黏土蠕变量的试验研究

对黏土的原状和重塑土样在三轴排水剪切试验条件下的蠕变性进行研究,试验方法包括改变加载应变率的单向加载试验、单向加载试验过程中的蠕变试验、卸载和重复加载过程的蠕变试验等。试验结果表明:1)黏土蠕变量大小与黏土试样的状态有关;2)黏土蠕变量大小与蠕变之前的应变率有关,蠕变前应变率越大,蠕变量就越大;3)黏土蠕变量大小与蠕变时所处的应力水平有关,所处的应力水平越高,蠕变量就越大;4)在重复加载过程中,随着应力水平的提高,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。     

单轴试验条件下粘土的粘塑性与模拟计算

利用饱和、湿润、风干以及烘干的藤森粘土的单轴排水固结试验进行了粘土粘塑性研究。试验过程中采用计算机应变控制的控制式三轴仪来实现加载速率，可在不同加载阶段实现不同的恒应变率加载试验。试验结果表明：各种不同饱和度藤森粘土的粘塑性都表现为等时特性；经过一定时间的蠕变后，粘土弹性模量会有较大的提高，且藤森粘土所表现出的粘塑性与粘土含水量的大小无关。研究结果表明，粘土的粘塑性应力-应变特性可以采用非线性三要素模型进行较好的描述。     

不同含水率粘土在不同试验条件下的应力-应变特性

采用单轴固结试验和三轴剪切试验对饱和、湿润、风干以及烘干的藤森粘土进行了应力-应变特性研究。在试验过程中,进行了不同轴向应力水平下的恒应变率加载试验、蠕变试验以及卸载与重复加载试验。局部变形测量传感器(LDT)被用于三轴剪切试验中的轴向变形测量以提高小应变测量的精度。试验结果表明,不同含水率与饱和度的藤森粘土在单轴固结试验与三轴剪切试验条件下都表现出明显的粘塑性。它们的加载速率效应、蠕变效应都相似;而且在蠕变结束后的一小段邻域内藤森粘土的强度有明显的提高。笔者提出用参数β来描述藤森粘土的粘性。对试验结果的计算分析表明,在单轴和三轴试验条件下不同含水率与饱和度的藤森粘土的β值介于0.034到0.064之间;而且,含水率高的藤森粘土的β值大于含水率低的藤森粘土的β值,单轴试验条件下的β值大于三轴试验条件下的β值。     

Modelling and Simulation of Rate-Dependent Stress-Strain Behaviour of Granular Materials in Shear

A constitutive modelling of the elasto-viscoplastic stress-strain behaviour of geomaterials in shear that has been developed within a non-linear three-component model framework is validated by simulating a comprehensive series of drained triaxial compression (TC) and direct shear (DS) tests on a wide variety of granular materials. Illustrative simulations of rate-dependent stress-strain behaviour of geomaterial under typical laboratory test conditions were performed to analyse the structure of the model. The versatility of the proposed model and its applicability to a wide variety of shear loading histories is examined and demonstrated by these simulations. The following results are shown. Commonly with different basic viscosity types, Isotach, TESRA and P&N, the viscous stress component has a positive component that increases with an increase in the irreversible strain rate, which makes feasible stable and realistic simulations of rate-dependent stress-strain behaviour, including creep deformation, based on the proposed model. With different unbound granular material types having similar relative densities, the creep strain in TC tests and creep shear displacement in DS tests that develop by sustained loading at a given shear stress level for a given period tends to decrease with an increase in the particle roundness. This trend of behaviours is explained by a decrease in the viscosity type parameter, θ, associated with an increase in the particle roundness based on the simulations of these tests.     

Residual Deformation of Geosynthetic-Reinforced Sand in Plane Strain Compression Affected by Viscous Properties of Geosynthetic Reinforcement

A series of plane strain compression (PSC) tests were performed on large sand specimens unreinforced or reinforced with prototype geosynthetic reinforcements, either of two geogrid types and one geocomposite type. Local tensile strains in the reinforcement were measured by using two types of strain gauges. Sustained loading (SL) under fixed boundary stress conditions and cyclic loading (CL) tests were performed during otherwise monotonic loading at a constant strain rate to evaluate the development of creep deformation by SL and residual deformation by CL of geosynthetic-reinforced sand and also residual strains in the reinforcement by these loading histories. It is shown that the creep deformation of geosynthetic-reinforced sand develops due to the viscous properties of both sand and geosynthetic reinforcement, while the residual deformation of geosynthetic-reinforced sand during CL (defined at the peak stress state during CL) consists of two components: i) the one by the viscous properties of sand and reinforcement; and ii) the other by rate-independent cyclic loading effects with sand. The development of residual deformation of geosynthetic-reinforced sand by SL and CL histories had no negative effects on the subsequent stress-strain behaviour and the compressive strength was maintained as the original value or even became larger by such SL and CL histories. The local tensile strains in the geosynthetic reinforcement arranged in the sand specimen subjected to SL decreased noticeably with time, due mainly to lateral compressive creep strains in sand during SL of geosynthetic-reinforced sand. This result indicates that, with geosynthetic-reinforced soil structures designed to have a sufficiently high safety factor under static loading conditions because of seismic design, it is overly conservative to assume that the tensile load in the geosynthetic reinforcement is maintained constant for long life time. Moreover, during CL of geosynthetic-reinforced sand, the residual tensile strains in the geosynthetic reinforcement did not increase like global strains in the geosynthetic-reinforced sand that increased significantly during CL. These different trends of behaviour were also due to the creep compressive strains in the lateral direction of sand that developed during CL of geosynthetic-reinforced sand.     

Stress–strain relations of cement-mixed gravelly soil from multiple-step triaxial compression test results

The peak compressive strengths at different confining pressures of cement-mixed gravel (CMG) that are very similar to those obtained by single-step loading (SL) drained triaxial compression (TC) tests using multiple specimens can be obtained by a multiple-step loading (ML) test using a single specimen. However, only the unload/reload stress–strain relations at different confining pressures (except for the primary loading pressure at the first step) can be obtained from an ML test, and the reloading relations become softer with the increases in negative irreversible axial strain increments that have taken place during the respective immediately preceding unloading regimes. This phenomenon is formulated by a unique empirical equation for the CMG tests. Undamaged reloading stress–strain curves (URCs) were inferred by removing the damage effects from measured reloading curves (MRCs) in the ML TC test based on this correlation. A method was developed within the framework of the proportional rule to infer primary loading curves (PLCs) at different confining pressures from the URCs and the PLC at the first step obtained from a given ML TC test. A procedure was formulated to modify the PLCs obtained by this procedure based on the PLCs measured at stresses exceeding the yield stress for large-scale yielding during reloading at different confining pressures in the ML test. This method was validated by comparing the PLCs obtained from the results of a pair of ML tests, increasing and decreasing the confining pressure, with those obtained from a set of SL TC tests at different confining pressures.     

Rate Effects on the Stress-Strain Behaviour of Eps Geofoam

The rate-dependency of the stress-strain behavior of EPS (Expanded Polystyrene) geofoam with densities of 19.3 and 28.0 kg/m³ was investigated by performing unconventional unconfined compression tests. A set of monotonic loading (ML) tests were performed at different constant values of vertical (axial) strain rate, ε_v. The ε_v value was stepwise changed many times and several sustained loading (SL) tests were performed during otherwise ML at a constant ε_v in other tests. A number of SL tests were performed during global unload and reload cycles to infer the stress-strain relation when ε_v=0. The elastic properties were evaluated by applying minute unload/reload cycles during otherwise ML. The rate-dependent stress-strain behaviour observed in these tests was described by an elasto-viscoplastic model (i.e., a non-linear three-component model), for which the vertical (axial) stress, σ_v, consists of inviscid and viscous components, σ_v^f and σ_v^v, while ε_v consists of elastic and irreversible components, ε_v^e and ε_v^ir. It is shown that the viscous property of EPS geofoam is of Isotach type in that, under the loading conditions where ε_v^ir is always positive, the current σ_v^v value is a unique function of instantaneous ε_v^ir and ε_v^ir, therefore the strength increases with ε_v. This viscous property was quantified based on the test results and incorporated into the model. The rate-dependent stress-strain behaviour, including the creep behaviour, observed in the experiment is simulated very well by the proposed model. In particular, the fact that the creep strain becomes significant when the sustained σ_v value becomes larger than the inviscid yield vertical stress is well simulated.     

侧向卸荷条件下软土典型力学特性试验研究

 针对侧向卸荷条件下软土变形、强度、蠕变等典型力学特性,选取珠三角洲相沉积的典型灰黑色淤泥质软土进行室内三轴试验研究。试验结果表明：侧向卸荷条件下的应力–应变规律呈应变硬化现象,可采用平均应力进行归一;土样在侧向卸荷条件下进行不排水剪切时会产生负的孔隙水压力,使得有效应力强度反而低于总应力强度,与轴向加荷条件下的强度规律不同;侧向卸荷条件下的初始卸荷模量小于轴向加荷条件下的初始卸荷模量,与平均固结压力呈线性关系;任一时间的卸荷变形模量仍可表示为切线模量,并可通过试验成果推导得出;侧向卸荷条件下软土蠕变变形比轴向加载显著,即使在偏应力不大时仍占总变形的较大比例;应变率随时间的下降呈较好的双对数线性关系,可以建立考虑剪应力水平对初始应变率影响的线性经验公式来估算某级荷载下应变率随时间的发展。     

Effects of Viscous Property and Wetting on 1-D Compression of Clay and Model Simulation

A series of one-dimensional (1D) compression tests on compacted kaolin powder were performed to evaluate the combined effects of the viscous property and wetting on the elasto-viscoplastic deformation of soil. In the tests, both creep deformation and collapse deformation due to wetting were allowed to take place at various fixed stress states during otherwise monotonic loading at a fixed strain rate. Combined effects of the viscous property and wetting on the stress-strain behaviour observed during 1-D compression were described by incorporating the wetting effects into a non-linear three-component elasto-viscoplastic model (a 3C model). Based on the experimental results, the effects of wetting on the inviscid stress and the irreversible strain relation of the plastic component of the 3C model and the property of the viscous component, having an Isotach property, are formulated as a function of the degree of saturation. Complicated rate- and time-dependent stress-strain behaviour observed during saturation at a fixed stress state and subsequent monotonic loading at a constant strain rate were successfully simulated.     

加载速率变化条件下砂土的黏塑特性及本构模型

 分析并研究饱和的、风干的砂土在平面应变压缩试验条件下的弹黏塑性特征,尤其是黏性特性。加载速率效应、蠕变以及应力松弛都是砂土材料本身黏性的反映,而与超孔隙水压力的消散无关。试验不仅实现了应变率逐步变化加载过程,同时也实现了蠕变加载和应力松弛过程。试验结果表明,在加载应变率发生突变时,对砂土应力–应变关系有明显的影响,呈现出刚性很高、近似于弹性的特性,而后随着应变的进一步增加,在明显屈服之后黏性应力逐渐衰减至基本惟一的应力–应变曲线。类似现象同样也发生在蠕变加载或应力松弛后以一恒定应变率突然重新加载的情况。基于非线性三要素模型框架,针对所观察到的砂土黏性特性,提出一种弹黏塑性本构模型。该模型可以描述砂土在任意加载历史下的黏性效应,包括加载应变率发生任意变化时的应力–应变响应、蠕变以及应力松弛。最后,利用该模型对上述砂土平面应变压缩试验结果进行模拟,所提案的三要素弹黏塑性本构模型能够很好地模拟砂土的黏性特性。     

Time-Dependent Shear Deformation Characteristics of Sand and their Constitutive Modelling

Time-dependent (viscous) characteristics of the shear stress and shear strain behaviour of water-saturated or air- dried specimens of clean sands (Hostun and Toyoura sands) observed in a series of drained plane strain compression tests are presented. The overall shear stress-shear strain relationships were very similar in a set of monotonic loading tests performed at constant axial strain rates that were different by a factor of up to 500. Despite the above, significant viscous effects on the stress-strain behaviour were observed, a) when the strain rate was changed stepwise or at a constant rate, b) at creep and stress relaxation stages, and c) immediately after loading was restarted at a constant strain rate following a creep stage. One type of constitutive model was developed in the framework of the general three-component model to simulate these behaviours. According to this model, the strain is decomposed into elastic and irreversible components, while the stress is decomposed into time-independent and dependent components. This model was developed to simulate such experimental results in that the time-dependent stress component changes not only when the irreversible shear strain rate changes but also when loading continues at a certain constant irreversible shear strain rate, while these viscous effects decay with an increase in the irreversible shear strain. The rationales for the structure of the proposed model obtained from the experiment are presented. It is shown that this model can simulate well the experimental observations described above, although they were obtained under certain limited test conditions.     

Circumferential strain behavior during creep tests of brittle rocks

Creep tests were carried out on Inada granite under confining pressure and on dry and wet specimens of Kamisunagawa sandstone. Distinctive points Q_A (the point where the axial strain rate reaches a minimum), Q_C (the point where the magnitude of circumferential strain rate reaches a minimum) and R_C (the point where the magnitude of circumferential strain acceleration begins to increase) were defined. Dependency of the strain values at the distinctive points on creep stress, confining pressure and water presence was examined. Circumferential strain at Q_C or R_C was not affected by creep stress for both rocks. Peak load points were observed in reloading stress–strain curves when reloading was carried out before Q_C for Kamisunagawa sandstone. Circumferential strain values at peak load during reloading coincided with critical extensile strain, which is defined as circumferential strain value at peak load during constant strain rate tests and is insensitive to confining pressure, water presence, and anisotropy, although slightly affected by strain rate. Load increased slightly or did not increase when reloading was carried out after R_C for both rocks. R_C in creep tests scattered around unloading stress–strain curves from peak load during constant strain rate tests for Kamisunagawa sandstone. These observations suggest that Q_C and R_C in creep tests are closely related to the peak load points during constant strain rate tests. Circumferential strain would be used as a condition-insensitive damage indicator of rock in creep tests as well as in constant strain rate tests.     

Unloading behavior of clays measured by CRS test

The unloading behavior of clays was studied by the Constant Rate of Strain (CRS) test, for three clays: two of them are reconstituted and the other was intact. In the conventional CRS test where the stress monotonically increases, the distribution of the pore water pressure in a specimen is assumed to be parabolic, the effective stress is calculated and then the compression behavior is evaluated. However, this assumption cannot be directly applied the unloading condition. In this study, the pore pressure distribution under unloading was simulated by a cubic polynomial under the assumption that hydraulic conductivity does not change in the unloading process. A unique relation in the e–log σ′_v relation was found, irrespective of both the magnitude of stress or strain and the compression index, C_c, at the unloading test, when the consolidation pressure is normalized by σ′_vmax, which is the consolidation pressure before the unloading test. In addition, the creep strain, which is gained by constant loading before the unloading test, was shown to have a great effect on the unloading behavior: that is, the soil behaves stiffly when subjected to a constant load for a prolonged period of time. A strain rate dependency in the unloading process was also observed particularly for heavily unloaded specimens. The unloading behavior was also investigated by the conventional constant load test. The test results show reasonable agreement with those obtained from the CRS test.     

粗粒土回弹特性试验研究

对2种粗粒土4种级配料进行了卸载–再加载的三轴固结排水剪试验,比较分析其强度变形特性与单调加载三轴试验条件下特性的差异。重点研究不同试验粗粒料的回弹模量取值规律,分析了回弹模量与初始模量、围压和应力水平等之间的关系。结果表明,围压对回弹模量影响较大而应力水平的影响相对较小;粗粒土的回弹模量约为初始模量的2.5～5.0倍,比目前对黏土等认识的1.5～3.0倍明显大。试验结果为邓肯–张模型等非线性弹性模型甚至弹塑性模型的回弹模量的合理取值提供了重要试验依据。     

Creep and time-dependent damage in argillaceous rocks

This paper presents the results of laboratory tests on the time-dependent behaviour of three rocks characterized by a high proportion of clay particles. The viscosity of these sedimentary rocks was studied under different loading conditions in uniaxial compression: static or cyclic creep tests and quasistatic tests (low-loading strain rate) were performed across various orientations of fabric planes.The quasistatic tests showed similarities in the mechanical response of these three argillaceous rocks: a late phase of dilation and a linear development of volumetric deformation before the beginning of unstable crack propagation. The development of secondary and tertiary creep phases during the creep tests highlighted the existence of a deviatoric stress threshold, below which only primary creep is observed. Long-term creep tests also showed that the volumetric variation is not constant during the development of viscoplastic deformations.A microstructural analysis of thin sections extracted from specimens after the tests, gave evidence of cataclastic and granular creep. Damage to the argillaceous matrix occurs and no cracks were observed in the quartz and carbonate grains. This evidence was also demonstrated for tests with loading at a high strain rate.Finally, this study highlights the significant viscoplasticity of argillaceous rocks. The mechanical properties deteriorate rapidly when crack propagation becomes unstable and the viscoplastic strains seem to be due to clay particle slips, known as granular creep.     

考虑坑侧堆载基坑开挖土体应力路径试验研究

基坑开挖过程中,基坑坑侧各区域土体受到不同程度的卸荷或堆载作用,从而呈现出复杂的应力路径。因此,开展了常规三轴固结不排水剪切试验、K₀固结不排水剪切试验、K₀固结侧向卸荷试验和K₀固结侧向卸荷轴向加荷试验等不同的应力路径试验,来研究基坑开挖前后基坑坑侧土体的不同应力路径变化情况,比较分析了各应力路径条件下土体的抗剪强度、应力应变的变化情况,归纳了基坑开挖前后复杂应力路径条件下土体的应力变形及强度特征。研究表明,在不同的应力路径条件下,土体所对应的应力变形及强度参数,具有明显的差异性;软黏土在不同的侧向卸荷应力路径情况下,应力应变曲线均呈现应变软化型,且侧向卸荷对土体强度影响较大,坑侧堆载引起的轴向加荷对土体强度变化也有一定影响。     

一个能够模拟软土时效特性的简单弹黏塑性模型(英文)

基于Perzyna超应力理论与修正剑桥模型,建立了一个能够模拟软土时效特性的简单的弹黏塑性本构模型,提出了参数的实验室确定方法。以室内试验为基础,模拟了不同试验条件下软土的时效特性:应变速率对先期固结压力和不排水抗剪强度的影响;一维固结与次固结特征及竖向应力对次固结系数的影响;不同应力水平下的不排水蠕变特征;不同应变水平下的应力松弛特征。通过实验数据与数值模拟的比较,对模型进行了验证,发现上述本构模型能够较好地描述不同加载路径下的正常固结与微超固结土的时效特征。同时,通过对同一试样的多阶段加卸载三轴实验、现场压力仪实验及实验室压力仪实验的模拟,发现此模型可以较好地拟合实验过程中复杂应力路径下软土的时效特征。     

Time-dependent tests on intact rocks in uniaxial compression

The influence of strain history on rock specimen deformation during multi-level loading and unloading cyclic uniaxial compression creep tests is studied with a creep testing machine. An experimental data processing method for such creep tests is suggested. The correction formulas to determine the rheological model parameters are derived for the case when load relaxation is considered. Creep and relaxation tests under uniaxial compression on four types of rocks are conducted using an electronic hydraulic servo-controlled stiff testing machine. The creep and relaxation laws of the different rocks are compared. The complete stress–strain curves for red sandstone specimens are obtained at nine strain rates from 2.43×10⁻⁶ to 4.38×10⁻³/s. The effects of strain rates on rock strength and limit strain are discussed. Empirical equations to evaluate the strain rate dependence of rock mechanical properties are presented.     

Effects of Reinforcement type and Loading History on the Deformation of Reinforced Sand in Plane Strain Compression

To evaluate the effects of reinforcement type in terms of stiffness, viscous property, rupture strength, shape and loading history on the stress-strain behaviour during primary, sustained and cyclic loading of reinforced sand, a series of drained plane strain compression tests were performed on Toyoura sand. The sand specimens were reinforced with two types of polymer geogrid as well as two types of metal grid, having largely different stiffness values and surface conditions. Despite that the effects of reinforcement type on the overall stress-strain characteristics of reinforced sand and their rate-dependency are significant during primary loading, the effects are much smaller than the difference in the stiffness of reinforcement. The effects of reinforcement type on the global unloading behaviour and the residual strain by cyclic loading during otherwise global unloading are generally insignificant or negligible. The residual strains by cyclic loading of reinforced sand became substantially small by preloading as well as pre-sustained loading and pre-cyclic loading at higher load levels. With this procedure, polymer geosynthetic reinforcement, which is much more extensible and viscous than metal reinforcement, can be used to reinforce soil structures allowing very limited residual deformation.     

Effect of incremental loading on the creep behaviour of chemically grouted sand

 This paper describes the results of a study on the stress-strain time dependent (creep) behaviour of silicate-Hardener 600B grouted Leighton Buzzard sand specimens when subjected to incremental loading at certain time intervals. Unconfined and confined multi-stage creep tests were conducted on sand specimens injected with different silicate content grouts which cover the range generally used in engineering practice. The laboratory results showed that creep movements increase with the increase in stress ratio causing a rise in strain rate which ultimately leads to creep rupture (complete failure). However, an increase in the stress ratio does not affect the linear relationship between logarithm of strain rate and the logarithm of time suggesting that the three parameter phenomenological relationship of Singh and Mitchell (1968) may be suitable to describe the multi-stage creep behaviour of silicate-Hardener 600B grouted sand. Received: 25 September 1998 · Accepted: 19 February 1999