Creep stress analysis of frozen soils under multiaxial states of stress

Creep behaviour of the so called Emscher-Marl is investigated in uniaxial compression tests under a temperature of −10°C. Similar to the results of creep tests published elsewhere, it is shown that the secondary creep controls the deformation. In cases of stationary creep the material behaves like a nonlinear elasto-viscous Maxwell fluid. Assuming no hydrostatic pressure effect, the power law applied to the uniaxial case can be transformed to multiaxial states of stress. For structures of frozen soil with a long-term deformation process Norton's power relationship between stress and rate of steady flow leads to reasonable results. As a numerical solution a finite-element computer program in connection with incremental procedure is developed for treatment of creep problems in groundfreezing technology. The calculation of a simply supported beam demonstrates the important influence of the high nonlinearity in stress and strain for frozen soils.     

Effect of specimen shape on creep response of a frozen sand

The stress—strain and strength behavior of frozen soils is most often studied in a conventional uniaxial or triaxial compression test. However, available experimental evidence shows that the results obtained in such a test may be considerably affected by the test conditions, i.e., by the specimen shape and the method of axial load transfer to the specimen ends, both of which affect the uniformity of stress and strain distribution in the sample. The methods proposed for solving the problem of load transfer to the specimen ends differ considerably for brittle and ductile materials, respectively. Since a frozen soil may behave either as a brittle or as a ductile material, depending on strain rate, temperature and confining pressure, it is considered that the load transfer method selected should correspond to the expected sample behavior. In this paper, the results of a series of compression and creep tests are presented, in which the effects of specimen shape and end conditions on the behavior of a frozen sand were systematically investigated and compared. a frozen sand were systematically investigated and compared.     

Design of tunnel support systems using ground freezing

One of the more promising techniques in soft ground tunneling through urbanized areas is the use of artificially frozen ground for temporary tunnel support. This paper describes the general design considerations involved in the ground freezing method. Various factors are discussed which influence the selection of the freezing temperature, the thickness of the frozen zones and the spacing of the freeze pipes. The time required to achieve freezing is discussed in addition to the amount and rate of frost heave caused by the freezing.

To illustrate the applicability of the freezing method, various considerations in the design of an 8-ft. diameter tunnel in upstate New York, a 75-ft. diameter tunnel in Georgia, and a 12 1/2-ft. diameter tunnel in Washington, D.C. are discussed. All three of the tunnels were to pass immediately beneath mainline railroad tracks. A laboratory testing program was implemented to determine the effects of the repetitive train loads on the zone of frozen soil around the tunnel perimeter. Stress-controlled repeated load triaxial tests were performed on both undisturbed and remolded samples frozen from temperatures of −7°C for the New York tunnel to −10°C for the Atlanta and Washington tunnels. Static testing consisted of both quick triaxial tests and creep tests on frozen samples of the various soil types.

It was found that there was little difference between the cumulative strain response from repeated load tests and static tests for the low frequencies investigated (one-quarter to one-half cycles per minute). Hyperbolic stress—strain functions were developed to simulate the stress—strain relationship for various cumulative loading times.

The stresses and strains in the frozen soil tunnel configuration were computed by the finite element method, using both linear and hyperbolic stress—strain functions. Tangent modulus values were varied to reflect the decreasing modulus with increasing loading time. The analyses indicated that zones of frozen soil of approximately 3 ft. thick were required for both the New York and Washington tunnels. However, high tensile stresses were calculated for the Atlanta tunnel, precluding the use of the freezing method.     

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.     

不同应力状态下软黏土蠕变特性试验研究

土的蠕变性质受到许多因素的影响,在不同的应力条件下土会呈现出不同的性状。天津滨海吹填软土同样具有一般软土的蠕变特性,以其为研究对象,利用土体三轴流变试验机进行了三轴不固结不排水蠕变试验和单轴压缩蠕变试验,并选取了10个时间节点取样进行微型十字板剪切试验,研究抗剪强度变化规律。试验结果表明,单轴压缩蠕变和三轴蠕变历时曲线都具有非线性的特征,随应力水平的提高,单轴压缩蠕变的应力-应变等时曲线向应力轴靠拢,三轴蠕变的应力-应变等时曲线向应变轴靠拢;线性蠕变变形的情况下黏滞系数主要与时间有关,非线性的情况下与时间和应力水平都相关;在三轴蠕变状态下压缩模量随时间增长而减小,长期强度随时间先降低再趋于稳定;单轴压缩时压缩模量随时间增长而提高,长期强度随时间先增长后稳定。最后经过推导得出了不同应力状态下的长期强度随时间、偏应力的函数关系式。     

高温冻结粉土力学特性试验研究

为了探讨高温冻结粉土的力学特性,进行了一系列常规三轴压缩和三轴压缩加卸载试验。结果表明：（1）随着围压的增大,应力应变曲线先表现为应变软化,后应变硬化,最后又应变软化。（2）体积应变曲线均表现为先体缩后体胀,且围压越大,体胀越弱。（3）通过引入改进的双曲线模型,能较好地模拟应变软化和硬化特性。在q-p平面内建立了一个新的强度准则,并利用修正Mohr-Coulomb屈服准则,探讨了黏聚力c和内摩擦角φ的变化规律。根据试验结果,确定了不同围压及不同加卸载循环次数下高温冻结粉土的回弹模量及其损伤变化规律。最后,建立了塑性体积变形和塑性剪切变形之间的关系式和表达式,进一步探讨了高温冻结粉土的剪胀特性。     

基于硬化-损伤机制的贫煤蠕变本构模型研究

岩石蠕变是其内部硬化效应和损伤效应共同作用演化的结果。通过分级加载下贫煤单轴压缩蠕变试验,基于试验中瞬时弹性模量增大和黏滞系数减小的现象,分析贫煤蠕变中的硬化-损伤机制。结果表明,较低应力下煤样的变形仅为瞬时应变,煤样仅有硬化效应;当应力达到蠕变起始应力阈值时煤样逐渐出现损伤,次生裂纹出现并扩展,硬化效应与损伤效应共同作用下煤样表现出衰减蠕变和稳定蠕变特征。长期蠕变过程中煤样强度经历先硬化再弱化的过程,最终因累计损伤过大而失稳破坏,依据各级应力下煤样的变形特征,构建了符合其变形规律的黏弹塑性组合模型,并引入了硬化函数和损伤函数,推导得到其蠕变本构方程。采用该模型描述分级加载下贫煤单轴压缩蠕变变形曲线,试验曲线和模型曲线吻合良好。该模型元件简单,物理意义明确,能很好地反映贫煤单轴压缩下的硬化–损伤蠕变特性。     

Particle orientation and its influence on the mechanical behaviour of isotropically consolidated reconstituted clay

The behaviour of naturally occurring geological materials such as clay and sand depends on many factors. For example, stresses, strains, previous stress history, mineralogy and the depositional environment all contribute in some degree to a characteristic that all natural soils share, namely “structure”. The structure of clay, or more generally, the microstructure of microscopically sized clay mineral particles, is just as important as the many other parameters that are used to quantify the performance of clays. This paper examines the microstructure that results from the particle arrangement brought about during reconstitution in the laboratory and considers its relevance to the resulting stress–strain behaviour.

Samples of reconstituted kaolin clay were produced using two different procedures. In the first series of tests, kaolin slurry was simply isotropically compressed in one increment. In the second series, the slurry was first isotropically compressed to a low pressure and then completely remoulded. This was followed by isotropic compression to the same pressure as the other series. Specimens were taken from the two series of samples, reconsolidated at various isotropic pressures, and sheared under undrained conditions.

Scanning Electron Microscope (SEM) images indicated that the monotonically compressed samples (Series 1) exhibited an anisotropic microstructure that was distinct from the remoulded (Series 2) samples. Significant differences were also found in the consolidation and stress–strain characteristics of the samples produced in the two series.     

冻土旁压试验与单轴试验对比

为了认识冻土旁压试验结果与常规试验结果之间的关系,在室内分别开展了冻结重塑黏土的旁压试验和单轴压缩试验,并对试验结果进行了对比分析。结果表明,在各级荷载作用下旁压曲线一般都呈现应变速率衰减的趋势,而单轴曲线在冻土破坏时会出现渐进流动阶段。旁压试验的应力-应变曲线呈现应变硬化型,而且出现初始拟弹性阶段;而单轴试验的应力-应变曲线则属于应变软化型,并在轴向应变大约为10%时达到剪应力峰值。温度相同时,旁压试验的剪切强度以及初始弹性模量都要大于单轴试验,且温度越低差值越大。     

球模仪测试冻土松弛模量的非线性Kelvin解答及其试验研究

冻土的松弛模量是进行蠕变力学分析的关键参数。但在理论方面, 尚未基于球模仪试验建立较完善的松弛模量表征公式。基于半空间黏弹性理论和分数阶微积分理论, 获得了松弛模量公式的非线性Kelvin模型解答并进行了材料参数影响分析。在给出恒载条件下, 可以用单轴蠕变试验的分析结果评估基于球形压痕试验的冻土松弛模量来预测准确性。随后分别以冻结细砂的球模仪试验曲线、 冻结砂和冻结砂质黏土的单轴蠕变曲线为例, 预测了各条试验曲线和冻土的松弛模量, 并通过其他试验实测数据进行了检验。结果表明： 该松弛模量表征公式的材料参数最少, 随各材料参数变化具有较佳单调性; 基于球形模板试验, 非线性Kelvin解答能对冻结细砂的松弛模量和位移曲线做出合理可靠的预测。非线性Kelvin模型能很好地吻合单轴蠕变试验曲线, 基于单轴蠕变试验的冻结砂松弛模量随时间变化规律和数值范围与球模仪试验分析结果较一致, 冻结砂质黏土的松弛模量随应力和温度的变化规律再现了已有研究结论。     

Uniaxial creep tests on a morainic material from switzerland

To determine deformation and strength behaviour of a frozen morainic material uniaxial creep tests with constant load on remoulded samples have been carried out.

From the test results an analytical method describing the mechanical behaviour could be found.

In the following the results of this investigation are presented and an analytical solution is derived. In addition, other experimentally determined curves are compared with the calculated ones. For the material tested the proposed method leads to quite satisfactory results and is expected to do so also for other materials.     

A fractional order creep constitutive model of warm frozen silt

A series of triaxial creep tests were conducted on warm frozen silts extracted from Qinghai–Tibet Plateau at temperature of ?1.5 °C under confining pressures of 0.5, 1.0, and 2.0 MPa, respectively. The applied test stress levels were 30, 50, 60, and 70% of triaxial shear strength, respectively. The test results indicate that the creep strain increases with the increase in applied stress level and there is a stress threshold, based on which the test results can be classified into two types of creep strain curves. The creep strain curve only includes primary and secondary creep stages when the stress level is less than the threshold value. When the stress level exceeds the threshold value, the creep strain velocity gradually increases and the specimen quickly fails in tertiary creep stage. Based on the creep test results, a fractional order rheological element model is established for warm frozen silt, which is also generalized from uniaxial stress state to the three-dimensional stress state. From the analysis on the features of the stress threshold, a creep strength criterion is also proposed simultaneously. Comparing the calculated results of the warm frozen silt with the tested ones, it is found that the predicted results of the proposed model are in good agreement with the test results. In the proposed fractional order model, the relationship between the damage factor and time is established to describe the damage degree of the specimen. Compared with the existing creep constitutive model of frozen soil, the proposed fractional order model has advantages of fewer model parameters, higher simulation precision and wider applicability in analyzing the mechanical properties of warm frozen silt.     

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...     

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.     

The mechanical behaviour of synthetic, poorly consolidated granular rock under uniaxial compression

In order to isolate the effect of grain size and cementation on the mechanical behaviour of poorly consolidated granular rock, we prepared synthetic rock samples in which these two parameters were varied independently. Various proportions of sand, Portland cement and water were mixed and cast in a mold. The mixture was left pressure-free during curing, thus ensuring that the final material was poorly consolidated. We used two natural well-sorted sands with grain sizes of 0.22 and 0.8 mm. The samples were mechanically tested in a uniaxial press. Static Young's modulus was measured during the tests by performing small stress excursions at discrete intervals along the stress–strain curves. All the samples exhibited nonlinear elasticity, i.e., Young's modulus increased with stress. As expected, we found that the uniaxial compressive strength increased with increasing cement content. Furthermore, we observed a transition from grain size sensitivity of strength at cement content less than 20–30% to grain size independence above this value. The measured values of Young's modulus are well explained by models based on rigid inclusions embedded in a soft matrix, at high cement content, and on cemented grain-to-grain contacts, at low cement content. Both models predict grain size independence in well-sorted cemented sands. The observed grain size sensitivity at low cement content is probably due to microstructural differences between fine- and coarse-grained materials caused by small differences in grain sorting quality.     

冻结粘性土的变形分析

通过三轴压缩试验条件下冻结粘性土变形分析,发现冻结粘性土的体应变明显不为零,随荷载的变化而变化。加荷初期试样体积收缩。但随着荷载的进一步增大而变为膨胀;轴向应变则随偏应力的增大而磁大,表现出明显的粘塑性特征。应力－应变曲线上的准弹性段、屈服处和粘塑性段刚好分别对应体变曲线上的体缩段、体缩量大处和膨胀段。根据冻结粘性土变形曲线。给出了考虑体变形的应力－应变关系。     

Stress–strain behaviour of a loosely compacted volcanic-derived soil and its significance to rainfall-induced fill slope failures

The stress–strain behaviour of a soil is crucial to the recognition of the mechanism of slope failure. Triaxial tests, composed of isotropically consolidated drained (ICD) and undrained compression (ICU) tests and anisotropically consolidated constant deviatoric stress path (CQD) tests, were carried out with the aim of investigating the stress–strain behaviour of loosely compacted volcanic-derived soils. The fact that the critical states are the same for ICU, ICD and CQD tests may show that the critical state is independent of the above three stress paths. The critical state line, as defined in critical state soil mechanics, is obtained from the e–log p′ and q–p′ plots based on the results of the above tests. The initial state of the consolidated specimens at initiation of failure may be classified as dilative or contractive in the light of the locations of the soil state relative to the critical state line. For contractional soils, the increased pore water pressure generated by rainfall infiltration leads to a contractive failure in a drained manner, giving rise to high excess pore water pressure. The excess pore water pressure caused by contraction cannot be dissipated instantly, resulting in a decrease in the shear resistance of the soil. The failure process is rapid. The failed soil mass is prone to flow after failure under the action of gravity due to its high moisture content and inflow of surface runoff and rainwater. For dilational soils, the increased pore water pressure resulting from infiltration leads to dilation, which reduces pore water pressure and thus increases the shear resistance of the soil. However, continued rainfall infiltration may be able to equilibrate the reduction in pore water pressure caused by dilation and, therefore, the dilation or displacement can continue. In Hong Kong, volcanic-derived soil is characterized by high permeability. Both the high permeability of volcanic-derived soil and a shallow failure surface make it possible for the reduction in pore pressure to equilibrate relatively quickly. Therefore, the failure is also rapid, at least for poorly compacted fill slopes.     

An improved hardening-damage creep model of lean coal: a theoretical and experimental study

Creep is an important rheological behavior of coal, impacting pillar stability, gateway support performance, and construction costs in underground mining. It is of great significance to study the creep mechanism of coal to prevent creep failure. In this paper, uniaxial compressive creep tests of raw lean coal under multiple stages are conducted, the increasing instantaneous elastic modulus and decreasing viscosity coefficient are calculated to analyze the hardening-damage creep mechanism of coal, and some new conclusions are as follows. When the stress is low, the hardening effect exists, and coal strain shows an instantaneous response. At high stress, both the damage effect and hardening effect work and coal strain shows time-dependent transient creep and steady creep. During multistage creep, coal is first hardened, then weakened, and finally fails due to well-developed cracks. Based on the strain evolution laws of lean coal at different stress levels, an elastic viscous-plastic model is established, and the hardening function and damage function are introduced into the model to obtain an improved hardening-damage creep model. This model is used to fit the creep data, and the model curves match the testing data very well, showing much better accuracy than other models. This model is simple with clear physical meanings of the elements and can reflect the nonlinear hardening-damage creep of raw lean coal under uniaxial compression very well.