能提高试件的开裂、屈服和极限荷载,但应综合考虑其与变形能力、延性的关系。截面配筋率具有其特殊性,配筋率在1.4%1.6%之间时试件的承载能力、变形能力及延性较好。墙肢截面高厚比是不稳定因素但在高厚比为6.57.1时,延性及变形能力较强。     

固结过程对冻土应力-应变特性的影响

选用四种不同固结方式,并在－５０℃条件下,对冻结砂土样进行三轴压缩试验,试验发现固结过程对冻土的强度与变表有明显影响。经历Ｋ０ＤＣＦ的冻土其强度最大,友坏变形最小,经历ＦＣ的试样强度最小,破坏变形最大;经历ＥＤＣＦ和ＥＵＤＣＦ的产介于中间。     

Effects of Frequency on the Dynamic Properties of Intact Rock Samples Subjected to Cyclic Loading under Confining Pressure Conditions

A series of laboratory tests was performed to assess the effects of frequency on the dynamic properties of sandstone samples subjected to cyclic loading in the confining stress state. Three levels of confining pressure (2.0, 10.0, and 40.0 MPa) and three sets of frequencies (0.1, 1.0, and 3.0 Hz) were applied for the axial cyclic loading tests by the MTS-815 Rock and Concrete Test System. The results from the cyclic loading tests indicate that frequency has a strong influence on the dynamic deformation, the dynamic stiffness, and the failure mode at the same confining pressure. With an increase in the frequency, the axial strain and the number of cycles at failure increased at the same confining pressure, the residual volumetric strain increased when dilatancy occurred at the same confining pressure, and the number of cycles at failure increased. A new damage variable D was defined that describes the degradation process of sandstone samples upon dynamic cyclic loading. The larger the frequency, the wider the localized band. Sandstone samples subjected to dynamic cyclic loading responded with a significantly higher initial stiffness. The higher the initial stiffness, the greater the frequency.     

Effects of initial static shear on cyclic resistance and pore pressure generation of saturated sand

In practical engineering, cyclic shear stresses induced by earthquakes, traffic, and waves are superimposed on the initial static shear stress in sand fills or deposits, leading to complex responses of soils such as their deformation characteristics, pore pressure generation, and susceptibility (or cyclic resistance) to liquefaction. To experimentally investigate the undrained cyclic response of saturated sand, a series of triaxial tests were performed, covering a broad range of initial static and cyclic deviatoric stress levels. The results indicate that different stress conditions lead to two types of cyclic behavior: cyclic mobility and residual deformation accumulation. The compressional static stress is beneficial to the cyclic resistance of the dense sand, whereas the extensional static stress is regarded as detrimental as it tended to reduce the cyclic strength. Moreover, by comparing the available test data obtained from the same sand with varying initial densities and confining pressures, the static shear effect on cyclic resistance was found to be dependent on the state of the sand. Compared to the interpretation made using the limiting pore pressure-based criterion, the conventional failure criterion using a cyclic axial strain of 5% may lead to a substantial overestimation of the cyclic resistance, thus resulting in unsafe assessment and design. Hence, by employing the pore pressure criterion, the pore pressure generated in the cyclic tests was investigated and was found to be significantly influenced by the static shear stress. A pore pressure generation model is proposed to obtain the pore pressure characteristics of sand under various static shear stress conditions.     

Study on dynamic properties and shakedown behaviors of coarse-grained fillers in Qinghai-Tibet Railway subgrade北大核心CSCD

对冻结路基填料开展一系列恒围压和动围压分级循环加载三轴试验,采用轴向及径向双向同步循环加载方式来模拟原位列车荷载下路基填料的复杂循环应力状态,试验结果表明：轴向累积塑性应变随冻结负温的升高而增大,随粗颗粒含量的增大而减小。体变随冻结负温的降低先减小再增大,而体变在粗颗粒含量增加下的三阶段演化特征被试验确定。最后采用现有的三类路基安定性评估理论对冻结路基填料的累积塑性变形进行评估,Werkmeister-准则和马-准则分别在动围压和恒围压的条件下易进入塑性蠕变阶段,陈-准则在两条件下的评判结果均属于塑性安定范围,后循环压实阶段与二次循环变形阶段间临界点不同的确定方法对安定性的评估结果有不可忽视的影响。     

循环荷载作用下软黏土刚度软化特征试验研究

循环荷载作用下,软黏土将发生软化现象,而以往的研究中大多未考虑初始偏应力的影响。通过对萧山正常固结饱和软黏土进行应力控制的循环三轴试验,研究了循环荷载作用下土体的刚度软化情况。通过分析每一次循环过程中割线剪切模量的变化规律,探讨了循环次数、循环应力水平、初始偏应力对刚度软化的影响。结果表明,随着循环次数的增加,土体刚度逐渐减小;循环应力水平的提高,初始偏应力的施加将加快刚度软化。在试验的基础上推导了反映土体刚度软化规律的经验公式,得到了萧山软黏土的破坏刚度比。     

高含冰量冻结粉土单轴受压损伤特性试验分析

为了研究高含冰量冻结粉土的应力-应变关系及结构的内部损伤演化过程,利用SOMATOM-PLUS X射线螺旋CT机对-1、-2、-4℃下的冻结粉土进行了单轴压缩实时CT扫描试验,探讨了温度对高含冰量冻结粉土的宏观力学性能及微观损伤演化过程的影响。通过试验结果可以看出：冻结粉土的应力-应变关系曲线大致经历线弹性变形阶段、损伤演化阶段与峰后软化阶段;高负温下温度对冻结粉土初始弹性模量影响不是很明显,而对冻结粉土强度的影响则比较大;温度从-1℃降到-2℃,强度大约可以提高63%,而从-2℃降到-4℃,强度可以提高约为36%;CT扫描各层初始密度损伤并不完全相同,但各层密度随着应变变化的趋势大致相同,中环、全区的密度随着应变的增加而减小,外区密度则随着轴向应变的增加而增大。     

Mechanical Properties of Weak Porous Sandstone Under General Triaxial Stress States

The mechanical properties of weak porous sandstone with uniaxial compressive strength of about 300kg/cm2 , porosity as high as 27% and bulk density 1.85 g/cm3 , were investigated experimentally by using Mogil’s true triaxial compressive apparatus. The properties of deformation and failure, in the whole process from brittleness to ductility, were observed by bringing confining pressure up to 500 kg/cm2 under room temperature . The intermediate principal stress was successfully brought up from the lowest limit. σ2=σ3 to the highest limit , σ2=σ1 under constant confining pressure, 50 and 100 kg/cm2 respectively. The whole process that the strength increases gradually to a maximum and then decreases gradually with increasing of intermediate principal stress under constant σ3 was observed . Failure and flow were effecfed not due to volume dilatancy but rather du to compression of volume under constant confining stress of 100 kg/cm 2 and a general triaxial stress state of σ3 =100 kg/cm2 , σ2=295kg/cm2 . For hard rocks , this behavior can be observed possibly only under higher temperatures and pressures. The experimental result shows that the ductility and coefficient of strain hardening are increased in proportion to the intermediate stress σ2 under constant σ3 . This is different from the behavior of hard rocks.     

高孔隙性软弱砂岩在一般三轴应力状态下的力学特性

本文作者利用茂木清夫教授的岩石真三轴压缩仪,研究了抗压强度为300公斤/匣米2左右,孔隙度高选27%,容重仅为1.85克/厘米3的高孔隙性软弱砂岩的力学特性.在室温下,把围压加到500公斤/厘米2,就观测到变形和破坏特性,从脆性过渡到韧性的全过程.同时,分别在50和100公斤/厘米2的恒定围压下,成功地把中间主应力σ2从σ2=σ3的下限加到σ2=σ1的上限,观测到强度随中间主应力的增长而连渐增加到最大值后又逐渐下降的全过程;在100公斤/厘米2的恒定围压下以及σ3=100,σ2=295公斤/厘米2的一般三轴应力状态下。     

Static Liquefaction Analysis Considering Principal Stress Directions and Anisotropy

Static liquefaction failure of a sloping ground occurs when the shear stress applied by a monotonic triggering load exceeds the undrained yield (peak) shear strength of the saturated liquefiable cohesionless soil. Current practices for determining the in-situ undrained yield strength for ground subject to static shear stress either rely on a suite of costly laboratory tests on undisturbed field samples or empirical correlations based on in-situ penetration tests which do not account for the effects of anisotropic consolidation, intermediate principal stress, and mode of shear on the degree of strain-softening and brittleness of cohesionless soils. This study investigates the effects of variations in the direction and relative magnitudes of principal stresses associated with different modes of shear and ground slopes on static liquefaction failure of cohesionless soils. Empirical relationships are developed between soil brittleness index and maximum excess pore water pressure ratio to characterize soil shearing behavior observed in a database of 271 undrained laboratory shear tests collected from the past literature. The application of these relationships for estimating the static liquefaction triggering strength of cohesionless soils under sloping grounds is described for plane-strain boundary conditions and the results are compared with those back-calculated for several cases of static liquefaction flow failures. The proposed procedure incorporates variations in mode of shear and initial stress anisotropy in an empirical formulation based on in-situ penetration tests.