盐岩循环加卸载过程中的速率效应试验研究

作为一种典型软岩,盐岩力学特性的时间相关性显著,准确预测和评估盐穴储气库加卸载过程中的变形损伤能力,关系到能源储备库的长期安全稳定运行。为研究盐岩的速率效应,进行了不同加卸载速率的疲劳试验。基于现有蠕变本构模型,构建了速率效应方程,区分了加卸载过程中的蠕变塑性变形与加载塑性变形。研究结果表明：（1）盐岩的整体疲劳寿命、稳定变形阶段（即第2阶段）疲劳寿命占比基本随加卸载速率的增大而增大。（2）减速变形阶段（即第1阶段）疲劳寿命占比、稳定变形阶段平均残余应变基本随加卸载速率增大而减小。（3）速率耦合疲劳试验中,加卸载速率越大,残余应变越小,蠕变塑性变形随加卸载速率的增大而减小,当加卸载速率由0.04 kN/s增加至5 kN/s时,蠕变塑性变形在循环中的占比由85%下降至36%,呈现出负指数的关系。（4）整个循环加卸载过程中,加载塑性变形、蠕变塑性变形均呈现U型发展趋势,与盐岩疲劳过程的三阶段相对应。该研究结果可以为进一步分析盐岩蠕变塑性与加载塑性奠定基础。     

吸力式沉箱底部土体循环特性及其等效循环蠕变模型研究

吸力式沉箱基础作为张力腿平台(TLP)的锚固基础,其底部土体不仅存在轴向卸荷作用,而且还存在循环荷载作用。现有研究很少涉及在卸荷条件下进行的软黏土循环累积变形研究。因此,通过循环三轴试验研究了轴向卸荷条件下软黏土的应变累积变形及应变软化特性。试验结果表明:在低静偏应力以及动偏应力条件下,土体应变累积及软化程度较低;随着动偏应力增加,循环累积变形逐渐变大且前期发展较快后期趋于稳定,同时应变软化程度逐渐增大;土体循环累积变形随动偏应力提高而增大,其循环累积应变曲线呈衰减-稳定的蠕变特性。在试验基础上,提出与静偏应力和动偏应力有关的软化系数公式。构建了考虑软化系数与静偏应力、动偏应力等条件下软黏土等效循环蠕变模型。将该模型推广至三维,并开发了有限元子程序。建立了循环荷载作用下吸力式沉箱基础循环抗拔承载力有限元分析法,将其用于吸力式沉箱基础循环累积变形分析中,并与沉箱模型循环试验结果进行了对比验证。     

砂岩分级加卸载蠕变试验过程能量演化分析

对陕西柠条塔砂岩样品进行单轴分级加卸载蠕变试验，分析样品变形破坏过程中的能量演化规律。结果表明：随循环级数增加，各级耗散能非线性增长，各级塑性应变能相对稳定；耗散能超过塑性应变能可作为样品破坏的先兆能量特征。通过定义相关系数，建立能量与变形之间的关系后发现：随循环级数增加，各级塑性应变能与各级新增塑性应变呈正相关性，各级耗散能与塑性应变累积量呈正相关性。分别对加载、蠕变、卸载及恢复等4个阶段的能量计算后得出：各阶段能量演化速率均随循环级数的增加而增大。加、卸载速率恒定时，加载段能量变化率大于同级卸载段能量变化率，蠕变段能量变化率大于同级恢复段能量变化率。通过分析各级循环加载段能量变化规律，提出能量衰减系数 ，该值随应力水平增长呈幂函数形式降低，以此提出一种可以有效预测破坏应力的方法。     

西安地区地裂缝带Q3原状黄土流变特性试验研究

汾渭盆地是我国乃至世界上地裂缝最为发育、灾害最为严重的地区。考虑在漫长的地质历史过程中黄土的流变特性对地裂缝破裂扩展模式的影响,采用分级循环加卸载方式,对西安地区长安地裂缝（fc1）带Q3原状黄土进行了不同围压下的固结不排水三轴流变特性试验,得到了地裂缝带Q3原状黄土的蠕变加、卸载曲线和应变速率与时间关系曲线。试验结果表明,不同围压下地裂缝带Q3原状黄土表现为低应力下为减速蠕变,高应力下为减速蠕变和等速蠕变的特征。并基于统一流变力学模型理论,建立了适用于西安地区地裂缝带Q3原状黄土的三维蠕变本构模型,获得了相应的本构模型参数。该成果为研究隐伏地裂缝破裂扩展模式及其力学机制奠定了基础。     

低含水量粘土的加载速率效应与蠕变变形

通过对低含水量藤森粘土(Fujinomori clay)试样的单向固结试验,研究了低含水量粘土的加载速率效应与蠕变变形。所采用的藤森粘土土样有湿润(含水量小于15%)、风干(含水量小于4.5%)与烘干(含水量小于1%)3种。试验中,应变率逐步改变,同时进行了蠕变及加、卸载试验以及小幅度的循环加载试验。试验结果表明:湿润土样、风干土样与烘干土样都有明显的加载速率效应与蠕变变形;在大范围的卸载-重复加载过程中,蠕变变形会由卸载初期的正变形逐步发展为后期的负变形,在这个发展过程中存在着中性点(即蠕变为零,或者无蠕变的点);藤森粘土的加载速率效应随含水量的降低有所降低,即湿润藤森土样的加载速率效应更明显。     

软土地基桶形基础循环累积变形研究

针对软土地基桶形基础在循环荷载作用下的变形问题进行研究。首先, 通过桶形基础循环模型试验获得在动、静荷载作用下地基的破坏特性。然后,从模型土池中取土样,进行不固结不排水（UU）循环三轴试验,测其循环累积应变特性,建立了累积应变与其相关因素间的关系。最后,采用有限元的方法模拟模型试验,将循环累积变形等效为蠕变,在理想弹塑性的基础上加入蠕变来计算循环荷载作用下桶形基础的变形,并与模型试验结果进行比较。结果表明,软土地基桶形基础在循环荷载作用下的变形与静力下的蠕变有相似的特点,桶形基础循环累积变形可以用等效为蠕变的方法计算。     

Effects of cyclic loading on the rheological properties of sandstones

This paper presents some results of the laboratory tests on the time-dependent behaviour of three types of sandstone. Research into the rheological properties of rocks is mostly done under static (quasistatic) loading. In nature, however, rocks are also often subjected to cyclic (vibration) loading as a result of vibrations from traffic or the propagation of seismic waves due to, for example, earthquakes, rock bursts, blasting operations and such like. On this account, the research presented is focused on the study of rheological rock properties during vibration (cyclic) loading. The tests of the rheological properties of a cyclic load were conducted both in creep regime and in relaxation regime, at various levels of average load or strain and for various parameters of vibration (frequency, amplitude). The measured values of different parameter settings were compared and evaluated so that it was possible to set the dependence of cyclical rock loading in the rheological regime. For the purpose of the comparison of rock behavior during vibration and static loading, other tests were also carried out. The results of the research show that, in the course of cyclical loading, the rock rheological properties depend, to a significant extent, on the parameters of this loading, especially the amplitude and frequency.     

考虑卸荷扰动的黏土蠕变特性试验研究

盾构施工过程中,由于盾尾建筑空隙的存在,土体会受到较大的卸荷扰动,尤其是对于结构性较强的黏土。通过卸荷拉力来模拟土体受到的卸荷扰动,进行排水剪切蠕变试验。试验结果表明:卸荷拉力越大,土体的扰动程度增大,同级荷载作用下土样的变形越大,且加载压力较小时土样的变形属于稳定型蠕变,蠕变速率随着卸荷拉力的增大而增大,加载压力较大时土样的变形属于衰减型蠕变;与Singh-Mitchell蠕变模型进行比较,发现卸荷拉力较小土样Singh-Mitchell蠕变模型的计算值与试验值相比吻合较好,但当卸荷拉力较大时,模型计算值与实验值相比有较大的误差;修正了Singh-Mitchell蠕变模型的参数,提出考虑卸荷扰动的修正Singh-Mitchell蠕变模型,计算结果与试验数据在加载压力和卸荷拉力较大时吻合较好,对工程实际有一定的指导意义。     

深部围岩流变特性试验研究及其模型辨识

采用分级增量循环加卸载方式,对湖南湘煤集团牛马司矿业公司水井头煤矿-300m东大巷二煤层底板粉砂岩进行了单轴蠕变特性试验,详细探讨了轴向蠕变变形特征,蠕变速率与时间和应力水平的关系,瞬时弹性模量和变形模量变化特征.试验结果表明,当应力水平低于岩石的长期强度时,粉砂岩只发生衰减蠕变,应力水平对蠕变速率变化的影响不明显;当应力水平高于岩石的长期强度时,岩石等速蠕变阶段明显.对流变试验数据进行深入分析的基础上,进行了流变模型辨识和基于最小二乘法的参数确定.根据不同应力水平下加卸载过程中岩石的蠕变特点,提出了采用改进的两原模型模拟岩石的蠕变特性.模型计算结果表明,改进后的西元模型既能够较好地描述低应力下的衰减蠕变,又能较精确地描述高应力下岩石的衰减蠕变和等速蠕变特性.     

矽卡岩单轴循环加卸载试验及声发射特性研究

利用MTS岩石力学试验系统和PAC声发射信号采集系统,研究了单轴循环加卸载作用下矽卡岩强度变化及声发射特征。在矽卡岩循环加卸载过程中,若加载过程中岩样没有明显局部破坏,则随着循环次数的增加,弹性模量逐渐增大,岩样强度略有提高;若加载过程中岩样内部已经出现明显的局部破坏,产生了不可恢复的宏观裂纹,则循环加卸载会促进宏观裂纹层面间的滑移,从而加速岩样失稳破坏。矽卡岩在单轴压缩作用下声发射曲线大体可分为压密、弹性、塑性、峰后屈服4个阶段,并伴随有不同的声发射情况。此外,矽卡岩在循环加卸载作用下具有反凯塞效应,并且随着加卸载循环次数的增加,费拉西蒂比逐渐变小;岩样卸载阶段仍然有大量声发射产生。     

黏土的蠕变特性试验研究

研究了黏土的未扰动和重塑土样在三轴排水剪切试验条件下的蠕变特性。既进行了初始加载过程中的蠕变试验,也做了卸载与重复加载过程中的蠕变试验。试验结果表明：(1) 在初始加载过程中,经过一定时间蠕变后黏土的强度有很大的提高;(2) 在卸载过程中,随应力值下降黏土的蠕变表现出由正蠕变、中性蠕变到负蠕变的逐渐转化;(3) 在重复加载过程中,随应力值增加黏土的蠕变表现出由负蠕变、中性蠕变到正蠕变的逐渐转化。     

饱和软黏土动力学特性循环扭剪试验研究

以饱和软黏土的循环扭剪试验研究为基础,阐明不固结不排水条件下饱和软黏土的动力学特性。在循环扭剪和循环三轴两种不同试验应力状态下,通过研究不同围压和不同静、动应力组合下饱和软黏土的应力等效破坏关系和应变等效破坏关系,提出在循环荷载作用下饱和软黏土的循环破坏同样遵循Mises屈服准则,且在Mises屈服准则下,饱和软黏土的循环强度趋于不变量。循环破坏的过程可以等效为一种拟静力弹塑性循环蠕变,建立了饱和软黏土循环累积变形随静荷载、循环荷载以及循环破坏振次之间的关系式。上述结论分别从应力和变形两个方面阐述饱和软黏土的动力学特性,与试验应力状态和围压无关,可以推广到一般应力状态下。     

Clay minerals in fluvial shaley sandstone of upper Triassic reservoir (TAGS)—Toual field SE Algeria: identification from wireline logs and core data

The creep property of rock under cyclic loading is very important in civil engineering. In order to establish a novel constitutive equation for rock under cyclic loading, a fractional-order viscoplastic body under cyclic loading was constructed based on fractional-order viscous element. A fractional-order visco-elastoplastic model (FVEPM) for rock was established by connecting constructed fractional-order viscoplastic body with Burgers model. The model was a Burgers model when the maximum value of cyclic loading was less than the critical strength of rock; otherwise, it was a FVEPM which can be used to reflect the transient, steady-state, and tertiary creep phases of rock. The cyclic loading was decomposed into a static load and a cyclic loading with a zero average stress. According to rheological mechanics theory, the rheology constitutive equation of rock under the static load can be derived. According to viscoelastic mechanics theory, the constitutive equation under cyclic loading with a zero average stress was established by introducing the variation parameters of energy storage and energy dissipation compliance caused by rock damage and fracture. Finally, a new dynamic constitutive equation of rock cyclic loading can be obtained by superimposing the constitutive equation under static load and cyclic loading with a zero average stress. Compared with existing test results of rock under cyclic loading, the proposed constitutive model can be used to describe the creep characteristics of rock under cyclic loading and reflect the presented fluctuation of strain curve of rock under cyclic loading.     

卸荷蠕变条件下软硬相接岩层非协调变形规律研究

软硬相接岩层在长时间跨度内的蠕变变形可能极为不协调,这对开挖工程安全稳定造成严重威胁。对某水电站引水隧洞同一开挖断面内砂质泥岩和泥质砂岩两种软硬相差较大的岩石进行卸荷条件下的蠕变试验和数值模拟,结果表明：在卸围压条件下,试样侧向蠕变变形较轴向发展更为迅速,呈现出显著的侧向扩容;初始围压水平较高时,试样卸荷蠕变破坏的侧向扩容效应更加显著;泥质砂岩在稳态蠕变阶段变形速率比砂质泥岩更慢,蠕变破坏时的应变值均比砂质泥岩蠕变破坏时的应变值要小,征兆性更弱。基于Burgers模型,引入损伤变量,建立带损伤的蠕变本构模型,模型曲线较好地描述了岩石破坏偏应力水平之前的线性蠕变特征和破坏偏应力水平下的非线性加速蠕变特征;软硬相接岩层蠕变数值模拟表明,蠕变变形初期,软硬相接点变形差别幅度不大,随着时间增长,在交接面处发生层间错动,对围岩整体稳定十分不利,工程实践中应予以足够关注。     

基于分数阶导数理论的Q3黄土流变本构模型研究

荷载的时间效应诱发的黄土蠕变和渐进破坏会显著影响其工程构筑物的长期服役性能。特别是在我国西北地区黄土广泛分布,在荷载的长期作用下一些典型工程建构筑物(如西安地铁开挖的黄土隧道工程)可能因黄土蠕变和渐进破坏而失稳,进而对工程建设的顺利实施以及工程建设完成后其安全运营维护产生极为不利影响。可见,对黄土流变特性进行分析并揭示其流变机理进而建立其本构模型具有重要的理论和实际意义。为此,以Q₃黄土为研究对象,基于分数阶导数理论,通过对典型蠕变变形过程进行分析,提出了一种可模拟加速蠕变阶段非线性变化特征的分数阶元件模型,对其进行理论分析进而建立了分数阶改进西原模型并推导出本构方程。在此基础上对Q₃黄土的原状与重塑样开展了三轴分级循环加-卸载流变试验,以验证分数阶改进西原模型的有效性。相应的理论计算结果表明:分数阶改进西原模型既能实现对Q₃黄土减速蠕变、等速蠕变和加速蠕变3个流变阶段的模拟,又可弥补整数阶改进西原模型无法描述加速蠕变阶段的不足,而且相比于整数阶改进西原模型,其在减速蠕变及卸载阶段的预测效果更佳。研究结果为考虑黄土流变问题的建构筑物安全运营维护设计与施工方案提供了重要的理论依据。     

饱和红层泥岩填料累积变形特性及安定界限研究

最优含水率下红层泥岩作为高速铁路路基填料的适用性已经在西南地区得到了初步验证,但饱和状态下红层泥岩填料的累积变形特性仍不明确。为此,开展了不同围压及动应力下饱和红层泥岩填料的动三轴试验,讨论了围压和动应力对填料累积变形特性的影响。结果表明：不同动应力幅值下,饱和红层泥岩填料的等效模量随振次增加,先迅速减小后趋于稳定。稳定后填料的等效模量具有应力水平相关性,随围压增大而增大,随动应力比增大而呈指数减小。不同动应力幅值下,填料变形可分为塑性稳定型、塑性蠕变型和增量破坏型3种类型。填料应变随振次增加而不再处于稳定阶段的应力状态定为塑性稳定界限应力状态。动应力进一步增加后,填料应变速率发生突增,对应的应力状态为塑性蠕变界限应力状态;塑性稳定与塑性蠕变界限应力状态对应的动应力比均低于静强度,且随围压增加呈指数衰减的规律。基床表层如用红层泥岩作填料,应考虑降雨的影响,并采取排水和改良措施以提高路基稳定性。     

伯格斯模型参数调试与岩石蠕变特性颗粒流分析

岩石作为地下工程的主要研究对象,其物质组成和所赋存的复杂地质环境使其具有明显的流变特性。基于三维颗粒流程序(PFC3D),采用由开尔文模型和马克斯伟尔模型串联组成的伯格斯（Burgers）流变模型,通过控制变量法,分析了伯格斯模型中弹性系数(Em和Ek)、黏性系数(?m和?k)和摩擦因数f对瞬时强度特性和流变特性的影响;分析得出数值试样的瞬时强度特性主要受马克斯伟尔体弹性系数Em和摩擦因数f影响：弹性模量和单轴抗压强度与Em和f均呈正相关,泊松比与Em呈正相关,与f呈负相关;而流变特性与各参数均呈现负相关性。在此基础上,进行了红页岩分级增量蠕变试验的模拟,通过与室内蠕变试验结果的对比,验证了PFC3D中的伯格斯模型用于岩石蠕变试验模拟的可行性,并将该方法用于分级增量循环加卸载蠕变试验和三轴蠕变试验数值研究。该研究内容是利用离散单元法研究岩石流变特性的一种新的尝试。     

A cyclic elastoplastic-viscoplastic constitutive model for soils

An elastoplastic-viscoplastic constitutive model for soils is presented in this study, based on an original approach concerning viscous modelling. In this approach, the viscous behaviour is defined by internal viscous variables and a viscous yield surface. The model has been developed from a basic elastoplastic model (CJS model) by considering an additional viscous mechanism. The evolution of the viscous yield surface is governed by a particular hardening called ‘viscous hardening’. This model is able to explain the time-dependent behaviour of soils such as creep (primary, secondary and un-drained creep rupture), stress relaxation and strain rate effects in static and cyclic loadings. The existing problems in the classical elasto-viscoplastic models related to the plasticity failure, the rapid loading and the cyclic loading are solved in the proposed model. The physical meanings and the identification strategy of model parameters are clearly given. The validation on certain triaxial test results and the simulation of cyclic triaxial test indicate the capacity of this model in prediction of time-dependent behaviour of clayey soils.     

Superplastic flow in finegrained limestone

Creep of Solnhofen limestone at temperatures between 600° and 900° C was found to fall into three different flow regimes: regime 1 with an exponential stress-dependence of strain rate, regime 2 with power-law creep and n ~ 4.7 and finally a superplastic regime 3 with n ~ 1.7. Within the superplastic regime the creep behaviour is strongly grain-size dependent, the strain rate increasing markedly with decrease in grain size at a given stress. Microstructural observations indicate that in regimes 1 and 2 intracrystalline plasticity is dominant whereas the superplastic regime is characterized by grain-boundary sliding. The crystallographic preferred orientation within the superplastic regime is weaker and of different geometry when compared with that in flow regimes 1 and 2. In a discussion on the deformation mechanisms it is suggested that flow regimes 1 and 2 are regimes of dis location creep in which the rate controlling step is diffusion assisted; for the superplastic regime existing models of grain-boundary sliding are compared with the observations Finally, the tectonophysical importance of superplasticity is discussed and by extra polating the observed creep behaviour to geological strain rates it is found that super plasticity in rocks is to be expected under a wide range of conditions, particularly at smal grain sizes.     

Comparison of the characteristics of rock salt exposed to loading and unloading of confining pressures

Rock mechanical behaviors and deformation characteristics are associated with stress history and loading path. Unloading conditions occur during the formation of a salt cavity as a result of washing techniques. Such conditions require an improved understanding of the mechanical and deformation behaviors of rock salt. In our study, rock salt dilatancy behaviors under triaxial unloading confining pressure tests were analyzed and compared with those from conventional uniaxial and triaxial compression tests. The volume deformation of rock salt under unloading was more than under triaxial loading, but less than under uniaxial loading (with the same deviatoric stress). Generally, under the same axial compression, the corresponding dilatancy rate decreased as the confining compression increased, and under the same confining compression, the corresponding dilatancy rate increased as the axial compression increased. The dilatancy boundary of the unloading confining pressure test began with unloading. This is different from the dilatancy of the uniaxial and triaxial compression tests. The accelerated dilatancy point boundary stress value was affected by confining and axial compressions. The specimens entered into a creep state after unloading. The associated creep rate depends on the deviatoric stress and confining compression values at the end of the unloading process. Based on unloading theory and the experimental data, we propose a constitutive model of rock salt damage. Our model reflects the dilatancy progression at constant axial stress and reduced lateral confinement.