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。     

循环加载时围压对岩石动力特性的影响

针对砂岩岩样,探讨了在不同围压作用下和轴向循环加载时岩样的动力力学性质,研究围压对砂岩岩样的动力力学性质的影响。采用的试验设备是MTS－815岩石和混凝土试验系统,5组围压分别为10、20、30、40、50 MPa。岩样为汶川地区的干砂岩,轴向荷载施加的频率为1 Hz。结果表明,（1）随着围压的逐渐增加,试样的残余轴向应变和体变逐渐增加,且剪胀发生时的残余体变也逐渐增加;（2）动力荷载作用时岩样在高围压比低围压下的初始刚度和强度高;（3）不同围压作用下,当应力比Rs较大时,岩样在较小的循环次数下发生破坏;（4）试样的破坏模式为剪切破坏,动力荷载作用下试样形成的局部化破坏带更宽。     

循环剪切荷载作用下软土的力学性状研究

利用从现场取得的原状土样,通过室内动三轴试验对循环荷载作用下的变形、孔压和强度特性进行研究。试验研究主要考虑了周围固结压力、循环剪切应力比、荷载频率和循环次数等因素的影响。研究表明,在循环荷载作用下,孔隙水压力和轴向应变均是一个波动上升的过程。当循环应力比增大时,动孔隙水压力变化幅值显著增加,而残余孔压也较大。同时,孔隙水压力值也随着周围压力的增大而明显增大。当作用荷载频率比较大时,需要更多的循环次数才能达到小频率作用荷载能达到的孔隙水压力值。但是,随着循环荷载作用次数的增加,频率对孔隙水压力的影响有减小的趋势。     

Study of rock joints under cyclic loading conditions

Summary A conceptual model for the behaviour of rock joints during cyclic shear and under constant normal stresses was proposed according to results from shear tests with 50 concrete replicas of rock joints. The shear strength and deformability of joint samples were found to be both anisotropic and stress dependent. Based on these experimental results, a two-dimensional constitutive model was developed for rock joints undergoing monotonic or cyclic loading sequences. The joint model was formulated in the framework of non-associated plasticity, coupled with empirical relations representing the surface roughness degradation, appearance of peak and residual shear stresses, different rates of dilatancy and contraction, variable normal stiffness with normal deformation, and dependence of shear strength and deformability on the normal stress. The second law of thermodynamics was represented by an inequality and used to restrict the values of some of the material parameters in the joint model. The new joint model was implemented into a two-dimensional Distinct Element Method Code, UDEC, and its predictions agreed well with some well-known test results.     

含二阶起伏体的模拟岩体节理试样剪切特性试验研究

利用规则的小尺寸锯齿构造二阶起伏体,对不同二阶起伏体高度的人工节理进行常法向应力下的直剪试验,研究了节理剪切力学特性。对节理的剪切强度、变形特征进行了分析,并对破坏特征做出解释。试验结果表明：二阶起伏体对节理剪切力学特性有重要影响,含二阶起伏体节理的剪切力学特性与只含有一阶起伏体节理的剪切力学特性不同,只含一阶起伏体的节理剪应力只有一个峰值,含二阶起伏体的节理会出现波浪状的剪胀曲线和多峰值剪应力,且峰值剪应力随剪切位移增大依次出现并逐个减小;随着二阶起伏体高度增大,节理峰值剪切强度增大,节理依次出现磨损破坏、多次性剪断破坏、一次性剪断破坏。     

Laboratory Investigation into the Effects of Silty Fines on Liquefaction Susceptibility of Chlef (Algeria) Sandy Soils

In a number of recent case studies, the liquefaction of silty sands has been reported. To investigate the undrained shear and deformation behaviour of Chlef sand–silt mixtures, a series of monotonic and stress-controlled cyclic triaxial tests were conducted on sand encountered at the site. The aim of this laboratory investigation is to study the influence of silt contents, expressed by means of the equivalent void ratio on undrained residual shear strength of loose, medium dense and dense sand–silt mixtures under monotonic loading and liquefaction potential under cyclic loading. After an earthquake event, the prediction of the post-liquefaction strength is becoming a challenging task in order to ensure the stability of different types of earth structures. Thus, the choice of the appropriate undrained residual shear strength of silty sandy soils that are prone to liquefaction to be used in engineering practice design should be established. To achieve this, a series of undrained triaxial tests were conducted on reconstituted saturated silty sand samples with different fines contents ranging from 0 to 40 %. In all tests, the confining pressure was held constant at 100 kPa. From the experimental results obtained, it is clear that the global void ratio cannot be used as a state parameter and may not characterize the actual behaviour of the soil as well. The equivalent void ratio expressing the fine particles participation in soil strength is then introduced. A linear relationship between the undrained shear residual shear strength and the equivalent void ratio has been obtained for the studied range of the fines contents. Cyclic test results confirm that the increase in the equivalent void ratio and the fines content accelerates the liquefaction phenomenon for the studied stress ratio and the liquefaction resistance decreases with the increase in either the equivalent void ratio or the loading amplitude level. These cyclic tests results confirm the obtained monotonic tests results.     

盾构施工与波浪荷载耦合作用后软土力学特性

针对波浪循环荷载作用和盾构穿越海堤过程中的施工扰动对下卧软黏土地基的弱化效应问题,研究了盾构施工与波浪荷载耦合作用后海洋软土的强度和刚度变化规律。通过逐步降围压的方式模拟盾构施工过程中的地层损失所引起的应力释放,循环加载3 000次模拟波浪荷载长期作用,开展了波浪和盾构施工耦合作用后的软土空心圆柱扭剪不排水剪切单元体试验。试验结果表明:(1) 在盾构施工与波浪荷载耦合作用下,软黏土的强度和刚度均随围压变大而增强,在同一围压下,二者均随波浪循环次数和土体损失率（δ）的增大而减小。(2) 在相同的波浪循环剪切次数下,软黏土初始切线模量随δ的增加而呈线性递减趋势,而软黏土黏聚力和内摩擦角随δ的增加而呈现非线性减小。波浪和盾构施工耦合作用后软黏土的强度和刚度均呈现不同程度的衰减,二者耦合扰动作用对软黏土的弱化效应在实际工程中应充分重视。此研究对探讨复杂扰动条件下海洋结构性软土力学特性规律,丰富和发展扰动状态下结构性软土的计算理论,以及对海岛基础设施建设具有重要的参考价值。     

Effects of Cyclic Loading on the Shear Behaviour of Infilled Rock Joints Under Constant Normal Stiffness Conditions

The variation of the shear strength of infilled rock joints under cyclic loading and constant normal stiffness conditions is studied. To simulate the joints, triangular asperities inclined at angles of 9.5° and 18.5° to the shear movement were cast using high-strength gypsum plaster and infilled with clayey sand. These joints were sheared cyclically under constant normal stiffness conditions. It was found that, for a particular normal stiffness, the shear strength is a function of the initial normal stress, initial asperity angle, joint surface friction angle, infill thickness, infill friction angle, loading direction and number of loading cycles. Based on the experimental results, a mathematical model is proposed to evaluate the shear strength of infilled rock joints in cyclic loading conditions. The proposed model takes into consideration different initial asperity angles, initial normal stresses and ratios of infill thickness to asperity height.     

圆形应力路径下软黏土的动力特性试验研究

实际交通荷载作用下,路基土单元内的竖向应力和水平应力大小不断发生变化,剪应力幅值和方向也不断变化,从而导致土体中的应力路径呈现出主应力轴连续旋转的现象。通过GDS空心圆柱扭剪仪模拟类似交通荷载作用下的应力路径,开展不同围压和不同循环应力比下的主应力轴连续旋转试验,旨在研究在交通荷载类轴向纯压缩条件下主应力轴方向连续旋转时循环应力比与围压对原状软黏土的强度、累积应变、回弹应变、软化等因素的影响。试验结果表明:随着孔压的不断累积,原状饱和软黏土试样逐渐软化,轴向模量和剪切模量均随着循环应力比和围压的增加而逐渐降低,并在主应力轴旋转一定的循环次数后达到稳定。当循环应力比较小时,轴向和剪切应力-应变滞回曲线均呈线性,不同主应力轴循环旋转次数下的轴向和剪切应力-应变滞回曲线近乎重合。随着主应力轴循环旋转次数的增加,轴向和剪切应力-应变滞回曲线越来越表现出明显的非线性,不同循环次数下试样的轴向和剪切应力-应变滞回圈不再重合且滞回圈逐渐向x轴倾斜。随着循环应力比的增加,在主应力轴连续旋转初期,轴向模量和剪切模量迅速衰减,且随着循环次数的增加而达到稳定,并且试样的轴向模量和剪切模量达到稳定时的主应力轴...     

循环剪切荷载作用下含二阶起伏体模拟岩石节理力学特性研究

利用人工材料浇注含二阶起伏体的模拟岩石节理试样,进行常法向荷载循环剪切试验,研究节理剪切力学特性在循环剪切过程中的劣化规律。试验结果表明：二阶起伏体对节理循环剪切力学特性有重要影响,剪切强度、剪切刚度、剪胀角随剪切循环次数增大而衰减,衰减趋势随着二阶起伏度的增大而加快;法向应力、二阶起伏度较大时,二阶起伏体对剪切力学特性的影响主要体现在第1轮剪切循环中,在随后的剪切循环中影响不明显;法向应力、二阶起伏度较小时,二阶起伏体的影响在前几轮循环剪切过程中均有较清晰的体现。基于Hertz接触力学理论,提出了节理面微凸体球面接触细观模型,揭示了节理循环剪切宏观试验现象的力学机制     

复杂应力路径下饱水砂岩宏细观力学特性研究

饱水作用对砂岩变形及强度特征影响显著,考虑岩体实际赋存环境,为了解复杂应力路径下饱水砂岩宏细观力学特性,利用RMT-150C岩石力学多功能试验机及SEM电镜扫描技术,对饱水砂岩进行循环荷载作用后卸荷破坏试验,研究不同影响因素作用下的疲劳损伤特征,重点分析了加载频率、上限应力等因素对卸荷变形、强度及细观损伤特征的影响规律。研究表明,根据试验设计方案获得的饱水砂岩应力-应变曲线大致可以划分为5个典型阶段,其中,同一应力状态下,疲劳损伤阶段轴向不可逆应变在等速变形阶段的应变速率随加载频率的增大迅速增加,这表明上限应力的“门槛值”极有可能是变化的,且与加载频率及循环次数密切相关;同时,卸围压变形破坏阶段的总应变及围压卸荷量均随加载频率（或上限应力）的增大而减小,与该阶段持续时间随加载频率（或上限应力）的变化规律一致。不仅如此,通过分析饱水砂岩破坏面细观损伤特征发现,加载频率与上限应力作用下的细观损伤特征差异显著,其中破坏面微裂隙面积占比随加载频率增大而减小,上限应力则恰好相反。     

干湿-冻融循环下延吉膨胀岩的力学特性及其应力-应变归一化

为了研究干湿、冻融和干湿-冻融循环作用对延吉膨胀岩的应力-应变关系、体变特性、抗剪强度的影响,开展了一系列的固结排水剪切试验。结果表明：未循环试样的应力-应变关系曲线表现为稳定型或弱应变软化型,3种环境作用后试样应力-应变关系曲线呈现出一定的应变软化特性,剪胀明显,而且围压越小、循环次数越高的试样软化和剪胀的程度越高;随着循环次数的增加,3种环境作用下试样的黏聚力急剧减小,而内摩擦角略有增加;干湿-冻融循环作用下试样的应力-应变关系软化程度最高,剪胀最显著,试样的黏聚力降低的幅度最大。基于Konder双曲线模型,对循环处理前后试样的应力-应变特性进行归一化分析,建立了考虑干湿-冻融循环次数和围压影响,能同时描述应变软化型和硬化型的应力-应变关系式,并对不同干湿-冻融循环次数试样的应力-应变曲线进行预测,预测值与实测值较为接近,预测效果较好。     

峰前循环剪切作用下岩石节理剪切力学特性

微震、工程爆破等低应力循环剪切荷载作用对节理岩体工程失稳破坏具有重要影响。为研究峰前循环剪切加卸载作用下岩石节理剪切力学特性,采用RDS-200型岩石节理剪切试验系统对人工劈裂黄砂岩节理进行了峰前循环剪切下的直剪试验。通过与未进行峰前循环剪切加卸载时岩石节理力学参数预测值对比,得到峰前循环剪切加卸载作用对峰前剪切刚度、峰值剪切强度、峰值剪切位移与残余剪切强度的影响。结果表明：（1）峰前循环剪切加卸载后,当法向应力为2 MPa时,岩石节理峰前剪切刚度增大,当法向应力为4～10 MPa时,岩石节理峰前剪切刚度在循环剪切应力幅值范围内增大,在超出循环剪切应力幅值时减小;（2）峰前循环剪切加卸载后,峰值剪切强度降低了10%～20%,降低百分比随法向应力增大整体呈对数函数增大;峰值剪切位移增加了2%～40%,增加百分比随法向应力增大整体呈对数函数减小;（3）峰前循环剪切加卸载后,岩石节理残余剪切强度无明显变化,峰值剪切强度与残余剪切强度差值减小,峰后剪切应力做功损失百分比降低。     

循环荷载作用下岩石劈裂结构面剪切力学特性演化与影响因素研究

基于现有结构面剪切力学特性研究中对简单几何形态结构面研究多,天然形态结构面研究少,力学性质演化研究多,几何形态演化研究少的研究现状,通过巴西劈裂试验制备近天然形态岩石结构面,并采用模拟材料批量复制劈裂结构面试样的方法,开展循环荷载作用下岩石劈裂结构面的剪切力学特性演化规律与影响因素研究,分析法向应力、循环剪切次数、岩壁强度以及结构面粗糙度对循环剪切作用下岩体结构面力学特性和形貌特征的影响。最终通过在黏着摩擦理论-Barton经验公式中引入与循环剪切次数Nd有关的负指数假定劣化参数,提出了岩体结构面循环剪切强度公式。研究结果表明：法向应力、岩壁强度、结构面粗糙度越大,结构面的最大剪应力越大;法向应力、循环剪切次数、结构面粗糙度越大,岩壁强度越小,则归一化粗糙度参数越小。提出的循环剪切强度公式较好地验证了试验结果,可为工程安全设计提供理论参考。     

An Elasto-Plastic Constitutive Model for Rock Joints Under Cyclic Loading and Constant Normal Stiffness Conditions

An elasto-plastic constitutive model is introduced for rock joints under cyclic loading, considering the additional shear resistance generated by the asperity damage in the first forward shear cycle and sliding mechanism for further shearing. A series of cyclic loading direct shear tests was conducted on artificial joints with triangular asperities and replicas of a real rock asperity surface under constant normal stiffness (CNS) conditions. The model was calibrated and then validated using selected data sets from the experimental results. Model simulations were found to be in good agreement with the rock joints behaviour under cyclic loading and CNS conditions both in stress prediction and dilation behaviour. In addition, dynamic stability analysis of an underground structure was carried out, using Universal Distinct Element Code and the proposed constitutive model.     

Experiment and Study on Mechanical Property of Sandstone Post-peak Under the Cyclic Loading and Unloading

Engineering research has shown that the surrounding rock of deep roadways experienced many times of post-peak cyclic loading and unloading. So studying on rock mechanical characteristics of post-peak loading and unloading is helpful to control the deep surrounding rocks. The test using RMT-150B rock mechanics testing system, to experiment on the mechanical properties of sandstone of post-peak cyclic loading and unloading. The results show that: the stress–strain curves of post-peak cyclic loading and unloading have significant plastic hysteretic loops. The area of plastic hysteretic loops gloss back. The enveloping outer enclosure of cycle loading curve is the similar as the stress–strain curves of strain softening stage when the samples failure, which shows that post-peak failure of rocks have significant memory. With the increase of cycles, the cumulative deterioration parameters are gradually increased, and the ultimate bearing capacity, elastic of loading section of samples and cumulative deterioration parameters conform with the exponential attenuation function with constant term. With the increase of surrounding pressure, the total energy, dissipated energy and elastic energy of samples are gradually increased. With the increase of cycles, the total energy, dissipated energy and elastic energy of samples all are gradually decreasing, the rate of reduction decreases gradually. The samples exist in vertical splitting and transverse shear combination failure under the post-peak uniaxial cycles, or exist in shear failure under the post-peak triaxial cycles. The shear plane exists slip traces.     

基于统计岩体力学的岩体强度特征分析

结构面的存在改变了岩体力学性质,影响了岩体强度特征。基于统计岩体力学强度判据,结合摩尔-库仑准则,得到了含单组结构面岩体破坏的4种不同方式和相应的结构面倾角范围,推导了岩体强度由结构面控制转化为应力控制的临界围压表达式。在此基础上,根据岩石和结构面参数之间的关系,将含单组结构面岩体分为4类,并探讨了可能破坏方式和发生破坏的条件。最后,举例分析了含单组结构面闪长岩的强度特征,结果表明,该岩体属于第I类岩体,在垂向受压时先沿结构面后沿岩块破坏,临界围压为9.12 MPa;从全空间分析,该岩体强度各向异性显著,围压增大可使岩体在一些方向上受力时强度由结构控制转化为应力控制。     

Experimental Study on Failure Characteristics of Schist Under Unloading Condition

The mechanical properties of rock under unloading condition have significant differences with those under conventional loading condition. To investigate the failure characteristics of schist under complex stress state, uniaxial and trixial compression and unloading confining pressure tests have been carried out on samples in the paper. The results show that the rock samples failed in coupling mode of shear and tension under uniaxial compression, and failed in shear mode under conventional trixial compression. However, the failure modes of the samples can be classified as shear, multiple fracturing, and multiple extension, and the strength of schist is closely related with the failure mode under unloading confining condition. For the unloading condition, the peak strengths of schist samples with shear failure mode are close to those under conventional trixial compression, but the peak strengths with multiple fracturing or multiple extension mode are much higher than those under trixial compression. The internal structure characteristics and stress paths were found to have major influences on the failure modes of schist. Therefore, the failure modes must be considered when the strength characteristics of schist are investigated.     

Pore pressure generation in silty sands during cyclic loading

Recent developments in studies of soil response to earthquake loadings have made it possible to incorporate the rates of pore water pressure build-up in soils in to nonlinear response analyses of the grounds. Such pore pressure changes help in computing the changes in stress-strain behaviour of soils in the deposit progressively as the earthquake progresses. The rate and magnitude of pore pressure generation in soils during seismic loading will have important effects on the shear strength, stability, and settlement characteristics of a soil mass, even if the soil does not liquefy. The results in terms of pore pressure response in soils from a series of experimental investigations using strain-controlled cyclic triaxial tests on soils samples collected from liquefied sites are presented in this paper. The effect of relative density, amplitude of cyclic shear strain, number of loading cycles, confining pressure and frequency of cyclic loading on the pore pressure build-up are studied. Analytical expressions are proposed using regression analysis to define mean relationships between normalized pore water pressure and normalized cycles for the prediction of pore water pressure build-up in silty sands. Also, the pore water pressure build-up in soils is independent of frequency of loading.     

Laboratory Investigation of the Resistance of Tailings and Natural Sediments to Cyclic Loading

A number of cyclic triaxial tests were carried out on mine tailings and natural sediment samples under undrained conditions to investigate their resistance to cyclic loading. The tests were performed on more than 100 samples with a cyclic shear stress ratio ranging from 0.10 to 0.40 under varying void ratio and the same confining pressure. It was observed that the axial strain and excess pore water pressure increased with the number of loading cycles while the effective stress decreased with increasing number of loading cycles. The liquefaction resistance of the tailings was also observed to be higher than that of natural soils with similar particle size distribution, void ratio and plasticity index. It was observed that the influence of specific gravity on the cyclic strength of mine tailings is significant. The results showed that the cyclic resistance of the tailings was not strongly influenced by plasticity index for low plasticity tailings. A boundary relationship between void ratio and normalized cyclic resistance ratio was established based on the results.