红黏土固结-蠕变特性及其耦合模型

红黏土的蠕变特性直接影响到红土边坡的长期稳定性。为了深入研究红黏土的蠕变特性,设计改制了一套基于杠杆加载原理的三轴蠕变试验装置。采用分级加载,对红黏土试样进行室内排水三轴蠕变试验,获得了不同偏应力水平下的蠕变全过程曲线,采用"陈氏加载法"将分级加载曲线转化成分别加载曲线,并利用等时曲线法得出红黏土蠕变破坏阈值。将固结理论与Kelvin蠕变模型相结合以描述红黏土在偏应力作用下的固结-蠕变特性,然后利用一部分偏应力水平下的蠕变试验结果进行回归建模以确定各模型参数,并以此建立出考虑固结-蠕变耦合作用的红黏土元件蠕变模型,最后采用所建蠕变模型对另一部分蠕变试验结果进行预测,结果表明:本文模型无论是拟合还是预测的精准度都很高。     

各向异性弹黏塑性模型在ABAQUS中的研发

通过将Perzyna过应力理论与临界状态理论相结合,并引入Wheeler旋转硬化法则,提出一个能描述土体初始各向异性及应力诱发各向异性的三维弹黏塑性本构模型。模型考虑流变发生的下限,在三维应力空间,模型存在形状相似的静屈服面及动态加载面。采用缩放形式的幂函数。本构模型数值算法采用回映算法,借助ABAQUS软件UMAT子程序接口实现。并通过对三轴不排水蠕变试验的模拟,确定合适的积分步长。此后,分别对三轴不排水蠕变试验及常应变率三轴不排水剪切试验进行了模拟。模拟中通过设置不同参数值,可将模型退化为各向同性模型,并对这两种模拟结果进行了比较。模拟结果表明：(1) 对于三轴不排水蠕变,在低剪应力水平下,各向同性模型和各向异性模型模拟的结果相差不大,而在高剪应力水平下,各向异性模型模拟结果更接近试验结果;(2) 对于常应变率加载试验的模拟,模型合理反映了土体不排水强度随着加载速率的增大而增大现象。     

盐岩蠕变特性和本构关系研究

对金坛盐岩蠕变规律进行了试验研究,分析了不同应力状态和温度对盐岩蠕变特性的影响,通过试验获取了蠕变曲线和蠕变特征参数。根据其蠕变曲线特征,用黏弹塑性理论建立了盐岩蠕变本构模型。试验结果表明,稳态蠕变率是偏应力和温度的非线性函数关系,并提出盐岩的稳态蠕变本构方程。通过试验数据拟合得到本构方程中的参数,将理论和试验结果进行比较,结果表明两者的吻合性较好。     

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.     

Effect of Intermediate Microfabric on Shear Strength and Strain Localization Response of Kaolin Clay Under Compression and Extension Loading

The present paper evaluates the role of microfabric in strain localization patterns observed in soil specimens during its shear deformation in compression and extension triaxial testing. A series of compression and extension lubricated end triaxial tests are performed on Kaolin clay with extreme as well as intermediate microfabrics, which are obtained using slurry consolidation technique by varying calagon content from 0 to 3 %. Intermediate microfabric is the geometric arrangement of particles within the soil mass, which lies in between the particle orientation during two extreme microfabrics; flocculated and dispersed. Flocculated has random orientation of particles with face-to-edge particle contacts and dispersed has parallel orientation of particles with face-to-face particle contacts. When the specimen is subjected to large stress levels in triaxial testing, the particle orientation and geometric arrangement get affected due to the force acting on the clay platelets. In this experimental research, the variation in clay’s stress–strain and pore pressure response and initiation, propagation and formation of shear bands at different levels of compression and extension shearing is evaluated using digital image analysis setup associated with triaxial system.     

深部人工冻结黏土三轴蠕变试验研究

通过将试样等向固结再冻结, 然后保持轴压不变进行径向卸载的实验方法, 获得深部冻结黏土在复杂应力状态下的蠕变变形规律. 对冻黏土进行了大量的三轴蠕变试验, 得到了在冻结温度不同和固结围压不同条件下的蠕变试验结果. 结果表明: 在较低的偏应力水平下, 试样只发生前两个阶段的蠕变变形, 且蠕变变形量超过总变形量的70%; 当试样偏应力值高于某一临界值时会出现加速蠕变阶段, 蠕变变形量超过总变形量的80%. 可以用改进的Zienkiewicz-Pande抛物线型屈服准则来描述加速蠕变阶段的临界应力.     

Predictions of a continuous hyperplasticity model for Bangkok clay

The performance of a new constitutive model called ‘kinematic hardening modified Cam clay’ (KHMCC) is presented. The model is described using the ‘continuous hyperplasticity’ framework. Essentially this involves an infinite number of yield surfaces, thus allowing a smooth transition between elasticity and plasticity. The framework allows soil models to be developed in a relatively succinct mathematical form, since the entire constitutive behaviour can be determined through the specification of two scalar potentials. An implementation of the continuous hyperplasticity model is also described. The model requires eight parameters plus a viscosity coefficient for rate-dependent analysis. The model is defined in terms of triaxial stress–strain variables for this study, and is used to model monotonic triaxial tests on Bangkok clay. Comparisons of the theoretical predictions with the results of cyclic undrained triaxial compression tests on Bangkok clay are also presented.     

冻土三轴蠕变非线性数学模型研究

采用MTS-810液压伺服材料试验机, 对人工配制的冻黏土试件进行了三轴蠕变试验, 获得了冻黏土在复杂应力状态下的蠕变曲线. 结果表明: 冻土的蠕变变形具有较强的温度敏感性, 温度越高, 这种温度敏感性越强; 相同温度下, 荷载越大, 变形越大. 运用相关理论, 推导了冻黏土在复杂应力状态下的三轴蠕变非线性数学模型, 采用MATLAB软件的数据拟合功能得到了模型方程参数的数值, 模型参数与温度之间存在密切关系, 建立了二者之间的数学表达式. 冻土三轴蠕变非线性数学模型的曲线与试验曲线拟合精度较高, 建立的数学模型可以精确体现冻土的蠕变规律, 能够为实际冻土工程的变形发展预测提供有效的理论指导.     

不同应力路径下剪切带的数值模拟

采用回映应力更新算法,编写了基于伏斯列夫面的超固结黏土本构关系模型子程序,嵌入非线性有限元软件ABAQUS。通过对单元试验进行三轴压缩、三轴伸长及平面应变等问题的模型预测,再现了超固结黏土在不同初始超固结比和应力路径时的变形和强度特性,从而验证了子程序的正确性。借助该本构模型,对三轴压缩、三轴伸长及平面应变应力路径下超固结黏土体变形局部化问题,进行了三维数值模拟。分析结果表明：超固结黏土在三轴压缩及伸长状态时,土体变形局部化在应力-应变关系软化时出现,而平面应变状态时,在应力-应变关系硬化阶段出现,其超固结黏土的剪胀特性在剪切带的形成过程中起重要作用。     

New Viscoplastic Model for Design Analysis of Tunnels in Squeezing Conditions

Summary  This paper is intended to describe the SHELVIP (Stress Hardening ELastic VIscous Plastic) model, a new viscoplastic constitutive law which has been developed to incorporate the most important features of behaviour observed in tunnels excavated in severe to very severe squeezing conditions. This model couples the elastoplastic and time-dependent behaviour by using a plastic yield surface, as frequently adopted in tunnel design analysis, and the definition of a state of overstress referred to a viscoplastic yield surface. The model is formulated in all its detailed aspects. The related analytical closed-form solution for representing triaxial creep deformations is developed. Also developed is an incremental numerical solution for describing the triaxial stress–strain behaviour under constant strain rate conditions. The model is shown to fit very satisfactorily the results of creep tests on clay shales and relaxation tests on coal specimens, as recently performed for design analysis of tunnels in squeezing conditions. Correspondence: D. Debernardi, Department of Structural and Geotechnical Engineering, Politecnico di Torino, Torino, Italy     

冻土三轴蠕变特性试验研究及平面冻土墙厚度的确定

主要是对冻土的三轴蠕变特性进行分析研究,从而进一步确定具有明显流变特性的平面冻土墙的厚度。通过对冻土的流变特性进行理论分析,建立了冻黏土在复杂应力状态下的对数型蠕变方程。采用“低温箱-三轴压力室”轻型试验设备系统对人工配制的冻黏土试件进行了三轴蠕变试验,获得了冻黏土在复杂应力状态下的蠕变曲线。根据试验结果,对冻黏土的对数型非线性蠕变方程进行回归分析,得到了冻黏土对数型蠕变方程参数的数值。根据冻土流变理论和所建立的蠕变方程,以及平面冻土墙的厚度计算公式,利用Visualc++ 6.0和Matlab 6.0技术开发了冻土墙厚度计算的计算机应用软件。分析研究了平面冻土墙厚度与跨度、基坑暴露时间、基坑开挖深度的关系。平面冻土墙厚度随时间的延长在短期内具有急速增长的趋势,而后随时间的延长逐渐趋于稳定;平面冻土墙厚度受其跨度的影响较小,但随基坑开挖深度的加深具有逐步增长的趋势;温度对平面冻土墙厚度的影响显著,温度越高,厚度越大,所以,控制温度是平面冻土墙设计中的关键。从而为蠕变变形较大的平面冻土墙的厚度确定提供了依据。     

A density‐dependent elastoplastic hydro‐mechanical model for unsaturated compacted soils

This paper presents a three‐dimensional elastoplastic constitutive model for predicting the hydraulic and mechanical behaviour of unsaturated soils. It is based on experimental results obtained from a series of controlled‐suction triaxial tests on unsaturated compacted clay with different initial densities. Hydraulic hysteresis in the water‐retention behaviour is modelled as an elastoplastic process, with the elastic part modelled by a series of scanning curves and the elastoplastic part modelled by the main drying and wetting curves. The effect of void ratio on the water‐retention behaviour is studied using data obtained from controlled‐suction wetting–drying cyclic tests on unsaturated compacted clay with different initial densities. The effect of the degree of saturation on the stress–strain‐strength behaviour and the effect of void ratio on the water‐retention behaviour are considered in the model, as is the effect of suction on the hydraulic and mechanical behaviour. The initial density dependency of the compacted soil behaviour is modelled by experimental relationships between the initial density and the corresponding yield stress and, thereby, between the initial density and the normal compression line. The model is generalized to three‐dimensional stress states by assuming that the shapes of the failure and yield surfaces in the deviatoric stress plane are given by the Matsuoka–Nakai criterion. Model predictions of the stress–strain and water‐retention behaviour are compared with those obtained from triaxial tests with different initial densities under isotropic compression, triaxial compression and triaxial extension, with or without variation in suction. The comparisons indicate that the model accurately predicts the hydraulic and mechanical behaviour of unsaturated compacted soils with different initial densities using the same material constant. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of Shear Deformation on Microfabric of Clay Using XRD Technique

The present paper deals with the impact of shear deformation on the geometric arrangement of particles within the soil specimen, which is termed as the microfabric of soil. A series of compression and extension lubricated end triaxial tests are performed on cylindrical specimens of Kaolinite clay with two extreme microfabrics; dispersed and flocculated, which are obtained using slurry consolidation technique. Flocculated microfabric has random orientation of particles within the soil mass having face-to-edge particle contacts; however, dispersed microfabric has parallel orientation of particles containing face-to-face particle contacts. When the specimen is subjected to large stress levels during its shear deformation, the particle orientation and the geometric arrangement within the soil specimen gets affected due to the force acting on the clay platelets. The variation in microfabric of soil before and after shear deformation process is evaluated by obtaining X-ray diffraction patterns of the clay specimen at three different locations using standard X-ray diffractometer. The discussion includes an analysis of the orientation of soil particles located at shear banding zones of the clay specimens, which may be useful for understanding the strain localization development in clays.     

Triaxial behaviour of transversely isotropic materials: application to sedimentary rocks

Failure and long‐term behaviour of oriented solids are studied. Transversely isotropic materials are considered and a mathematical formulation that respect the material symmetry is developed and applied to model the triaxial behaviour of sedimentary rocks. Two failure criteria and a viscoplastic constitutive model that describe, respectively, triaxial failure and triaxial creep tests are presented and discussed. The application of the developed models to describe the mechanical behaviour of Tournemire shale shows that theoretical predictions are in good agreement with the experimental data. In the present paper, the developed approach is applied to sedimentary rock materials, nevertheless, it can be generalized to any material that exhibits transverse isotropy. Copyright © 2007 John Wiley & Sons, Ltd.     

Deformation properties of frozen soils

This paper gives a brief account of the results of a series of oedometer tests and triaxial creep tests on frozen soils.

The oedometer tests are performed on artificially frozen sand, silt and clay. The tests also include ice as a reference material. The results are given by parameters describing the tangent deformation modulus and the time resistance against creep.

The triaxial creep tests are carried out on the artificially frozen clay, and on a permafrost clay from Spitsbergen. The data are interpreted in accordance with Hult's theory. The artificially frozen clay shows much higher creep resistance than the permafrost clay.

In addition to deformation moduli and creep parameters the routine data of the materials tested are also included.     

Delayed plastic model for time‐dependent behaviour of materials

A delayed plastic model, based on the theory of plasticity, is proposed to represent the time‐dependent behaviour of materials. It is assumed in this model that the stress can lie outside the yield surface and the conjugate stress called static stress is defined on the yield surface. The stress–strain relation is calculated based on the plastic theory embedding the static stress. Thus, the stress–strain relation of the model practically corresponds to that of the inviscid elastoplastic model under fairly low rate deformation. The delayed plastic model is coupled with the Cam‐clay model for normally consolidated clays. The performance of the model is then examined by comparing the model predictions with reported time‐dependent behaviour of clays under undrained triaxial conditions. It is shown that the model is capable of predicting the effect of strain rate during undrained shear and the undrained creep behaviour including creep rupture. Copyright © 2001 John Wiley & Sons, Ltd.     

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.     

粘弹塑本构模型及用于冻土数值计算的柔度矩阵

随着寒区经济发展及冻结法施工技术在工程中的推广应用,对冻土蠕变的深入研究显得尤为重要.对冻土单轴流变试验结果分析,用非线性牛顿体替代线性牛顿体,从而对西原模型进行了改进,得到了冻土三向应力状态下的本构方程;并利用D-P屈服准则推导出应用与数值计算的柔度矩阵.通过有限元程序的二次开发,将本构模型添加到大型非线性程序ADINA中.用三轴蠕变试验检验了粘弹塑本构模型,获得相应的试验拟合曲线.结果表明:三轴蠕变试验与理论计算结果吻合良好,说明粘弹塑本构模型可以描述冻土在高应力下反映的非衰减性蠕变变形特征.     

Numerical prediction of the creep behaviour of an unstabilised and a chemically stabilised soft soil

This paper examines the ability of volumetric and deviatoric creep laws associated with constitutive models to simulate the creep behaviour of a soft soil in its natural state or chemically stabilised state. Initially, the models/laws are validated by oedometer and triaxial creep tests, for the stabilised and unstabilised soils. Finally, the long-term behaviour of an embankment built on a soft soil reinforced with deep mixing columns is predicted based on the properties for a curing time of 28 days. The results show that the creep phenomenon should be considered in a long-term analysis of deep mixing columns.     

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.