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1.
A landslide in stiff,intact clay 总被引：1，自引：0，他引：1
A landslide in a stiff clay formation, interrupting the excavation of a tunnel for a major railway in Sicily, is reported. Limit equilibrium and FEM undrained and drained analyses of the slope before tunnel excavation agree in showing that the slope was stable with a relatively high factor of safety and the critical slip surface is located well above the tunnel. The undrained stability of the tunnel checked both via FEM and via standard analytical solutions for face stability is also verified. The FEM analyses of the slope have been repeated considering the excavation of the tunnel in undrained and drained conditions. The advancement of the tunnel face is simulated in a plane strain analysis by the Panet method. In undrained conditions, the system keeps stable. In drained conditions with a stress release factor of 50 %, the slope is on the verge of failing with a very low safety factor, while with a stress release factor of 80 %, it fails following a complex mechanism that matches the observed failure surface. These findings are discussed and some conclusions attempted. 相似文献
2.
In this article we present closed‐form solutions for the undrained variations in stress, pore pressure, deformation and displacement inside hollow cylinders and hollow spheres subjected to uniform mechanical pressure instantaneously applied to their external and internal boundary surfaces. The material is assumed to be a saturated porous medium obeying a Mohr–Coulomb model failure criterion, exhibiting dilatant plastic deformation according to a non‐associated flow rule which accounts for isotropically strain hardening or softening. The instantaneous response of a porous medium submitted to an instantaneous loading is undrained, i.e. without any fluid mass exchange. The short‐term equilibrium problem to be solved is now formally identical to a problem of elastoplasticity where the constitutive equations involve the undrained elastic moduli and particular equivalent plastic parameters. The response of the model is presented (i) for extension and compression undrained triaxial tests, and (ii) for unloading problems of hollow cylinders and spheres through the use of appropriately developed closed‐form solutions. Numerical results are presented for a plastic clay stone with strain hardening and an argilite with strain softening. The effects of plastic dilation, of the strain softening law and also of geometry of the cavity on the behaviour of the porous medium have been underlined. Analytical solutions provide valuable benchmarks enabling various numerical methods in undrained conditions with a finite boundary to be verified. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
3.
This paper presents a strain-rate dependent plastic constitutive model for clays. Based on the concepts of critical-state soil mechanics and bounding surface plasticity theory, the model reproduces the mechanical response of clays under triaxial and simple shear loading conditions. The model parameters are determined for Boston Blue Clay, London Clay and Kaolin Clay, and the performance of the model in simulating the mechanical response of these clays is demonstrated for low to medium strain rates. The sensitivity of each model parameter is checked by perturbing the calibrated values by ±20 %. Subsequently, a probabilistic analysis using Monte Carlo simulations is performed by treating the model parameters as random variables and the impact of the statistics of the parameters on the undrained shear strength is investigated. 相似文献
4.
The modified Cam clay (MCC) model is used to study the response of virgin‐compressed clay subjected to undrained triaxial compression. The MCC constitutive relationship is obtained in a closed form. Both elastic and plastic deviatoric strains are considered in the analysis. The solution allows to obtain total and effective stress paths followed by the clay in undrained spherical expansion. Pore water pressures are determined from the difference between total and effective mean stresses. For illustration purposes, the analysis is also applied to the well‐known reconstituted normally consolidated London clay and the results are compared with the recently published data obtained by a numerical approach. In addition, the Almansi large strains are used in the analysis, as these allow to obtain limit expansion and pore pressures, whereas both small‐strain and logarithmic‐strain approaches do not permit to determine them. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
5.
For evaluation of slope stability in materials displaying strain-softening behavior, knowledge concerning the failed state material response is of importance. Here, soft sensitive clay is studied. Such clays behave contractant at failure, which for undrained conditions yields a strain-softening behavior governed by the generation of excess pore water pressure. Strain softening is further linked with material instability and the phenomenon of strain localization. In the case of shear band formation, internal pore pressure gradients are then expected to be present for globally undrained conditions in the sensitive clay due to its low permeability. In the present study, this hypothesis and its implications on the global response and shear band properties are investigated. Utilizing an experimental setup with a modified triaxial cell allowing for shear band formation, the effect of varying the displacement rate is studied. Onset of strain localization is interpreted to occur just before or at the peak shear strength. A strong rate dependency of the softening response is observed. Increasing displacement rates give raised brittleness in terms of the slope of the global softening curve due to accumulating pore pressure. Also, reduced shear band thickness and a shear band inclination approaching 45° are obtained for increasing rates. In the context of slope failure in such materials, the rate dependency in the post-peak state opens up for a large variation in behavior, all depending on time as an important factor. 相似文献
6.
For numerical studies of geotechnical structures under earthquake loading, aiming to examine a possible failure due to liquefaction, using a sophisticated constitutive model for the soil is indispensable. Such model must adequately describe the material response to a cyclic loading under constant volume (undrained) conditions, amongst others the relaxation of effective stress (pore pressure accumulation) or the effective stress loops repeatedly passed through after a sufficiently large number of cycles (cyclic mobility, stress attractors). The soil behaviour under undrained cyclic loading is manifold, depending on the initial conditions (e.g. density, fabric, effective mean pressure, stress ratio) and the load characteristics (e.g. amplitude of the cycles, application of stress or strain cycles). In order to develop, calibrate and verify a constitutive model with focus to undrained cyclic loading, the data from high-quality laboratory tests comprising a variety of initial conditions and load characteristics are necessary. It is the purpose of these two companion papers to provide such database collected for a fine sand. Part II concentrates on the undrained triaxial tests with strain cycles, where a large range of strain amplitudes has been studied. Furthermore, oedometric and isotropic compression tests as well as drained triaxial tests with un- and reloading cycles are discussed. A combined monotonic and cyclic loading has been also studied in undrained triaxial tests. All test data presented herein will be available from the homepage of the first author. As an example of the examination of an existing constitutive model, the experimental data are compared to element test simulations using hypoplasticity with intergranular strain. 相似文献
7.
Experimental evidence has indicated that the critical state line determined from undrained compression tests is not identical to that determined from undrained extension tests. The purpose of this paper is to investigate a modelling method that accounts for the non‐uniqueness of critical state lines in the compression and the extension testing conditions. Conventional elastic–plastic cap models can predict only a unique critical state line for the compression and the extension tests. A new micromechanical stress–strain model is developed considering explicitly the location of critical state line. The model is then used to simulate undrained triaxial compression and extension tests performed on isotropically consolidated samples with different over‐consolidated ratios. The predictions are compared with experimental results as well as that predicted by models with kinematic hardening of yield surface. All simulations demonstrate that the proposed micromechanical approach is capable of modelling the undrained compression and the undrained extension tests. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
8.
The estimated undrained shear strength (su) is often not a unique value because it can be evaluated by various test types and/or procedures, such as different failure modes, shear strain rates, and boundary conditions. This study explores (1) the relationship between reference undrained shear strength and in situ shear wave velocity in terms of the effective overburden stress, and (2) the independent relationships to evaluate the undrained shear strength with special consideration of different directional and polarization modes (VH, HV, HH shear waves), which has not been reported. This evaluation is done via a worldwide database compiled from 43 well-documented geotechnical test sites associated with soft ground. Finally, new correlation models are proposed to estimate the undrained shear strength based on the in situ shear wave velocity as well as the plasticity index or the overconsolidation ratio. The application of the shear wave velocity–undrained shear strength relation is illustrated through two independent case studies. The proposed relationships are expected to contribute to reasonable estimates of undrained shear strength as well as offer practical guidance on even extrapolation beyond the data that is available to geotechnical engineers. 相似文献
9.
A fuzzy algorithm, the Takagi–Sugeno model, is implemented to develop a fuzzy inference system for predicting the holding capacity of suction caisson foundations for offshore platforms. The premise parameters of the fuzzy model are optimized by using a subtractive clustering algorithm. The consequent parameters are optimally determined via a weighted least square estimation. The input variables used for training the fuzzy model include the aspect ratio of the caisson, the undrained shear strength of the clay, and the angle that the chain force forms with the horizontal. The output of the proposed fuzzy model is the capacity of the suction caisson anchor. To demonstrate the effectiveness of the fuzzy modeling framework, the results of extensive finite element analyses are investigated. Comparisons of the trained fuzzy model with the data demonstrate that the proposed modeling framework is an effective method to estimate the holding capacity of offshore suction caisson systems. Moreover, the performance of the fuzzy model is robust against higher levels of input data uncertainties. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
10.
The paper presents a simple constitutive model for normally consolidated clay. A mathematical formulation, using a single tensor-valued function to define the incrementally nonlinear stress–strain relation, is proposed based on the basic concept of hypoplasticity. The structure of the tensor-valued function is determined in the light of the response envelope. Particular attention is paid towards incorporating the critical state and to the capability for capturing undrained behaviour of clayey soils. With five material parameters that can be determined easily from isotropic consolidation and triaxial compression tests, the model is shown to provide good predictions for the response of normally consolidated clay along various stress paths, including drained true triaxial tests and undrained shear tests. 相似文献
11.
为了描述天然软土的时间相依以及结构性特征,提出了一种能考虑土体超固结和结构性的实用弹黏塑性本构模型。它以Asaoka和Hashiguchi的上下负荷面作为某一应变速率下的参考屈服面,按照相对过应力的基本思路,新引入了两个能通过不同应变速率三轴压缩试验测定的率敏性参数 和 ,建立了以当前应力、黏塑性应变以及黏塑性应变速率为状态变量的动屈服准则函数,并给出了基于Newton-Raphson迭代的应力积分算法,且成功地将其嵌入到大型有限元软件ABAQUS中。最终通过数值算例来验证模型的正确性以及应力积分算法的可靠性。结果表明：该模型能同时描述土体的率敏性、蠕变以及结构性特征,模型参数物理意义明确、易懂可测,预测结果与试验数据吻合良好,可用于复杂边值问题的有限元计算。 相似文献
12.
DEM study of fabric features governing undrained post-liquefaction shear deformation of sand 总被引：3，自引：3，他引：0
In an effort to study undrained post-liquefaction shear deformation of sand, the discrete element method (DEM) is adopted to conduct undrained cyclic biaxial compression simulations on granular assemblies consisting of 2D circular particles. The simulations are able to successfully reproduce the generation and eventual saturation of shear strain through the series of liquefaction states that the material experiences during cyclic loading after the initial liquefaction. DEM simulations with different deviatoric stress amplitudes and initial mean effective stresses on samples with different void ratios and loading histories are carried out to investigate the relationship between various mechanics- or fabric-related variables and post-liquefaction shear strain development. It is found that well-known metrics such as deviatoric stress amplitude, initial mean effective stress, void ratio, contact normal fabric anisotropy intensity, and coordination number, are not adequately correlated to the observed shear strain development and, therefore, could not possibly be used for its prediction. A new fabric entity, namely the Mean Neighboring Particle Distance (MNPD), is introduced to reflect the space arrangement of particles. It is found that the MNPD has an extremely strong and definitive relationship with the post-liquefaction shear strain development, showing MNPD’s potential role as a parameter governing post-liquefaction behavior of sand. 相似文献
13.
For numerical studies of geotechnical structures under earthquake loading, aiming to examine a possible failure due to liquefaction, using a sophisticated constitutive model for the soil is indispensable. Such a model must adequately describe the material response to a cyclic loading under constant volume (undrained) conditions, amongst others the relaxation of effective stress (pore pressure accumulation) or the effective stress loops repeatedly passed through after a sufficiently large number of cycles (cyclic mobility, stress attractors). The soil behaviour under undrained cyclic loading is manifold, depending on the initial conditions (e.g. density, fabric, effective mean pressure, stress ratio) and the load characteristics (e.g. amplitude of the cycles, application of stress or strain cycles). In order to develop, calibrate and verify a constitutive model with focus to undrained cyclic loading, the data from high-quality laboratory tests comprising a variety of initial conditions and load characteristics are necessary. The purpose of these two companion papers was to provide such database collected for a fine sand. The database consists of numerous undrained cyclic triaxial tests with stress or strain cycles applied to samples consolidated isotropically or anisotropically. Monotonic triaxial tests with drained or undrained conditions have also been performed. Furthermore, drained triaxial, oedometric or isotropic compression tests with several un- and reloading cycles are presented. Part I concentrates on the triaxial tests with monotonic loading or stress cycles. All test data presented herein will be available from the homepage of the first author. As an example of the examination of an existing constitutive model, the experimental data are compared to element test simulations using hypoplasticity with intergranular strain. 相似文献
14.
F. Molenkamp 《国际地质力学数值与分析法杂志》1991,15(3):169-180
To increase the insight into the phenomenon of liquefaction a theoretical study has been performed of material instability for undrained behaviour. Material instability concerns the instability of an equilibrium state of an infinite homogeneous solid for both uniform deformation and shear-band generation. In a companion paper shear-band generation has been considered. In this paper the case of uniform deformation is elaborated and combined with the previous results. It can be concluded that instability occurs sooner for increasing contraction and decreasing drainage. For zero plastic volume change and decreasing drainage the mode of instability changes from drained shear-band generation to undrained uniform deformation. For contractant materials the instability of undrained behaviour is mainly due to uniform deformation. 相似文献
15.
In this paper the Discrete Element Method (DEM) is coupled with the Lattice-Boltzmann Method (LBM) to model the undrained condition of dense granular media that display significant dilation under highly confined loading. DEM-only models are commonly used to simulate the micromechanics of an undrained specimen by applying displacements at the domain boundaries so that the specimen volume remains constant. While this approach works well for uniform strain conditions found in laboratory tests, it doesn’t realistically represent non-uniform strain conditions that exist in the majority of real geotechnical problems. The LBM offers a more realistic approach to simulate the undrained condition since the fluid can locally conserve the system volume. To investigate the ability of the DEM-LBM model to effectively represent the undrained constraint while conserving volume and accurately calculating the stress path of the system, a two dimensional biaxial test is simulated using the coupled DEM-LBM model, and the results are compared with those attained from a DEM-only constant volume simulation. The compressibility of the LBM fluid was found to play an important role in the model response. The compressibility of the fluid is expressed as an apparent Skempton’s pore pressure parameter B. The biaxial test, both with and without fluid, demonstrated particle-scale instabilities associated with shear band development. The results show that the DEM-LBM model offers a promising technique for a variety of geomechanical problems that involve particle-fluid mixtures undergoing large deformation under shear loading. 相似文献
16.
在探讨含细粒砂土内部结构和粒间相互作用的基础上,引入粒间状态变量概念,对含细粒砂土力学性状随细粒含量的变化进行了预测分析:当Drs >>0时含细粒砂土类似于e=ec时均质砂的不排水性状,具有较高的强度;当Drs→0时含细粒砂土最终性状具有不确定性;当Drs < 0时含细粒砂土表现为剪缩性状。将南京砂中粗粒和细粒进行重新配制获得含不同细粒的砂土,对它们进行不排水剪试验;在利用粒间状态变量对试验结果进行分析的基础上,将试验结果与预测分析进行比对,两者的吻合性较高。在ec-logSus空间内稳态线随粉粒的增加而向上移动,且各稳态线间呈平行状。因此,粒间状态变量能较好地反映无黏性土力链的变化,是描述砂土微观结构的新途径。 相似文献
17.
A large strain analysis of undrained expansion of a spherical/cylindrical cavity in a soil modelled as non‐linear elastic modified Cam clay material is presented. The stress–strain response of the soil is assumed to obey non‐linear elasticity until yielding. A power‐law characteristic or a hyperbolic stress–strain curve is used to describe the gradual reduction of soil stiffness with shear strain. It is assumed that, after yielding, the elasto‐plastic behaviour of the soil can be described by the modified Cam clay model. Based on a closed‐form stress–strain response in undrained condition, a numerical solution is obtained with the aid of simple numerical integration technique. The results show that the stresses and the pore pressure in the soil around an expanded cavity are significantly affected by the non‐linear elasticity, especially if the soil is overconsolidated. The difference between large strain and small strain solutions in the elastic zone is not significant. The stresses and the pore pressure at the cavity wall can be expressed as an approximate closed‐form solution. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
18.
Effects of Previous Deviatoric Strain Histories on the Undrained Behaviour of Hostun RF Loose Sand 总被引：1，自引：0，他引：1
The effects of previous deviatoric strain histories on the undrained behaviour of very loose and saturated Hostun RF clean sand are investigated in this paper. From an initial isotropic stress state in the triaxial plane, strain histories are generated by isotropic consolidation followed by standard drained triaxial preshear or presheared cycle, either in compression or in extension, up to a desired value of axial strain or mobilized stress ratio. Deviatoric strain histories are achieved by having nearly the same void ratio at the beginning of the undrained shearing for all tested samples. Subsequent undrained behaviour in triaxial compression and extension is analyzed in detail. Previous deviatoric strain histories can progressively transform the compressive and unstable behaviour of loose sand into a dilative and stable behaviour of dense-like sand, while being loose. Experiments show a common response induced by recent strain histories in terms of effective stress paths, independently of the axial strain attained during the drained presheared cycle, a unique initial gradient of the effective stress paths, a progressive appearance of dilatancy, an evolution the undrained behaviour and a systematic partial static liquefaction associated with softer behaviour when sheared in the opposite direction of the initial presheared direction. This paper offers a comprehensive understanding of the mechanisms of a specialized induced anisotropy created by simple linear stress paths in the classical triaxial plane. 相似文献
19.
We define a flow liquefaction potential for determining flow liquefaction susceptibility during proportional strain triaxial compression. The flow liquefaction potential is a function of inconsistency between the natural dilative tendency of the soil and the imposed dilatancy during proportional strain triaxial compression. It helps us analyze why given the right conditions, a loose soil that contracts during drained triaxial compression and liquefies under undrained triaxial compression may be stable under proportional strain triaxial compression. Conversely, we also use the flow liquefaction potential to analyze why a dense soil that dilates during drained triaxial compression and is stable under undrained triaxial compression may liquefy under proportional strain triaxial compression. The undrained loose case is a special case of proportional strain triaxial compression under which a soil can liquefy. The central objective of this paper was to investigate the origins of flow liquefaction instability. Hence, we also analyze stress evolution during proportional strain triaxial compression and discuss the mechanics of the test leading up to flow liquefaction instability. We arrive at a necessary precursor for instability, which can serve as a warning sign for flow liquefaction instability, while the soil is still stable. The precursor is not a condition of sufficiency and should also not be confused with the onset of instability itself. The same loading must be applied continuously to induce flow liquefaction instability. The current progress is encouraging and facilitates a deeper understanding of origin of flow liquefaction instabilities. 相似文献
20.
Experimental study of poromechanical behavior of saturated claystone under triaxial compression 总被引：1，自引：1，他引：0
This paper presents experimental results of drained and undrained triaxial compression tests of saturated Meuse–Haute/Marne claystone. The emphasis is to study the evolution of pore pressure with growth of microcracks and the effect of pore pressure on mechanical behavior. Basic mechanical responses are first investigated through drained triaxial compression tests, showing nonlinear stress strain relations, volumetric dilatancy and pressure sensitivity. In undrained triaxial compression tests, the pore pressure exhibits a transition from increase to decrease due to the transition from volumetric compressibility to dilatancy caused by the growth of microcracks. The failure surfaces, determined by total stress and Terzaghi’s effective stress under undrained condition, are compared with the one under drained condition. 相似文献