单轴压缩下岩石蠕变失稳破坏过程数值模拟

在岩石破裂过程分析（RFPA2D）系统的基础上,考虑岩石损伤过程的时间因素影响,引入岩石细观单元蠕变本构方程,建立了考虑流变效应的岩石破裂过程RFPA2D数值模型。应用该模型模拟了恒定荷载作用下岩石的蠕变破坏过程,得到了岩石蠕变破裂的3个典型阶段：初始蠕变阶段、稳定蠕变阶段和加速蠕变阶段,模拟结果同实验室试验所观察到的现象十分吻合。这表明考虑流变效应的RFPA2D数值模型适用于模拟岩石的蠕变破坏这一复杂的、非线性演化问题。此外,数值模拟还揭示了岩石的宏观蠕变破坏实质上是细观层次上单元损伤累计的结果,这些结论对岩石工程的长期稳定性研究具有重要的理论指导和实践意义。     

锦屏大理岩蠕变损伤演化细观力学特征的数值模拟研究

岩石的蠕变损伤和断裂是岩石流变效应的重要表现形式,但其损伤演化过程往往难以直观观测,为此,采用二维颗粒流数值模拟方法(PFC2D)对岩石的蠕变损伤和断裂的细观力学机制进行了分析。在锦屏大理岩室内试验基础上,利用颗粒流应力腐蚀模型(PSC),建立了能反映其短期和长期强度特征的柱状岩样数值模型,并开展了大量数值试验。结果表明,蠕变损伤的演化过程与暂态的损伤演化过程具有明显的差异。在岩样蠕变损伤过程中,其内部微裂纹多沿加载方向开裂且分布均匀,先快速增加再稳定扩展,最后则发生快速断裂。当荷载较小时,岩样宏观上呈现劈裂破坏特征,当荷载较大时,岩样呈现剪切破坏特征。在岩样蠕变损伤初始和稳定演化阶段的前期,荷载大小对岩样的损伤演化过程影响不大;在稳定演化阶段的后期至断裂过程中,低荷载下岩样的损伤增速比高荷载下快。     

软岩非线性蠕变损伤模型及其试验研究

为了反映软岩蠕变全过程,进行红层泥岩蠕变力学试验,试验发现弹性模量随时间的增长而逐渐衰减,黏滞系数在应力恒定且并未达到屈服应力的情况下,随时间的增长而逐渐增大。因此认为传统理论流变力学中的蠕变损伤处理方法不适用于将黏滞系数进行损伤演化,故引入分数阶微积分来描述软岩蠕变的黏弹性和黏塑性应变。通过构建一个考虑蠕变时效损伤的弹性体,并将其与基于分数阶微积分的黏滞和黏塑性体进行串联,从而建立一个新的非线性蠕变损伤模型。依据红层泥岩和相关文献中冻结软岩及红砂岩的蠕变试验数据,通过该软岩蠕变损伤模型对其进行辨识,充分显示出所建模型的合理性和适用性。     

基于颗粒流方法的堆积层滑坡运动过程模拟

以颗粒流离散元为研究方法对勉县杨家湾十组堆积层滑坡破坏方式与运动过程进行数值模拟研究。通过PFC2D双轴模拟试验所标定的岩土体宏观模拟参数与室内试验所获取的宏观实测参数进行对比,确定堆积层滑坡所需的颗粒细观参数,然后将标定的细观参数代入堆积层滑坡模型,对滑坡破坏方式及运动过程进行模拟研究。结果表明:滑坡破坏在初始阶段蠕滑变形累积,滑坡体挤压坡脚,直至坡脚产生剪切破坏,并向上牵引发展,使得滑坡整体顺接触面破坏下滑并堆积于坡脚,表现为典型的牵引式渐进破坏,结果与实际情况基本吻合。研究认为,采用颗粒流方法对堆积层滑坡破坏与运动过程的模拟研究具有较高的适用性,对该类滑坡防治具有一定的参考意义。      

Integration schemes with substepping algorithms for creep analysis in geomaterials

This paper describes a robust and efficient methodology for predicting displacements, deformations, and stresses in geomaterials that are susceptible to creep. The methodology is based on two integration schemes, which consider substepping algorithms. The first scheme is used for integrating space-time relations in a global sense, whereas the second scheme is used for integrating stress-stain relations in a local sense. Different from previous studies, both integration schemes are easy to implement and general in the sense that they can be applied to any type of creep law. Through an in-house finite element simulator, several numerical tests are performed. They include triaxial and wellbore closure analyses considering soft soils and salt rocks. The results show that the combination of both schemes leads to stable and accurate solutions with reduced computational time.     

Numerical and Laboratory Analysis of Relaxation Tests for Determining Time-Dependent Properties of Rock

Roof falls in coal mines may occur within a few months to a few years after excavation. In this paper, we proposed the use of relaxation tests as a substitute for time-dependent tests. The relation between creep behavior and relaxation behavior was numerically investigated and demonstrates that the material assigned with creep model can show relaxation behavior. Then the relaxation model was developed by modifying the Burgers creep model. Numerical simulation of a relaxation test on a simulated rock model in 3DEC yielded results that were similar to theoretical prediction. A relaxation test was performed on two groups of specimens under varying load conditions. Results from the laboratory tests validated the approach of using relaxation test to determine time-dependent properties. Finally, time-dependent properties were investigated by performing relaxation tests at different stages of a complete stress–strain curve. The relaxation test during strain-softening was unsuccessful; however, the relaxation behavior at residual stage in post-failure region was more significant than that in pre-failure region and the sudden drop in stress indicated there was strength deterioration due to the accumulation of viscous strain.     

单轴压缩下绿砂岩长期强度的尺寸效应研究

岩石的蠕变特性是影响岩体工程稳定性的重要因素,而岩石的长期强度是确定岩体工程长期稳定的一个重要指标。由于岩石材料的非均质性,其长期强度具有明显的尺寸效应。为了研究岩石长期强度的尺寸效应,首先,在幂函数模型基础上,基于损伤力学理论,建立了能够描述岩石蠕变全过程的非线性蠕变损伤模型;然后,把运用该模型计算得到的单轴压缩蠕变数值模拟结果与室内单轴压缩蠕变试验结果进行对比,验证了模型的正确性;最后,采用所提出的模型对7个不同尺寸的岩样进行了单轴压缩蠕变数值模拟,并对岩石长期强度尺寸效应进行了分析。数值模拟结果表明：随着试样尺寸的逐渐增大,岩石长期强度值逐渐减小,当试样尺寸增大到一定程度时,岩石长期强度稳定在一个特定值附近。     

山区高填方地基蠕变沉降特性及简化计算方法探讨

如何准确计算高填方地基的沉降变形,特别是工后沉降变形已成为目前日益增多的山区高填方工程中亟待解决的问题,但目前有关工后沉降的计算模型及计算分析方法还有待完善。在前人研究的基础上,首先对使用较为广泛的双曲线模型的特点、应用范围以及参数取值等进行了探讨,特别是初步分析了室内试验蠕变变形比现场完成较快的原因,进而在原有模型的基础上针对大面积填方问题提出了一个考虑蠕变过程中荷载变化的计算方法,并通过计算对比展示了其合理可行性。同时,通过分析室内试验成果,发现碎石料在干湿循环作用下发生的变形与循环次数关系明显,在此基础上提出了能考虑当地降雨情况的蠕变变形与干湿循环变形的耦合计算方法。最后应用所提方法对一实际工程进行了工后沉降预测。     

节理岩体等效流变损伤模型及其在卸载边坡中的应用

重大建设项目对施工过程中岩土体稳定性提出了更高的要求,常以一种动态设计施工模式来应对工程体的各类突发状况与病害。流变损伤模型既能反映岩土体在施工过程中变形的时效发展,又能反映其力学性质的时效劣化,继而能较准确地掌握工程体动态稳定性。基于以上考虑,为体现卸载边坡工程在卸载回弹阶段的瞬时塑性特征和时效演化阶段的黏塑性特征,因此,在流变模型中引入加载塑性元件和黏塑性元件,建立了复合黏弹塑（弹-黏-黏弹-黏塑-塑）模型,室内岩石压缩（卸载）蠕变试验证明了该流变模型的合理性,并对其参数进行辨识。在此基础上,从几何研究方法出发,引入反映节理分布的初始损伤张量及一种等效的依据黏塑性偏应变推导出的损伤演化方程,最终建立了一种新型的节理岩体等效流变损伤模型。将此模型用于川东红层某软硬岩互层型路堑边坡的卸载分析,结果表明：随不同的开挖阶段,易损部位（软岩集中段、软岩深埋段、软硬交接硬岩段）在瞬时卸载回弹阶段的塑性损伤和时效演化阶段的黏塑性损伤逐渐积累,边坡浅表部逐渐出现卸载损伤（松弛）带,在损伤累积中边坡各部位蠕变速率呈不同程度的增长。计算结果较好地反映了边坡变形、损伤发展与动态稳定性特征,其研究结果对于指导支护时机及相应的信息化施工具有一定的意义。     

考虑初始损伤和蠕变损伤的岩石蠕变全过程本构模型

考虑到天然岩石存在不同程度初始损伤以及蠕变过程中岩石受载后裂隙扩展而导致的新损伤,对具有初始损伤的岩石蠕变特性进行全面描述。根据不闭合结构面应力与法向变形之间的关系,提出裂隙岩石塑性变形体元件,描述岩石蠕变过程中的瞬时塑性变形。引入初始损伤影响因子,建立具有初始损伤的岩石损伤变量演化方程,构建模拟岩石加速蠕变的蠕变损伤体元件。将裂隙岩石塑性变形体和蠕变损伤体与描述瞬时弹性变形和黏弹性变形的广义开尔文模型进行串联组合,形成能够反映具有初始损伤的岩石瞬时弹-塑性变形、稳定蠕变和加速蠕变的蠕变全过程本构模型,提出了进行少量蠕变试验既能解析模型参数的方法,在不同应力水平下模型理论曲线与蠕变试验曲线吻合。     

深层膏质泥岩蠕变特性及非线性蠕变模型研究

泥岩作为油气储层常见复杂介质,在深部高温、高压环境下蠕变力学特性异常复杂,是引起套管损伤破坏的重要因素。以深部油气藏工程含膏质泥岩为研究对象,首先开展了高温为130 ℃、高围压为30 MPa、不同偏应力水平下多试样、单级加载三轴蠕变试验。该泥岩呈显著的时效力学特性,蠕变应力阀值较低,应力和持时对蠕变特性存在显著影响,低应力、长时间作用下泥岩亦呈现明显的稳态蠕变和加速蠕变破坏,且加速蠕变起始时间随应力增加呈指数降低关系。其次探讨了泥岩加速蠕变和强度损伤特性,提出了蠕变损伤起始时间概念,认为泥岩内部蠕变损伤变量可用指数函数予以描述。最后,基于蠕变损伤变量和起始时间,构建考虑泥岩加速蠕变的非线性改进Burgers蠕变模型,对蠕变参数开展辨识和分析,所选模型可准确地对深层膏质泥岩蠕变特性进行描述。研究成果旨在为深部泥岩油气藏套管工程长期稳定分析及安全评价提供可靠的依据。     

高填方地基的蠕变沉降计算方法

高填方地基的填方体量大、填方高度高的特征决定了高填方地基具有较大的工后沉降,可能会影响工程的实际应用,研究高填方地基工后沉降的计算方法对于控制工后沉降具有重要意义。蠕变沉降是高填方地基工后沉降的重要组成部分,试验表明,土的蠕变变形主要受时间和应力历史等因素影响。基于考虑时间效应的统一硬化模型,以瞬时正常压缩线为参考线,引入弹性瞬时压缩线,建立了可以同时考虑时间和应力历史影响的高填方地基一维蠕变沉降计算方法,并对室内试验进行了计算,计算结果与试验结果基本一致。所建计算方法所用参数除一个参数外其余都为常见参数,而新参数可以通过常规试验确定,该计算方法便于工程应用。     

FINITE ELEMENT MODELLING OF A DEEP SEA CLAY IN LONG-TERM LABORATORY CREEP TESTS

A new finite element program is introduced and its predictive capabilities are compared to results from two long-term, drained laboratory creep tests on a deep sea clay. The constitutive behaviour is based on Cam clay critical state plasticity theory with creep and time-dependent hardening. Creep is computed using either Singh–Mitchell's three-dimensional equation or Taylor's secondary compression relationship. The experimental creep data include a triaxial specimen subjected to two deviatoric stress increments and a one-dimensional consolidation specimen subjected to three vertical stress increments. In addition, the pore pressure behaviour following an increase in stress is examined in the triaxial sample. Predictions compare favourably to test data, which provide confidence for applying the chosen constitutive model and numerical formulation to solve seabed-related problems on the continental slope that are of interest to geologists, the oil industry and the navy, among others.     

Discrete element modelling of creep of asphalt mixtures

Creep tests on asphalt mixtures have been undertaken under four stress levels in the laboratory while the discrete element model (DEM) has been used to simulate the laboratory tests. A modified Burger’s model has been used to represent the time-dependent behaviour of an asphalt mixture by adding time-dependent moment and torsional resistance at contacts. Parameters were chosen to give the correct stress-strain response for constant strain rate tests in Cai et al. (2013). The stress-strain response for the laboratory creep tests and the simulations were recorded. The DEM results show reasonable agreement with the experiments. The creep simulation results proved to be dependent on both bond strength variability and positions of the particles. Bond breakage was recorded during the simulations and used to investigate the micro-mechanical deformation behaviour of the asphalt mixtures. An approach based on dimensional analysis is also presented in this paper to reduce the computational time during the creep simulation, and this analysis is also a new contribution.     

考虑温度影响的花岗岩蠕变全过程本构模型研究

以北山花岗岩为研究对象,采用MTS815岩石力学试验系统开展不同温度条件下的蠕变特性试验研究。考虑温度对花岗岩特征参数的影响,结合岩石蠕变破坏过程中的损伤演化规律,提出了一种新的高温损伤流变元件。通过将高温损伤流变元件代替经典西原模型中Newton元件的方法,构建了能够描述不同温度条件下花岗岩蠕变全过程的本构模型。分析了不同温度条件下花岗岩单轴蠕变试验结果,确定了模型参数,获得了温度对花岗岩蠕变关键参数的影响规律。通过对花岗岩高温蠕变模型进行参数敏感性分析,揭示了弹性模量、黏性系数等关键参数对花岗岩蠕变特性的影响规律,并验证模型在不考虑温度及损伤的影响条件下可退化为经典西原模型。研究表明,建立的花岗岩高温蠕变本构模型可以准确地描述花岗岩典型蠕变全过程的3个阶段,尤其是加速蠕变阶段。黏性系数受损伤的影响越大,花岗岩的稳定蠕变阶段越短,越容易发生加速蠕变。     

含水状态下软岩蠕变试验及损伤模型研究

为考虑时效劣化和含水弱化对岩石蠕变参数的劣化效应,提出了蠕变损伤模型,即对西原模型进行参数修正与完善,开展了不同含水状态的软岩三轴蠕变试验。首先,对不同含水率的岩石蠕变规律进行了对比分析,初步揭示了含水状态对岩石蠕变过程影响规律的机理。其次,为了能够较好地描述岩石蠕变过程,引入时效劣化效应和含水弱化效应,并通过试验数据拟合了损伤变量和蠕变参数。最后,将模型的计算值与试验值进行对比分析。结果表明,岩石随着含水率的增大,初始蠕变值和稳态蠕变值有所增大。通过FLAC3D验证了该模型的数值计算值与试验值具有很好的吻合性,提出的岩石时效蠕变损伤模型将为岩石流变特性的研究提供一定的理论基础。     

Time-dependent cracking and brittle creep in crustal rocks: A review

Rock fracture under upper crustal conditions is driven not only by applied stresses, but also by time-dependent, chemically activated subcritical cracking processes. These subcritical processes are of great importance for the understanding of the mechanical behaviour of rocks over geological timescales. A macroscopic manifestation of time-dependency in the brittle field is the observation that rocks can deform and fail at constant applied stresses, a phenomenon known as brittle creep. Here, we review the available experimental evidence for brittle creep in crustal rocks, and the various models developed to explain the observations. Laboratory experiments have shown that brittle creep occurs in all major rock types, and that creep strain rates are extremely sensitive to the environmental conditions: differential stress, confining pressure, temperature and pore fluid composition. Even small changes in any of these parameters produce order of magnitude changes in creep strain rates (and times-to-failure). Three main classes of brittle creep model have been proposed to explain these observations: phenomenological, statistical, and micromechanical. Statistical and micromechanical models explain qualitatively how the increasing influence of microcrack interactions and/or the increasing accumulated damage produces the observed evolution of macroscopic deformation during brittle creep. However, no current model can predict quantitatively all of the observed features of brittle creep. Experimental data are limited by the timescale over which experiments are realistically feasible. Clearly, an extension of the range of available laboratory data to lower strain rates, and the development of new modelling approaches are needed to further improve our current understanding of time-dependent brittle deformation in rocks.     

煤岩蠕变模型与破坏特征试验研究

在MTS岩石伺服试验机上对山东某矿3煤进行了短时流变试验,并对试验结果进行了分析。结果表明：该煤岩蠕变门槛系数为0.915、流变系数为0.383;煤岩流变破坏呈现空间不均匀性,整体以塑性破坏为主,但存在剪切主导破坏面;煤岩流变过程中有微破裂破坏即损伤存在,并且这种损伤在较低的应力水平就有所发生,大量微破坏的积累与贯通导致了煤岩试件破坏;煤岩蠕变过程中不仅存在损伤软化,而且存在蠕变硬化,蠕变的过程取决于这两种机制竞争的综合结果;可以用3次多项式经验蠕变模型较好地描述该煤岩的蠕变特性,西原模型可以较好地描述该煤岩的初始蠕变和等速蠕变阶段,但不能描述加速蠕变阶段。     

泸定大渡河桥康定岸重力锚边坡长期变形与稳定性分析

悬索桥重力式锚碇边坡的长期变形与稳定性是影响该类桥梁结构正常使用的一个关键要素。为合理讨论这一问题,以泸定大渡河桥康定岸重力式锚碇边坡为例,首先开展了现场边坡土体剪切、压缩试验,通过三维弹塑性数值模拟方法确定坡体应力场,基于此确定直剪蠕变试验加载条件,进而针对主要由冰碛土构成的该坡体,进行直剪蠕变试验,根据试验结果揭示的土体蠕变特性随时间逐渐减弱的特征,结合Mohr-Coulomb强度准则,采用Burgers体与广义Kelvin体组成的两时段蠕变本构模型进行锚碇边坡的黏弹塑性分析。考虑锚碇发挥正常使用功能的要求,提出了特征点（散索鞍点）位移容许值判据,将强度折减法扩展应用于黏弹塑性坡体的长期变形与稳定性分析。实例分析表明,该锚碇在正常运营100 a时散索鞍点朝河侧水平位移为13.91 cm,在正常使用极限状态的条件下,锚碇边坡的长期稳定系数为1.71。     

Nonstationary flow surface theory for modeling the viscoplastic behaviors of soils

This paper presents a three-dimensional elastic viscoplastic model that can describe the time-dependent behaviors of soft clays. The constitutive model is formulated based on the nonstationary flow surface theory and incorporates new developments, including (i) an improved definition of the nonstationary flow surface that is capable of capturing the stress–strain behaviors under different loading paths, (ii) a unique stress–strain—viscoplastic-strain-rate equation that is able to explicitly describe the nonstationary flow surface, and (iii) a final stable state concept that identifies the final equilibrium state at the end of creep and stress relaxation, which is also used to simplify the loading criteria. The consistency condition is validated for the proposed model, and the viscoplastic multipliers are calculated by solving the consistency equations. The model performance is investigated and validated via simulation of both oedometer and triaxial tests. The numerical results demonstrate that the proposed model is able to reproduce the main viscoplastic behaviors of soils, including creep, undrained creep rupture, stress relaxation, rate effect and accumulated effect.