Failure behavior of highly stressed rocks under quasi-static and intensive unloading conditions

Unloading failure of rocks, especially highly stressed rocks, is one of the key issues in construction of underground structures. Based on this, analytical models for rocks under quasi-static and intensive unloading conditions are established to study the failure behavior of highly stressed rocks. In case of rock failure under quasi-static unloading, the rock mass ahead of working face is regarded as an elasto-brittle material, and the stress–displacement curves are used to characterize the tensile fracture of peak-stress area. It is observed that, when intensive unloading happens, there is an elastic unloading wave (perturbation wave) propagating in the rock mass. If the initial stress exceeds the critical stress, there will be a fracture wave, following the elastic unloading wave. To study the propagation feature of fracture wave, the conservation laws of mass, momentum and energy are employed. Results show that the post-peak deformation, strength and energy dissipation are essential to the failure process of highly stressed rocks.     

K0固结软黏土双向耦合循环加载真三轴试验研究

文章针对天津滨海K₀固结饱和软黏土,开展轴向及侧向耦合循环加载真三轴试验。重点探讨了循环中主应力系数及循环应力水平对软黏土动应力应变响应的影响。试验结果表明：同一循环应力水平,循环中主应力系数越大,累积永久大主应变越小,中主应力的增大对大主应变的发展有抑制作用;循环中主应力系数在0.4～0.6,存在某一临界值,其为中主应变发生正负反向的临界点。同一中主应力系数,循环应力水平越高,土样最终累积永久主应变越大;循环应力水平在0.4～0.46,存在某一临界值,其为土样循环振稳及失稳的临界点。在大主应力方向,循环中主应力系数越小,土体刚度弱化越显著;在中主应力方向,刚度弱化程度随中主应力系数的变化并非单调。循环应力水平越大,土体刚度弱化越显著。利用广义剪应变表征三维复杂应力状态下土单元的应变水平,建议了能够预测任意给定循环中主应力系数、循环应力水平及循环次数下累积永久广义剪应变的指数函数表达式。研究成果以期为波浪、交通等复杂动荷载作用下软黏土地基的稳定性分析及变形计算提供试验依据。     

软土的振动排水固结特性试验研究

以江苏镇江某现场软土为研究对象,室内利用自主设计的动态固结渗透仪模拟饱和软土在振动荷载作用下的响应。在不同排水条件、围压、静荷载、振动荷载和频率下进行排水固结试验,并同单一的静压排水固结试验进行对比,重点分析试验过程中的动应变和固结排水效应,探讨软土的动力固结排水机理。试验结果表明,振动作用可以明显地提高排水速度,增加排水量;固结压力、振动荷载和频率对软土的振动排水效应产生显著的影响。试验结果和振动排水的机理分析,对当前软土地基处理及其施工工艺设计具有重要的实用价值和指导意义。     

三轴应力条件下原状黄土的吸湿渗水特性

以原状黄土为研究对象,用改装的非饱和土三轴剪切渗透仪,在三轴应力条件下进行了直接向试样分级注水使吸力逐渐减小的增湿渗透试验,分析了相同净平均应力p下偏应力q变化、相同应力比η下净平均应力p变化及饱和度Sr变化时对渗水系数的影响。研究结果表明:偏应力q、净平均应力p及饱和度Sr对渗水系数皆有较大的影响,且影响程度存在差异性。p,q一定,随着Sr增大渗水系数呈先快速后缓慢增大的趋势。Sr一定,随着q增大(p相同)或p增大(η相同)渗水系数皆呈近似线性减小趋势,前者的斜率与p大小有关,而与Sr大小无关;后者的斜率与η大小无关,而与Sr大小有关,且皆小于前者。不同净平均应力下饱和渗水系数与应力比之间皆可用指数函数描述,且仅一个参数随着净平均应力而变化。不同应力状态下相对渗水系数与饱和度关系可近似归一,无法用VG-M模型来描述,但可用本文提出的模型描述。该模型直接把净平均应力与应力比作为变量,便于实际工程应用。模型预测与试验结果吻合较好。     

循环三轴应力路径下钙质砂颗粒破碎演化规律

钙质砂广泛分布于中国南海区域,是吹填造陆的重要材料。钙质砂颗粒容易破碎,使得其力学特性相比于普通的陆源硅质砂有显著差异。对取自中国南海西沙群岛某岛礁的钙质砂开展了三轴排水循环剪切试验,研究了围压、循环应力比、循环振次对钙质砂颗粒破碎发展过程的影响。在试验所采用的围压范围内,钙质砂在固结过程中产生的颗粒破碎较少,但是在随后的循环剪切过程中产生了显著的颗粒破碎。在循环剪切作用下,钙质砂的颗粒破碎形式主要是尖角的磨损,剪切后试样的颗粒中出现了一些碎屑和微细颗粒,大颗粒的棱角有一定程度的磨圆,但粒径无明显减小。在常围压下的等幅循环剪切中,颗粒破碎程度随着循环剪切次数的增大而增加,增长速率逐渐降低,可以采用对数曲线来描述相对破碎指数的发展过程。再考虑围压和循环应力比的影响规律,初步建立了一个描述颗粒破碎演化过程的数学模型。     

长期循环荷载作用下温州结构性软黏土的应变特性研究

针对软黏土存在的结构性特征,在不同围压下对天然温州软黏土进行了不排水静剪和大数目（50000次）循环加载试验,研究了结构性对土体强度的影响,并进一步分析了结构性软黏土在长期循环荷载作用下的应变特性。研究表明：围压较低时,土体的结构性在固结过程中没有或很少被破坏,软黏土表现为较高的归一化强度;随着围压的增大,归一化强度逐渐降低;当围压高于土体屈服压力时,土体结构性在固结过程中被完全破坏,此时土体的归一化强度达到稳定值,与重塑土强度相同。这种特性使得软黏土在循环加载试验中,即使在相同动应力比（mtml_13-0310_bak/img_0.png26.013.95= 0.45）下,不同围压下的应变行为也表现出明显的不同。这种区别表现在应力-应变滞回曲线、回弹特性和累积塑性应变等方面。     

三轴循环加卸载条件下砂性土变形特性研究

利用基于局部变形数字图像测量的三轴试验系统和可提供循环加卸载的试验机,设计了砂性材料的三轴循环加卸载试验,研究了砂性散体材料在不同状态下的轴向、径向和体积变形特性以及泊松比的变化。加载体胀和卸载体缩是砂性材料颗粒结构性的具体表现,在多次加卸载循环后,颗粒的滑移、滚动和破碎等结构性变形最终完全消除,结构变形达到稳定状态。泊松比的变化与试样的体积变形密切相关,从体积变化趋势中可以分析出泊松比的变化范围。     

微生物处理砂土不排水循环三轴剪切CFD-DEM模拟

微生物诱导碳酸钙沉积是一种新型的地基处理技术,处理后的土体可以看成一种结构性土。首先,在已有三维含颗粒抗转动和抗扭转模型及三维胶结破坏准则的基础上,通过考虑颗粒碰撞接触过程中颗粒本身的塑性变形及率相关性的接触黏滞阻尼,建立考虑循环荷载作用下的三维胶结模型;然后,参考已有研究,建立了反硝化反应在加固砂土中的时效性关系。并引入CFD-DEM耦合程序,用以模拟分析不同胶结含量以及不同气泡含量下,微生物处理砂土在固结不排水循环剪切试验中的力学特性;最后,从宏微观角度分析生物胶结与生物气泡对砂土抗液化性能的影响及其作用机理。研究表明,胶结和气泡共同存在对抗液化能力的提升并没有起到“1+1=2”的效果;胶结的存在提高了非饱和砂土的抗液化能力,明显抑制孔压比和轴向应变的发展,力学配位数得到了提升;而气泡的存在却降低了胶结砂土的抗液化能力,使得胶结砂土达到初始液化的振次减少,轴向应变向受拉方向大幅增长,力学配位数下降明显。     

循环荷载作用下结构性软粘土的变形和强度特性

采用萧山原状和重塑软粘土在不同压力下固结后进行动力试验,探讨结构性软粘土在循环荷载作用下,合适的动应变破坏标准。文中比较了按本文提出的破坏标准与破坏应变标准取为5%和10%时的动强度差异,结果表明,在破坏振次较小时,本文提出的应变破坏标准更为合理;随着振次的增大,不同应变标准下的动强度差异性逐渐减小并趋于同一个稳定值--土体的最小动强度。固结压力对结构性软土动强度的影响较大,当固结压力小于土体结构屈服应力时,原状土动强度明显高于重塑土;而当固结压力超过土体结构屈服应力时,两者的动强度基本趋于一致。     

Pullout behavior of polymeric strip in compacted dry granular soil under cyclic tensile load conditions

Assessment of the reinforcement behavior of soil under cyclic and monotonic loads is of great importance in the safe design of mechanically stabilized earth walls. In this article, the method of conducting a multistage pullout (MSP) test on the polymeric strip (PS) is presented. The post-cyclic behavior of the reinforcement can be evaluated using a large-scale pullout apparatus adopting MSP test and one-stage pullout (OSP) test procedures. This research investigates the effects of various factors including load amplitude, load frequency, number of load cycles and vertical effective stress on the peak apparent coefficient of friction mobilized at the soil-PS interface and the pullout resistance of the PS buried in dry sandy soil. The results illustrate that changing the cyclic tensile load frequency from 0.1 Hz to 0.5 Hz does not affect the pullout resistance. Moreover, the influence of increasing the number of load cycles from 30 to 250 on the peak pullout resistance is negligible. Finally, the effect of increasing the cyclic tensile load amplitude from 20% to 40% on the monotonic pullout resistance can be ignored. The peak apparent coefficient of friction mobilized at the soil-PS interface under monotonic and cyclic load conditions decreases with the increase in vertical effective stress.     

Resilient behavior of compacted subgrade soils under the repeated triaxial test

Subgrade soils are very important materials to support highways. Resilient modulus (M_r) has been used for characterizing stress-strain behavior of subgrades subjected to repeated traffic loadings. Recently the repeated triaxial test procedure has been upgraded through AASHTO T 307. Since the testing procedure is still complex, the testing has not been widely implemented in practice. In order to evaluate resilient behavior of compacted subgrades soils, the repeated triaxial test and the unconfined compressive test as well as some fundamental property tests were conducted. In this study, the applicability of a simplified procedure with a confining pressure of 13.8 kPa and deviator stresses of 13.8, 27.6, 41.4, 55.2, 69 and 103.4 kPa was investigated on the typical sandy–silty–clay and silty–clay subgrade soils encountered in Indiana. The results obtained from the simplified procedure are comparable with those obtained from AASHTO T 307 which calls for 15 combinations of stresses. This shows the simplified procedure to be feasible and effective for design purpose. Some soils compacted wet of optimum moisture content showed an excessive permanent deformation. This phenomenon was investigated by the comparison of the unconfined compressive test and the repeated triaxial test results. For soils exhibiting excessive permanent deformation, use of deformed length is desirable for more accurate calculation of M_r. Usually the soils compacted dry of optimum shows the largest M_r for sandy–silty–clay soils due to capillary suction, but it is not necessarily true for silty–clay soils. A predictive model to estimate regression coefficients k₁, k₂, and k₃ using 11 soil variables obtained from the soil property tests and the standard Proctor compaction tests was developed. The predicted regression coefficients compare well with measured ones.     

Experiments on mechanical properties of salt rocks under cyclic loading

The primary purpose of underground gas storages is to provide gas for seasonal consumptions or strategic reserve.The periodical operations of gas injection and extraction lead to cyclic loading on the walls and surrounding rocks of gas storages.To investigate the mechanical behaviors of different host rocks in bedded salt deposit,laboratory experiments were conducted on the samples of rock salt,thenardite,glauberite and gypsum.The mechanical properties of rock samples under monotonic and cyclic loadings were studied.Testing results show that,under monotonic loading,the uniaxial compressive stress(UCS) of glauberite is the largest(17.3 MPa),while that of rock salt is the smallest(14.0 MPa).The UCSs of thenardite and gypsum are 16.3 and 14.6 MPa,respectively.The maximum strain at the peak strength of rock salt(halite) is much greater than those of the other three rocks.The elastic moduli of halite,thenardite,glauberite and gypsum are 3.0,4.2,5.1 and 6.8 GPa,respectively.Under cyclic loading,the peak strengths of the rock specimens are deteriorated except for rock salt.The peak strengths of thenardite,glauberite and gypsum decrease by 33.7%,19.1% and 35.5%,respectively;and the strains of the three rocks at the peak strengths are almost the same.However,the strain of rock salt at the peak strength increases by 1.98%,twice more than that under monotonic loading.Under monotonic loading,deformation of the tested rock salt,thenardite and glauberite shows in an elastoplastic style.However,it changes to a ductile style under cyclic loading.Brittle deformation and failure are only observed for gypsum.The results should be helpful for engineering design and operation of gas storage in bedded salt deposit.     

Non-dilatant deformation and failure mechanism in two Long Valley Caldera rocks under true triaxial compression

We conducted laboratory rock strength experiments in two ultra-fine-grained brittle rocks, hornfels and metapelite, which together are the major constituent of the Long Valley Caldera (California, USA) basement in the 2025–2996 m depth range. Both rocks are banded, and have very low porosity. Uniaxial compression tests at different orientations with respect to banding planes reveal that while the hornfels compressive strength is nearly isotropic, the metapelite possesses distinct anisotropy. Conventional triaxial tests in these rocks reveal that their respective strengths in a specific orientation increase approximately linearly with confining pressure. True triaxial compression experiments in specimens oriented at a consistent angle to banding, in which the magnitudes of the least (σ₃) and the intermediate (σ₂) principal stresses are different but kept constant during testing while the maximum principal stress is increased until failure, exhibit a behavior unlike that previously observed in other rocks under similar testing conditions. For a given magnitude of σ₃, compressive strength σ₁ does not vary significantly in both Long Valley rock types, regardless of the applied σ₂, suggesting little or no intermediate principal stress effect. Strains measured in all three principal directions during loading were used to obtain plots of σ₁ versus volumetric strain. These are consistently linear almost to the point of rock failure, suggesting no dilatancy. The phenomenon was corroborated by SEM inspection of failed specimens that showed no microcrack development prior to the emergence of one through-going shear failure plane steeply dipping in the σ₃ direction. The strong dependency of compressive strength on the intermediate principal stress in other crystalline rocks was found to be related to microcrack initiation upon dilatancy onset, which rises with increased σ₂ and retards the failure process. We infer that strength independence of σ₂ in the Long Valley rocks derives directly from their non-dilatant deformation.     

Analysis of mechanical behavior of soft rocks and stability control in deep tunnels

Due to the weakness in mechanical properties of chlorite schist and the high in situ stress in Jinping II hydropower station, the rock mass surrounding the diversion tunnels located in chlorite schist was observed with extremely large deformations. This may significantly increase the risk of tunnel instability during excavation. In order to assess the stability of the diversion tunnels laboratory tests were carried out in association with the petrophysical properties, mechanical behaviors and waterlweakening properties of chlorite schist. The continuous deformation of surrounding rock mass, the destruction of the support structure and a large-scale collapse induced by the weak chlorite schist and high in situ stress were analyzed. The distributions of compressive deformation in the excavation zone with large deformations were also studied. In this regard, two reinforcement schemes for the excavation of diversion tunnel bottom section were proposed accordingly. This study could offer theoretical basis for deed tunnel construction in similar geological condition~     

循环荷载下软土地基固结沉降分析

软土地基在车辆长期作用下会产生较大的工后沉降,研究软土地基在车辆循环荷载下的变形及应力等特征,对控制工后沉降处理软土地基具有一定意义。以处于软土地基的某公路为例,应用ABAQUS建立饱和软土地基修正剑桥模型,不同时段通行车辆为循环荷载,得到公路在循环荷载下变形、应力、孔压等分布特性。结果表明,高峰通行时间越长地基的变形量越大,地基顶部孔压较底部孔压消散快,达到完全固结需要相当长的时间。     

双向激振下饱和软黏土应变软化现象试验研究

循环荷载作用下饱和软黏土将发生刚度软化现象。由于受试验条件的限制,以往对软黏土循环软化现象的研究大都基于单向循环荷载试验。通过GDS双向动三轴系统对双向循环荷载作用下饱和软黏土的刚度软化现象进行了初步的探讨。着重分析了循环偏应力、径向循环应力、初始剪应力等因素对双向循环作用下软黏土刚度变化规律的影响。研究结果表明:循环偏应力的增加、径向循环应力的提高都将加快刚度软化。随着径向循环应力比的增加,临界循环应力比减少。双向激振循环荷载作用下软黏土存在门槛径向循环应力比。当径向循环应力比小于该门槛值时,双向振动不能加速土体软化。随着初始固结剪应力的增加,刚度有所提高。在试验的基础上初步建立了双向循环荷载作用下饱和软黏土的软化模型。将该软化模型引入到修正的Iwan模型中来描述土体的应力–应变关系,证实了模型的准确性。