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1.
适用于砂土循环加载分析的边界面塑性模型 总被引：1，自引：0，他引：1
基于临界状态土力学框架,建立了一个适用于砂土排水循环加载的边界面塑性模型。采用了考虑虚拟峰值应力比的偏应变硬化准则,初始加载阶段应力点位于边界面上,反向加载阶段以历史最大屈服面作为边界面,同时实现了对密砂软化现象的模拟和对历史所受最大应力的记忆。边界面采用修正的椭圆形,引入考虑密度与应力水平的状态相关剪胀函数,采用非相关联流动法则和以应力反向点作为映射中心的径向映射准则。模型仅有10个参数,通过常规三轴试验即可确定,并且使用一套参数可以模拟不同围压、密度的单调和循环加载情况。分别对饱和砂土的单调、循环排水三轴试验进行模拟,结果表明,该模型能够合理地反映饱和砂土排水条件下的应力-应变特性。 相似文献
2.
This paper presents an elasto‐plastic model for unsaturated compacted soils and experimental results obtained from a series of suction‐controlled triaxial tests on unsaturated compacted clay with different initial densities. The initial density dependency of the compacted soil behaviour is modelled by establishing experimental relationships between the initial density and the corresponding yield stress and thereby between the initial density and the location and slope of normal compression line. The model is generalized to three‐dimensional stress states by assuming that the shapes of the failure surface and the yield surface in the deviatoric plane are given by the extended SMP criterion. A considerable number of the isotropic compression, triaxial compression and extension tests on unsaturated compacted clay with different initial densities were performed using a suction‐controllable triaxial apparatus, to measure the stress–strain–volume change in different stress paths and wetting paths. The model has well‐predicting capabilities to reproduce the mechanical behaviour of specimens compacted under different conditions not only in isotropic compression but also in triaxial compression and triaxial extension. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
3.
The clayfills are being produced in open-pit mining. The stress state in the stiff lumps of the clayfills is significantly lower than in situ level. As a result, their current states lie on the dry side of the critical state. The linear Hvorslev surface is widely used due to its simplicity and capability to model the limit stress condition of soils on the dry side. However, it may overestimate the strength at very low stress level. For this purpose, a series of drained triaxial tests were performed on a silty clay at very small stress levels. The failure points of the tested soil confirm a nonlinear relationship in \(p^{\prime }\)–q plane on the dry side of the critical state. The degree of nonlinearity increases after being normalized by the Hvorslev equivalent pressure, which can be well modeled by a nonlinear power law criterion proposed by Atkinson (Géotechnique 57(2):127–135, 2007). Based on the test data and the critical state concept, a new failure line is proposed with help of the equivalent Hvorslev pressure. The nonlinear Hvorslev surface is then incorporated into an elastoplastic model and a hypoplastic model. Comparisons between the experimental data and simulations reveal that the proposed models can well represent the behavior observed in the laboratory. 相似文献
4.
Limitations of suction‐controlled triaxial tests in the characterization of unsaturated soils 
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下载免费PDF全文 X. Zhang 《国际地质力学数值与分析法杂志》2016,40(2):269-296
Behavior of unsaturated soils is influenced by many factors, and the influences of these factors are usually coupled together. Suction‐controlled triaxial (SCTX) tests are considered to allow researchers to investigate influences of individual variables on unsaturated soils under specified stress path with controls of stresses, pore water, and air pressures. In the past 50 years, SCTX testing method has been established as a standard approach to characterize constitutive behavior of unsaturated soils. Most important concepts for modern unsaturated soil mechanics were developed upon results from the SCTX tests. Among these, one of the most important contributions in the constitutive modeling of elasto‐plastic behavior for unsaturated soils is the Barcelona basic model (BBM) proposed by Alonso et al. in 1990. The BBM successfully explained many features of unsaturated soils and received extensive acceptance. However, the SCTX tests are designed based upon the divide‐and‐conquer approach in which an implicit assumption is used: soil behavior is stress‐path independent. However, it is well‐established that unsaturated soil behavior is elasto‐plastic and stress‐path dependent. It is found that the SCTX tests in fact cannot control the stress path of an unsaturated soil during loading. This incapability, in combination with complicated loading/collapse behavior of unsaturated soils, makes the SCTX tests for characterizing unsaturated soil questionable. This paper discusses the limitations of the SCTX tests in the characterization of unsaturated soils. A possible solution to the problem was proposed based on a newly developed modified state surface approach. The discussions are limited for isotropic conditions. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
5.
The hydro-mechanical behaviour of a reconstituted unsaturated soil under different suctions and strain rates was studied through various rate-controlled unsaturated/undrained triaxial tests. The fully saturated reconstituted specimens were desaturated to four different initial suctions (s0?=?0, 100 kPa, 200 kPa and 300 kPa) and then triaxially sheared (conventional triaxial compression) at three different strain rates in undrained conditions (\(\dot{\varepsilon }_{1} = 0.001\) h?1, 0.01 h?1, and 0.1 h?1). The observed hydro-mechanical behaviour during shearing including the volumetric strain, deviatoric stress, degree of saturation and suction is presented and discussed in this paper. The results indicate that when the strain rate rises at the given initial suctions (or pore water pressures), the maximum deviatoric stress (qmax), critical net stress ratio (M) and critical state suction (sc) increase but the degree of saturation (Src) and volumetric strain at the critical state (εcv ) reduce. The critical effective stress ratio (M′) is not dependent on the strain rate for saturated and unsaturated samples. The critical state lines for unsaturated soils with the constant strain rates are parallel with each other in the e???lnp′ space.
相似文献6.
Modelling the mechanical behaviour of unsaturated soils has been the subject of many research works in the past few decades. A number of constitutive models have been developed to describe the complex behaviour of unsaturated soils. Despite the significant advances in the constitutive theories for unsaturated soils, none of the existing models can completely describe the various aspects of the real behaviour of unsaturated soils. In this paper, a new unified approach is presented, based on the integration of a neural network and a genetic algorithm, for the modelling of unsaturated soils. In the proposed approach, a genetic algorithm was used to optimise the weights of the neural network. A three-layer sequential architecture was chosen for the neural network. The network had eight input neurons, five neurons in the hidden layer and three neurons in the output layer. The eight input neurons represented the initial gravimetric water content, initial dry density, degree of saturation, net mean stress with respect to pore-air pressure, axial strain, deviatoric stress, soil suction and volumetric strain, and the three neurons in the output layer represented the deviatoric stress, suction and volumetric strain at the end of each increment. The network was trained and tested using a database that included results from a comprehensive set of triaxial tests on unsaturated soils from the literature. The predictions of the proposed model were compared with the experimental results. The comparison of the results indicates that the proposed approach was accurate and robust in representing the mechanical behaviour of unsaturated soils. 相似文献
7.
A plastic deviatoric model with hardening is developed on the basis of geomechanical tests performed in the saturated case on low permeable porous material such as argillite. This model is a generalized Mohr–Coulomb plastic criterion combined with a Drucker–Prager plastic potential and the hardening parameter is the plastic distortion. Three different hardening functions have been introduced on the basis of triaxial tests: an increase of friction angle, a decrease of cohesion after a threshold and a contractancy to dilatancy transition for volumetric plastic strain. This plastic model has been adapted to the partially saturated case. The effective stress is expressed thanks to the equivalent interstitial pressure π. Numerical results are presented for the excavation and monotonous ventilation of a deep cylindrical cavity. A first plastification due to excavation is followed by a second one due to desaturation. The extent of the non-saturated zone provokes an extent of a plastic zone in the rock mass. Analysis shows that the origin of the plastification can be found in the deviatoric stresses because mean effective stresses are compressive during drying. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
8.
This study presents a simple approach to modelling the effect of temperature on the deformation and strength of unsaturated/saturated soils by using the average skeleton stress and degree of saturation. The concept of thermo-induced equivalent stress is introduced to consider the influence of temperature on the pre-consolidated stress. A skeleton stress–saturation framework is applied to enable the model to describe the thermo-elastoplastic behaviour of both unsaturated and saturated soils, as the skeleton stress can smoothly shift to Terzaghi’s effective stress if saturation changes from the unsaturated to the saturated condition. The new model only employs seven parameters, of which five parameters are the same as those used in the Cam-Clay model. The other two parameters can be easily determined by oedometer tests and simple thermo-mechanical tests. Numerical simulations of isotropic loading tests and triaxial shear tests under different conditions are conducted to illustrate the performance of the proposed model. By comparing with experimental temperature controlled oedometer tests and triaxial tests, it is confirmed that the proposed model is able to capture the thermo-mechanical behaviour of unsaturated/saturated normally and over-consolidated soils with a set of unified parameters. 相似文献
9.
The paper presents a new single-surface elasto-plastic model for unsaturated cemented soils, formulated within the critical state soil mechanics framework, which should be considered as an extension to unsaturated conditions of a recently proposed constitutive law for saturated structured soils. The model has been developed with the main purpose of inspecting the mechanical instabilities induced in natural soils by bond degradation resulting from the accumulation of plastic strains and/or the changes in pore saturation. At this scope, the constitutive equations are used to simulate typical geotechnical testing conditions, whose results are then analysed in light of the controllability theory. The results of triaxial tests on an ideal fully saturated cemented soil and on the corresponding unsaturated uncemented one are first discussed, aiming at detecting the evidence of potentially unstable conditions throughout the numerical simulations. This is followed by similar analyses considering the combined effects of both the above features. For each analysed case, a simple analytical stability criterion is proposed and validated against the numerical results, generalizing the results, and highlighting the crucial role of state variables and model parameters on the possible occurrence of failure conditions.
相似文献10.
The cyclic behaviours of embedded offshore structures under different cyclic loading levels are related to the cyclic shakedown and degradation of the surrounding soils. In the present study, a damage-dependent bounding-surface model based on a newly proposed hardening rule was developed to predict the cyclic shakedown and degradation of saturated clay and the effect of the initial anisotropic stress state. By extending the Masing’s rule to the bounding-surface plasticity theory, the stress reversal point is taken as the generalised homological centre of the bounding surface. With movement of the generalised homological centre, at lower stress amplitudes, the cyclic process ends at a steady state, and cyclic shakedown is reached. At higher stress amplitudes, a damage parameter related to the accumulated deviatoric plastic strain is incorporated into the form of the bounding surface, which is hence able to contract to model degradations in stiffness and strength. To take into account the effects of initial anisotropic conditions on the cyclic behaviour of soils, an initial anisotropic tensor is introduced in the bounding surface. The developed model is validated through undrained isotropic and anisotropic cyclic triaxial tests in normally consolidated and overconsolidated saturated clay under both one-way and two-way loadings. Both cyclic shakedown and degradation are well reproduced by the model, as is the anisotropy effect induced by the initial anisotropic consolidation process. 相似文献
11.
A unified constitutive model for unsaturated soils is presented in a critical state framework using the concepts of effective stress and bounding surface plasticity theory. Consideration is given to the effects of unsaturation and particle crushing in the definition of the critical state. A simple isotropic elastic rule is adopted. A loading surface and a bounding surface of the same shape are defined using simple and versatile functions. The bounding surface and elastic rules lead to the existence of a limiting isotropic compression line, towards which the stress trajectories of all isotropic compression load paths approach. A non‐associated flow rule of the same general form is assumed for all soil types. Isotropic hardening/softening occurs due to changes in plastic volumetric strains as well as suction for some unsaturated soils, enabling the phenomenon of volumetric collapse upon wetting to be accounted for. The model is used to simulate the stress–strain behaviour observed in unsaturated speswhite kaolin subjected to three triaxial test load paths. The fit between simulation and experiment is improved compared to that of other constitutive models developed using conventional Cam‐Clay‐based plasticity theory and calibrated using the same set of data. Also, the model is used to simulate to a high degree of accuracy the stress–strain behaviour observed in unsaturated Kurnell sand subjected to two triaxial test load paths and the oedometric compression load path. For oedometric compression theoretical simulations indicate that the suction was not sufficiently large to cause samples to separate from the confining ring. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
12.
应变综合结构势是一种新的、可以方便应用于复杂应力状态下的土结构性参数。根据应变综合结构势参数的定义,用三轴剪切试验中原状黄土（或人工结构性黄土）、重塑黄土以及饱和黄土的应力-应变曲线求得原状黄土（或人工结构性黄土）的结构性参数曲线,分析了三轴试验条件下非饱和黄土的结构变化特性。结果表明,应变综合结构势作为一种新的土结构性参数,它与应力综合结构势参数对同一影响因素的变化趋势基本相同,可以合理解释土体在三轴剪切试验条件下土结构性参数曲线所表现出的基本特征,更适合描述在三轴复杂应力状态下土体结构性的变化和发展。 相似文献
13.
In many geotechnical systems, such as reinforced slopes and embankments, soil-structure interfaces are often unsaturated. Shear behaviour of unsaturated interfaces is strongly dependent on their matric suctions, as revealed by the results of extensive laboratory tests. So far, constitutive models for unsaturated interfaces are very limited in the literature. This paper reports a new bounding surface model for saturated and unsaturated interfaces. New formulations were developed to incorporate suction effects on the flow rule and plastic modulus. To examine the capability of the proposed model, it was applied to simulate suction- and stress-controlled direct shear tests on unsaturated soil–cement, soil–steel and soil–geotextile interfaces. Measured and computed results are well matched, demonstrating that the proposed model can well capture key features of the shear behaviour of unsaturated interfaces, including suction-dependent dilatancy, stress–strain relation and peak and critical state shear strengths. 相似文献
14.
Most existing hydromechanical models for unsaturated soils are not able to fully capture the nonlinearity of stress–strain curves at small strains (less than 1%). They cannot therefore, for example, accurately predict ground movements and the performance of many earth structures under working conditions. To tackle this problem, a state‐dependent bounding surface plasticity model has been newly developed. Particularly, the degradation of shear modulus with strain at small strains ranging from 0.001% to 1% is focused. The proposed model is formulated in terms of mean average skeleton stress, deviator stress, suction, specific volume and degree of saturation. Void ratio‐dependent hydraulic hysteresis is coupled with the stress–strain behaviour. Different from other elastoplastic models for unsaturated soils, plastic strains are allowed inside bounding surfaces. In this paper, details of model formulations and calibration procedures of model parameters are presented. To evaluate the capability of the new model, it is applied to simulate a series of triaxial compression tests on compacted unsaturated silt at various suctions. Effects of suction, drying and wetting as well as net stress on unsaturated soil behaviour are well captured. The model shows good predictions of the degradation of shear modulus with strain over a wide range of strains from 0.001% to 1%. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
15.
考虑体变对非饱和土土-水状态的影响,将状态曲面函数引入传统的Vanapalli强度公式得到与孔隙比相关的抗剪强度准则,新准则使用饱和土的强度参数和两条不同孔隙比对应的土-水特征曲线。选择一种尾矿砂和高岭土的混合土料为研究对象,进行一系列的土-水特征曲线试验、吸力控制的等向压缩和三轴剪切试验。试验结果表明,新准则能更准确地预测非饱和土的强度,证明了传统强度预测的误差主要来源于忽略了体变导致的土-水状态变化,并提出在不同应力空间内精确地获得抗剪强度包线的方法,合理地解释了强度包线斜率在净应力-强度平面内随吸力增大、强度包线形状在吸力-强度平面内随净应力发生变化的特性。 相似文献
16.
Influence of pore pressure on plastic deformation and strength of limestone under compressive stress
This study is devoted to experimental investigation of effects of pore pressure on plastic deformation and failure of a water-saturated limestone. The experimental study is composed of three different groups of laboratory tests. The basic mechanical behavior of the rock is first characterized by drained triaxial compression tests on water-saturated samples without pore pressure. The results are compared with those obtained in a previous study from triaxial compression tests on saturated samples with a constant pore pressure. In the second group, water injection tests under a confining pressure of 20 MPa and different values of deviatoric stress are realized to study the effect of pore pressure increase. Finally, undrained triaxial compression tests are carried out for investigating the coupling effect of plastic deformation and pore pressure variation. Based on experimental data, the validity of effective stress concept for plastic yielding and failure strength is discussed.
相似文献17.
A unified mechanical and retention model for saturated and unsaturated soil behaviour 总被引：1，自引：1，他引：0
The coupled mechanical and water retention elasto-plastic constitutive model of Wheeler, Sharma and Buisson (the Glasgow coupled model, GCM) predicts unique unsaturated isotropic normal compression and unsaturated critical state planar surfaces for specific volume and degree of saturation when soil states are at the intersection of mechanical (M) and wetting retention (WR) yield surfaces. Experimental results from tests performed by Sivakumar on unsaturated samples of compacted speswhite kaolin confirm the existence and form of these unique surfaces. The GCM provides consistent representation of transitions between saturated and unsaturated conditions, including the influence of retention hysteresis and the effect of plastic volumetric strains on retention behaviour, and it gives unique expressions to predict saturation and de-saturation conditions (air-exclusion and air-entry points, respectively). Mechanical behaviour is modelled consistently across these transitions, including appropriate variation of mechanical yield stress under both saturated and unsaturated conditions. The expressions defining the unsaturated isotropic normal compression planar surfaces for specific volume and degree of saturation are central to the development of a relatively straightforward methodology for determining values of all GCM parameters (soil constants and initial state) from a limited number of laboratory tests. This methodology is demonstrated by application to the experimental data of Sivakumar. Comparison of model simulations with experimental results for the full set of Sivakumar’s isotropic loading stages demonstrates that the model is able to predict accurately the variation of both specific volume and degree of saturation during isotropic stress paths under saturated and unsaturated conditions. 相似文献
18.
The unified three-dimensional (3D) critical state bounding-surface plasticity model gUTS enables clays, silts and sands to be treated within a single framework. Furthermore, loose and dense states of a particular soil subjected to a wide range of confinements are viewed as a single material defined by the same set of constants. The model is able to handle both monotonic and complex cyclic paths including those involving a rotation of the principal stress directions. The model incorporates the following features: combined use of radial and deviatoric mapping rules and the use of an apparent normal consolidation line for sands; use of a non-associated flow rule where the ratio of the rates of volumetric plastic strain to deviatoric plastic strain is a function only of the ratio of deviatoric to mean effective stresses and the Lode angle; adoption of a bi-linear critical state line projected onto the plane of the void ratio versus logarithm of mean effective stress; inclusion of a sub-elliptic, or super-elliptic, segment in the plastic dilatancy surface for stress ratios less than critical; use of elliptic segments in the deviatoric planes; movement of the projection centre in the deviatoric mapping region and incorporation of a plastic stiffening effect for cyclic paths which repeatedly load in the same deviatoric direction. 相似文献
19.
针对研究区(黑方台地区)黄土的非饱和蠕变特性,采用改进型的FSR-10非饱和三轴蠕变仪进行了基质吸力控制条件下的三轴蠕变试验,研究发现:（1）当施加的偏应力较小时,黄土的变形较小,并具有一定程度的可恢复性,随着偏应力的增加,土颗粒需经过较长的时间才能够移动到新的位置,达到新的平衡,当偏应力增加到一定程度时,颗粒移动难以达到平衡,土样发生破坏。（2）在其他条件相同的情况下,围压与试样产生的变形量呈负相关,且围压越小,变形达到稳定所需的时间也越长;（3）在围压和偏应力水平一定的情况下,土体的变形量随基质吸力呈负相关,且基质吸力越小,蠕变曲线达到稳定所需的时间越长。 相似文献
20.
This paper explores the possibility of using well-accepted concepts—Mohr-Coulomb-like strength criterion, critical state, existence of a small strain elastic region, hyperbolic relationship for representing global plastic stress–strain behaviour, dependence of strength on state parameter and flow rules derived from the Cam-Clay Model—to represent the general multiaxial stress–strain behaviour of granular materials over the full range of void ratios and stress level (neglecting grain crushing). The result is a simple model based on bounding surface and kinematic hardening plasticity, which is based on a single set of constitutive parameters, namely two for the elastic behaviour plus eight for the plastic behaviour, which all have a clear and easily understandable physical meaning. In order to assist the convenience of the numerical implementation, the model is defined in a ‘normalized’ stress space in which the stress–strain behaviour does not undergo any strain softening and so certain potential numerical difficulties are avoided. In the first part the multiaxial formulation of the model is described in detail, using appropriate mixed invariants, which rationally combine stress history and stress. The model simulations are compared with some experimental results for tests on granular soils along stress paths lying outside the triaxial plane over a wide range of densities and mean stresses, using constitutive parameters calibrated using triaxial tests. Furthermore, the study is extended to the analysis of the effects induced by the different shapes of the yield and bounding surfaces, revealing the different role played by the size and the curvature of the bounding surface on the simulated behaviour of completely stress- and partly strain-driven tests. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献