冻结砂土在动荷载下的蠕变特征

通过分析不同试验条件下的蠕变过程曲线,探讨了冻结砂土在动荷载下的蠕变模型,分析了最大加载应力,温度及加载频率对冻土蠕变破坏应变,破坏时间和最小蠕变速率的影响.结果表明,当最大加载应力变大时,破坏应变增加,破坏时间缩短,最小蠕变速率变快;加载条件相同时,温度越低,破坏应变越小,破坏时间越长,最小蠕变速率越小;加载频率变化时,最小蠕变速率的变化无明显规律,都在一个量级范围内,当频率变大时,最小蠕变速率略有变小的趋势.频率增加,破坏时间缩短,破坏应变减小.频率小于7Hz时,频率对破坏应变和破坏时间影响较大,而当频率大于7Hz时,随频率加快,破坏时间和破坏应变只略有减小.     

循环载荷对压实雪冰抗压强度的影响研究

世界强国针对南极科技前沿、地缘政治、开发利用权益的争夺日趋激烈。中国建设和运营南极航空基础设施,有利于提升中国在南极的综合影响力、扩大南极科学考察规模、满足人员和物资快速投送需求,因此,南极机场建设的战略发展价值突出,而压实雪层跑道是中国在南极建设大型机场路面的首选形式。采用压实雪的方法制备人工冰样,结合冰雪跑道所承受的飞机载荷特征,在（-10.0±0.3） ℃温度条件下,以1.0 MPa为平均加载应力,以0.03 Hz为加载频率,应力幅值为0.2~1.0 MPa,循环次数100~900为循环条件,开展在1.0×10^-3~1.0×10^-1 s^-1应变速率下循环载荷压实雪冰的抗压强度试验,以及5.0×10^-4~1.0×10^-1 s^-1应变速率下单调载荷压实雪冰的抗压强度试验。结果表明：单调载荷下的压实雪冰抗压强度在韧性区内随着应变速率的增加而增加,在韧性-脆性过渡区达到最大值,然后在脆性区中显著减少;在高应变速率和一定循环次数范围内,循环载荷对压实雪冰的抗压强度具有强化作用;当超过一定循环次数时,循环载荷对压实雪冰的抗压强度会产生弱化作用。以上试验结果可为压实雪层跑道的设计和维护提供理论依据。     

Test on cyclic creep behavior of mucky clay in Shanghai under step cyclic loading

Based on the cyclic triaxial test under step loading, the variation of cyclic creep and pore water pressure with increase in cyclic stress ratio (CSR) and load cycle of mucky clay in Shanghai were analyzed. The results show that there are two threshold values of stability cyclic stress ratio and failure cyclic stress ratio during the step loading which divide the development of cyclic creep strain into gradual stability stage, rapid growth stage and instantaneous failure stage. When the stress level is less than the failure cyclic stress ratio, the recoverable elastic strain is nearly linear with CSR. The cyclic creep properties of mucky clay are obviously influenced by the loading frequency under the same stress level.     

考虑温度影响的冻土蠕变本构模型

冰作为冻土的基本组成部分,对冻土蠕变的加速蠕变阶段有重要影响。温度通过影响冻土中冰的冻结与融化过程,及其黏塑性流动,引起冻土结构的强化与弱化,从而成为了决定冻土蠕变力学行为的关键因素之一。同时,外部应力也造成冻土的强化与弱化,影响着冻土的蠕变。通过引入硬化因子与损伤因子来考虑温度、应力造成的冻土材料强化与弱化。硬化因子H代表了蠕变过程中强化效应的大小,而损伤因子D则代表了由弱化效应造成的冻土材料相关参数的折减比例,进而提出了适用于冻土的改进西原蠕变本构模型。该模型预测值与试验数据的比较表明：改进的模型不仅能较好地描述初始蠕变阶段、稳定蠕变阶段,而且相比于传统模型,能更好地描述冻土加速蠕变阶段,具有合理性与一定的实用性。     

Creep behaviour of frozen soils in uniaxial compression tests

The stress—strain behaviour of frozen soils is often described by means of creep curves from uniaxial or triaxial creep tests. Ten or twelve tests of similar samples are required to obtain a good relationship between stress σ, strain ε, temperature T and time t or strain rate ε. To reduce the required number of samples it is possible to apply the compression load in creep tests stepwise. Therefore creep curves for different stress levels can be obtained from one sample.

In this paper the bearing capacity of cylindrical samples of frozen medium sand under constant uniaxial compression stress and under stepwise increased stresses is compared. It is shown how to use these different creep curves to describe the stress—strain behaviour of frozen soils.     

人造冰和冰芯冰蠕变和方位组构发展对比

本描述了单轴压缩蠕变实验结果。试样包括ｃ轴取向为随机分布和非随机分布的冰、人造冰和取自南极洛多姆冰帽的冰芯冰，各向同性冰芯冰和人造的最小应变率存在差别，未获得合理的解释。方位组构与应力状态相容时，各赂异性冰的最小应变率高于各向同性冰的最小应变率，随着第三应谱阶段的发展，冰芯冰和人造冰的第三应变率趋近相同的恒定值，实验后冰芯冰和人造冰ｃ轴方位组构类型相同，晶粒尺寸也接近。     

冻土蠕变指标试验研究

通过不同温度、不同加载应力作用下冻结兰州黄土、黏土、砂质黏土的蠕变试验,分析了蠕变曲线、初始应变、流变起始应变与流变起始时间、破坏应变与破坏时间及相对蠕变指标.结果表明：3种土质冻土的蠕变曲线变化规律大致相同,加载过程中,应变非线性增加,且加载应力越大、温度越高,初始应变越大;流变起始时间与破坏时间都与加载应力、温度有密切关系,加载应力越大、温度越高,越先出现流变和破坏. 对于相同的土质,加载应力和温度对流变起始应变、破坏应变的影响不大;对于不同土质的初始应变、流变起始应变和破坏应变,都是黏土最大、砂质黏土次之、兰州黄土最小. 3种土质冻土的初始加载段和非稳定蠕变段所占的时间较短,但产生的应变却较大;同时,温度越高,相对流变时间越短、相对破坏时间越长,说明非稳定蠕变阶段所占的时间随温度的升高而变短、稳定蠕变阶段所占的时间随温度的升高而变长.     

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.     

模拟人工冻结凿井状态下冻土强度特性研究

通过模拟人工冻结凿井中冻土冻结、受力的实际过程,对已冻结试样进行不同温度、不同初始围压状态下的减载试验研究.结果表明:温度和土层深度是影响深部冻土破坏强度和破坏应变的主要因素,当温度不变时,破坏强度和破坏应变随初始围压呈线性关系变化.破坏强度受温度的影响取决于初始围压,在低初始围压状态下,冻土的破坏强度受温度变化影响不明显,但随着初始围压增大即土层深度加深,破坏强度受温度的影响也逐渐明显.破坏应变随温度的降低而逐渐减小,且呈双曲线形变化,但当温度低于 - 7℃时,在不同初始围压下其破坏应变基本不随温度的变化而变化     

基于微裂隙变形与扩展的岩石冻融损伤本构模型研究

在冻融条件下岩石微裂隙中的水发生相变,体积膨胀,对微裂隙产生很大的冻胀力,当冻胀力超过岩石的抗拉强度时,微裂隙扩展。温度升高时,水又进入新的微裂隙,如此反复循环造成了岩石的损伤。据此,将岩石中的微裂隙等效为扁平状椭圆裂隙,基于断裂力学建立了单条微裂隙下裂隙扩展长度与冻胀力的关系,考虑岩石中微裂隙的分布,将岩石冻融条件下的应变分解为初始损伤应变、附加损伤应变和塑性应变,建立了弹塑性冻融损伤本构模型。最后,通过岩石冻融试验对该模型的合理性进行了验证,结果表明,该模型能够较好地模拟岩石在不同冻融次数下的应力-应变关系曲线。     

Life-time prediction for rocks under static compressive and tensile loads: a new simulation approach

The life time or time to failure of rocks under load is governed by microstructural defects, like microcracks, voids etc. The life time can be predicted either by empirical exponential laws or physical laws based on damage and fracture mechanics. The proposed numerical model is based on subcritical crack growth using the linear elastic fracture mechanical approach and is implemented as a numerical cellular automate. The algorithm considers both tensile and shear fracturing. Each cell contains a microcrack of random length according to a given probability function. Fracture growth is controlled by the Charles equation. Macroscopic cracks are the results of the coalescence of growing microcracks. Within the numerical approach elasto-plastic stress redistributions take place. If the stress intensity factors have reached the critical values or the microcrack has reached the zone dimension, the zone is considered as fractured and residual strength values are assigned. The proposed approach was applied to rock samples under uniaxial compressive and tensile loads (creep tests). Successful results were obtained in respect to the predicted life time, damage evolution and the fracture pattern. Conclusions for further improvements and extensions of this methodology were drawn.     

泥质砂岩蠕变特性与锚固控制效应试验研究

通过泥质砂岩单轴压缩蠕变试验,揭示岩石蠕变变形与破坏的特点,反映了不同应力路径下的岩石蠕变速率与发展过程,建立了泥质砂岩的非线性蠕变模型,并通过对试件施加侧向刚性约束和锚固约束,以反映地下工程围岩在平面应变状态和支护状态下的蠕变特性。研究结果表明,锚固约束可以显著提高岩石发生稳定蠕变的应力阀值,减缓蠕变速率。根据改进的蠕变方程进行试验数据的拟合,确定了改进的西原模型蠕变方程中的各物理量。与单轴蠕变试验结果对比表明,该模型的计算结果与试验数据吻合较好,说明改进的西原模型可较好地反映岩石加锚后的蠕变特征。研究结果对深部地下工程围岩的变形与稳定控制研究具有重要的指导意义。     

滑带土蠕变过程及微观结构演化分析

滑带土的蠕变过程与微观结构密切相关,前人对滑带土的蠕变性质及微观结构做了大量研究,但对土体在蠕变过程中因时间效应发生的微观结构变化研究较少。本文选取延安二庄科北区滑坡滑带土为研究对象,采用直剪蠕变试验研究了滑带土的蠕变特性,用环境扫描电子显微镜对蠕变过程中不同阶段的土样进行SEM图像扫描,并对其微观结构孔隙特征进行定量分析。结果表明:滑带土样在400 kPa正应力荷载下具有明显蠕变特性,每一级剪应力加载均出现衰减蠕变和稳态蠕变阶段;当剪应力值达到255 kPa后,试样发生蠕变破坏,且滑带土样在蠕变过程中由于时间效应发生了塑性破坏。在每一级加载后,土样剪应变率均逐渐减小并最终趋于稳定;在各级剪应力加载后的衰减蠕变阶段,随着加载剪应力的增大,剪应变率也增大;蠕变过程中,土样的孔隙数逐渐减少,土颗粒重新定位排列,孔隙定向性增强,趋向于长条形的孔隙越来越多,孔隙轮廓越来越光滑,但圆形度越来越差。随着蠕变破坏的发生,在蠕变过程中形成的结构被迅速破坏,孔隙定向性减弱,数目急剧增加。     

高温-高含冰量冻土原位旁压蠕变试验

为了研究高温-高含冰量冻土的蠕变特性,在青藏高原多年冻土区开展了旁压蠕变试验。试验结果表明：旁压蠕变曲线可以借鉴Ladanyi的工程蠕变理论来表示;每个试验中瞬时弹性应变在各级压力下的总应变中所占比例的平均值为3%～7%,冻土温度轻微的改变都会引起剪切模量的大幅变化;冻土的初始蠕变应变可以采用幂函数形式来计算,其强化系数k=1.87,应变系数与负温绝对值之间呈线性正相关关系。稳定蠕变阶段的应变速率可以用Glen蠕变模型来表示,其中参数n随体积含冰量的增加而线性增加;若体积含冰量外推至100%（纯冰）时,n=3.85,这与前人通过室内试验得出的结果有些差异。温度较低时,蠕变速率较小;当温度趋近于0 ℃时,蠕变速率迅速增大。     

万福煤矿不同含水率砂岩蠕变力学特性试验研究

为研究深部巷道围岩在地下水作用下的长期蠕变力学特性,采用自主研制的深部软岩五联流变试验系统,开展不同含水率(0%、0.8%、1.6%、2.4%、3.3%)下砂岩吸水软化单轴压缩试验及单轴蠕变试验。通过试验结果研究表明：砂岩的单轴抗压强度、弹性模量和蠕变破坏应力与含水率呈指数下降关系,蠕变破坏应力与单轴抗压强度的比值在0.76～0.84之间;砂岩衰减蠕变阶段时间随着含水率的增加而减少,随应力水平的增加而增加。径向应变比轴向应变先进入稳态蠕变阶段,破坏应力下径向应变的加速蠕变阶段开始时间要先于轴向应变;基于稳态蠕变速率曲线确定了砂岩的长期强度,径向稳态蠕变速率确定的值略小于轴向,长期强度与含水率之间满足负指数关系;将蠕变试验中径向应变与轴向应变之比定义为μ_c,提出了基于μ_c值的岩石长期强度确定方法且μ_c值与含水率无关,对于本次砂岩样品可以认为μ_c值大于0.3时样品会在一定时间内发生加速蠕变破坏;随着含水率的增加,样品破坏形态由单斜面剪切破坏逐渐演变至X状共轭斜面剪切破坏。研究结果为地下水作用下巷道长期稳...     

Deformation Behavior of Chalk Studied Close to In Situ Reservoir Conditions

A laboratory test program, which simulated reservoir conditions of pressure and temperature, was conducted on outcrop and reservoir chalk samples of various porosities. All the samples experienced a stress path following uniaxial strain condition K ₀ that led to compaction failure, i.e. pore collapse. The experiments were loaded by depletion of pore pressure conducted under load controlled conditions. This depletion phase was followed by a creep period, where time-dependent deformation was monitored. The intention of creating such reservoir condition in these laboratory experiments was to gain knowledge of the nature of chalk compaction. Chalk is an important reservoir rock for the oil and gas industry with unique storage capability with porosities up toward 50%. However, this rock is also very weak which has resulted in significant reservoir compaction and in turn severe seabed subsidence and casing failure. Mapping of the mechanical behavior of chalk in terms of deformation is thus decisive for a proper understanding of these reservoirs. The results of this study show that chalk is indeed a rate-dependent material under laboratory loading conditions as time effects were revealed as the loading rate was varied. However, the results raise uncertainty about the importance of rate dependency for chalk under completely drained conditions. Further, such high-porosity chalk suffers for substantial plastic strains and obvious strain hardening. Indeed, a relation between deformation/porosity and hardening is proposed by the introduction of real-time modulus values. Time-dependent deformation, also called creep was influenced by the depletion phase, as consolidation or transient creep influenced the deformation response for as much as 175 h after a change in load. This indicates that transient creep is dependent on the stress history. However, observations suggest the existence of a universal mechanism for steady state creep, governed by neither the initial porosity nor the stress history or chalk type, which thus seems to be an independent strain contributor. Finally, time dependence is found on the K ₀ development for chalk tested at typically laboratory rates, which has been discussed as a reflection of the nature of the grain re-arrangement during failure and plastic deformation. Ultimately, such time dependence of the K ₀ may contribute to the understanding of stress path data deduced from field data.     

多晶冰蠕变机制的研究进展

与绝大多数材料遵循基于位错攀移的指数蠕变机制不同,多晶冰遵循基于位错滑移的指数蠕变机制,这种机制引起了冰川学家极大的兴趣.以前人的研究为基础,综述了冰中质子无序及质子点缺陷的形成过程,质子点缺陷对位错滑移影响的微观机理,以及多晶冰蠕变的微观机制.研究表明:冰晶体中的氢原子(质子)无序使得位错在滑移过程中形成质子点缺陷(D、L、H₃O⁺和OH^-缺陷),从而降低了位错的滑移速率.质子点缺陷的形成需要氢原子(质子)跃迁,其激活能大于水分子自扩散所需的激活能.同时,多晶冰的蠕变激活能与质子跃迁的激活能相当,而大于水分子自扩散所需的激活能,因此多晶冰的蠕变控制机制是位错滑移,而不是位错攀移.     

膨胀土卸荷蠕变特性及其非线性蠕变模型

鉴于膨胀土滑坡往往表现为长期性、渐进性等与时间相关的特性,利用GDS应力路径三轴仪对膨胀土进行了三轴卸荷蠕变试验。试验结果表明：当偏应力较小时,膨胀土的蠕变曲线仅出现瞬时变形和衰减蠕变;当偏应力达到一定值时,其蠕变曲线也呈现衰减蠕变、稳态蠕变和加速蠕变3个阶段,但其加速蠕变阶段的特征与一般岩土体不同,其蠕变速度近乎常数。膨胀土的应力?应变等时曲线显示,膨胀土卸荷蠕变具有非线性特征,且其非线性程度与蠕变时间和应力水平相关,蠕变时间越长、应力水平越高,其非线性程度越高。基于非线性流变力学理论,提出了一种非线性四元件蠕变模型,将标准线性体与一个非线性黏壶串联,该模型可描述等围压三轴压缩应力状态下膨胀土轴向应变随时间的演变规律。根据膨胀土卸荷蠕变试验结果,采用曲线拟合法对三维非线性模型的参数进行反演识别。拟合曲线和试验曲线对比显示,两者吻合良好,说明该模型可以很好地描述膨胀土的蠕变特性。此外,基于该蠕变模型获取了膨胀土的临界破坏应力,其与常规剪切破坏应力的比值随着固结压力的减小而减小,表明越接近坡面的土层越容易发生蠕变破坏。     

基于断裂及高温损伤的岩石蠕变模型研究

为了反映热-力耦合作用下岩石蠕变变形的全过程,依据断裂力学原理提出了岩石裂纹扩展的临界损伤应力和一个新的可描述岩石在稳态蠕变阶段与临界损伤应力相关的非线性黏性分量,在传统西原模型和Burgers模型的基础上,将指数形式的损伤变量、临界损伤应力以及与其有关的非线性黏性分量引入到流变微分方程,通过叠加原理推导了考虑温度效应的单轴和三轴压缩条件下岩石的流变本构关系,建立了岩石的热-力耦合损伤蠕变本构模型。利用不同温度、不同应力条件下花岗岩的三轴蠕变试验曲线和本文蠕变模型的计算曲线进行比较,结果表明本文蠕变模型能够较好地模拟岩石在初始瞬态、稳态和加速蠕变阶段全过程的变形规律,验证了所建模型的有效性和合理性。该模型为分析高温、高应力环境下岩石工程的长期变形和稳定情况提供了理论依据。     

Laboratory determination of strength properties of frozen salt marine clay

In connection with the building of a railway tunnel through the City of Oslo, temporary improvement of the soft clay by freezing was considered in order to reinforce the tunnel ceiling in an area where the rock cover was insufficient. The same solution was also considered for another part of the tunnel where the problem was to prevent bottom heave failure in a deep excavation.

As this marine clay deposit has a very high salt content (about 25 g/l pore liquid), it was reasonable to believe that the strength of the frozen clay would be relatively low compared to previously published data concerning fresh clay soils. To investigate the stress—strain properties of frozen Oslo clay, a fairly comprehensive research programme was carried out; including two different temperature levels and using different testing techniques. The clay was tested in compression, tension, bending and shear, respectively. The tests were run very slowly, time to failure was normally 70–100 days.

This paper presents the main test results, which indicate that there exists a fairly well defined critical shear stress level for a given freezing temperature. As long as the stresses do not exceed this stress level, the creep deformations are small and the creep rates are constant or decreasing with time. On the other hand, higher shear stresses lead to very extensive deformations or failure.