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1.
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2.
Gu  Xiaoqiang  Li  Youhong  Hu  Jing  Shi  Zhenhao  Liang  Fayun  Huang  Maosong 《Acta Geotechnica》2022,17(8):3229-3243

Natural clays usually show anisotropic stiffness due to their deposition process and anisotropic in situ stress state. The stiffness anisotropy depends on both of the stress anisotropy and fabric anisotropy, while the latter can be quantified by the stiffness anisotropy at isotropic stress states. This paper measures the K0 value (i.e., stress anisotropy) and elastic shear stiffness anisotropy of natural Shanghai clay in a triaxial apparatus with horizontal and vertical bender elements. The results show that the K0 value of Shanghai clay lies in the range of 0.40–0.66, and an empirical equation is proposed to estimate the K0 value based on the plasticity index and initial void ratio. The fabric anisotropy of natural Shanghai clay lies in the range of 1.2–1.4 with a stronger fabric in the horizontal plane. Moreover, the experimental data of the stiffness anisotropy and fabric anisotropy of different clays in the literature are reviewed and analyzed. It reveals that the stiffness anisotropy generally increases, while the fabric anisotropy remains nearly the same during K0 consolidation. For normally consolidated clay, the fabric anisotropy generally lies in the range of 1.1–1.7. For overconsolidated clays, the fabric anisotropy generally increases as the overconsolidation ratio increases. Empirical equations are proposed to approximately estimate the fabric anisotropy of clays based on its stress normalized elastic shear stiffness.

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3.
The porosity of soils is considered to be a directional measure and its distribution is characterized by a functional form. This form has been used to extend the critical state soil mechanics framework to include the effects of structure in soils. A new internal plastic energy dissipation formulation has been proposed to account for fabric arrangement. New expressions for the yield locus, and the plastic stress–strain response of structural soils have been derived. The applicability of the concepts to model the plastic stress–strain behaviour of a number of soils is illustrated. The advantage of the new model is very well identified in modelling the stress–strain behaviour of K0 consolidated and natural clays. © 1998 John Wiley & Sons, Ltd.  相似文献   

4.
This paper presents a new generalized effective stress model, referred to as MIT-S1, which is capable of predicting the rate independent, effective stress–strain–strength behaviour of uncemented soils over a wide range of confining pressures and densities. Freshly deposited sand specimens compressed from different initial formation densities approach a unique condition at high stress levels, referred to as the limiting compression curve (LCC), which is linear in a double logarithmic void ratio, e, mean effective stress space, p′. The model describes irrecoverable, plastic strains which develop throughout first loading using a simple four-parameter elasto-plastic model. The shear stiffness and strength properties of sands in the LCC regime can be normalized by the effective confining pressure and hence can be unified qualitatively, with the well-known behaviour of clays that are normally consolidated from a slurry condition along the virgin consolidation line (VCL). At lower confining pressures, the model characterizes the effects of formation density and fabric on the shear behaviour of sands through a number of key features: (a) void ratio is treated as a separate state variable in the incrementally linearized elasto-plastic formulation: (b) kinematic hardening describing the evolution of anisotropic stress–strain properties: (c) an aperture hardening function controls dilation as a function of ‘formation density’; and (d) the use of a single lemniscate-shaped yield surface with non-associated flow. These features enable the model to describe characteristic transitions from dilative to contractive shear response of sands as the confining pressure increases. This paper summarizes the procedures used to select input parameters for clays and sands, while a companion paper compares model predictions with measured data to illustrate the model capability for describing the shear behaviour of clays and sands. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

5.
A compiled database of shear wave velocity measurements in a variety of clays, silts and sands shows directional hierarchies between downhole (VsVH), standard crosshole (VsHV), and rotary crosshole (VsHH) tests. The special in situ database has been collected from 33 well-documented geotechnical test sites. Expressions relating the small-strain shear modulus in terms of effective confining stress level, stress history and void ratio are explored for each of these three modes of directional shear wave velocity. The relationships are separated initially into soil groups (intact clays, fissured clays, sands and silts), and then generalised to consider all soil types together.  相似文献   

6.
Consolidation of sensitive clays: a numerical investigation   总被引：1，自引：1，他引：0  
Consolidation of sensitive clay layers, which have significant compressibility at different stress states, is investigated via a nonlinear one-dimensional consolidation approach with a piecewise linear e ~ log10σ′ model. The behaviour of sensitive clays during consolidation and the limitations of conventional consolidation theory are addressed. It is shown that (1) the sensitive clay layer can be divided by the preconsolidation pressure into two zones, that is, high- and low-compressibility zones. The progressive destruction of particle cementation bonding through the soil layer is shown by the moving front of the interface between these two zones; (2) the excess pore pressure dissipation primarily takes place in the low-compressibility zone, which results in a small settlement during the early stages of consolidation; (3) conventional consolidation theory highly overestimates the settlement and gives a poor prediction of effective stress distribution.  相似文献   

7.
The purpose of this paper is to present a simple, unified critical state constitutive model for both clay and sand. The model, called CASM (Clay And Sand Model), is formulated in terms of the state parameter that is defined as the vertical distance between current state (v, p′) and the critical state line in v–ln p′ space. The paper first shows that the standard Cam-clay models (i.e. the original and modified Cam-clay models) can be reformulated in terms of the state parameter. Although the standard Cam-clay models prove to be successful in modelling normally consolidated clays, it is well known that they cannot predict many important features of the behavior of sands and overconsolidated clays. By adopting a general stress ratio-state parameter relation to describe the state boundary surface of soils, it is shown that a simple, unified constitutive model (CASM) can be developed for both clay and sand. It is also demonstrated that the standard Cam-clay yield surfaces can be either recovered or approximated as special cases of the yield locus assumed in CASM. The main feature of the proposed model is that a single set of yield and plastic potential functions has been used to model the behaviour of clay and sand under both drained and undrained loading conditions. In addition, it is shown that the behaviour of overconsolidated clays can also be satisfactorily modelled. Simplicity is a major advantage of the present state parameter model, as only two new material constants need to be introduced when compared with the standard Cam-clay models. © 1998 John Wiley & Sons, Ltd.  相似文献   

8.
SANICLAY is a new simple anisotropic clay plasticity model that builds on a modification of an earlier model with an associated flow rule, in order to include simulations of softening response under undrained compression following Ko consolidation. Non‐associativity is introduced by adopting a yield surface different than the plastic potential surface. Besides, the isotropic hardening of the yield surface both surfaces evolve according to a combined distortional and rotational hardening rule, simulating the evolving anisotropy. Although built on the general premises of critical state soil mechanics, the model induces a critical state line in the void ratio–mean effective stress space, which is a function of anisotropy. To ease interpretation, the model formulation is presented firstly in the triaxial stress space and subsequently, its multiaxial generalization is developed systematically, in a form appropriate for implementation in numerical codes. The SANICLAY is shown to provide successful simulation of both undrained and drained rate‐independent behaviour of normally consolidated sensitive clays, and to a satisfactory degree of accuracy of overconsolidated clays. The new model requires merely three constants more than those of the modified Cam clay model, all of which are easily calibrated from well‐established laboratory tests following a meticulously presented procedure. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

9.
A critical state model for overconsolidated structured clays   总被引：1，自引：0，他引：1  
This paper presents a generalised critical state model with the bounding surface theory for simulating the stress–strain behaviour of overconsolidated structured clays. The model is formulated based on the framework of the Structured Cam Clay (SCC) model and is designated as the Modified Structured Cam Clay with Bounding Surface Theory (MSCC-B) model. The hardening and destructuring processes for structured clays in the overconsolidated state can be described by the proposed model. The image stress point defined by the radial mapping rule is used to determine the plastic hardening modulus, which varies along loading paths. A new proposed parameter h, which depends on the material characteristics, is introduced into the plastic hardening modulus equation to take the soil behaviour into account in the overconsolidated state. The MSCC-B model is finally evaluated in light of the model performance by comparisons with the measured data of both naturally and artificially structured clays under compression and shearing tests. From the comparisons, it is found that the MSCC-B model gives reasonable good simulations of mechanical response of structured clays in both drained and undrained conditions. With its simplicity and performance, the MSCC-B model is regarded as a practical geotechnical model for implementation in numerical analysis.  相似文献   

10.
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11.
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.  相似文献   

12.
Compression and consolidation anisotropy of some soft soils   总被引：1，自引：1，他引：1  
The compression and consolidation anisotropy of 11 soft soils were studied by conducting oedometer tests on sets of duplicate undisturbed specimens prepared in the vertical and horizontal directions from adjacent sections of carefully sampled borehole cores. The one-dimensional compression, yield and creep characteristics of the various silts, clays and amorphous peaty material tested were similar for the vertical and horizontal directions. The exception was the structured, coarse fibrous peaty material which was strongly cross-anisotropic. Drainage occurred more rapidly in the horizontal direction with horizontal-to-vertical permeability ratios r k of 1.0–1.7. Higher r k values were associated with more marked fabrics, namely for clays with fine sand partings, fibrous organic inclusions or fine root-holes and the laminated silts. The r k value was for practical purposes independent of the stress level. Brendan C. O’Kelly: Formerly Scott Wilson Consulting Engineers, UK  相似文献   

13.
The yield vertex non‐coaxial theory is implemented into a critical state soil model, CASM (Int. J. Numer. Anal. Meth. Geomech. 1998; 22 :621–653) to investigate the non‐coaxial influences on the stress–strain simulations of real soil behaviour in the presence of principal stress rotations. The CASM is a unified clay and sand model, developed based on the soil critical state concept and the state parameter concept. Without loss of simplicity, it is capable of simulating the behaviour of sands and clays within a wide range of densities. The non‐coaxial CASM is employed to simulate the simple shear responses of Erksak sand and Weald clay under different densities and initial stress states. Dependence of the soil behaviour on the Lode angle and different plastic flow rules in the deviatoric plane are also considered in the study of non‐coaxial influences. All the predictions indicate that the use of the non‐coaxial model makes the orientations of the principal stress and the principal strain rate different during the early stage of shearing, and they approach the same ultimate values with an increase in loading. These ultimate orientations are dependent on the density of soils, and independent of their initial stress states. The use of the non‐coaxial model also softens the shear stress evolutions, compared with the coaxial model. It is also found that the ultimate shear strengths by using the coaxial and non‐coaxial models are dependent on the plastic flow rules in the deviatoric plane. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

14.
Ultra-soft soil with high moisture content will experience large strain deformation under one-dimensional compression with little or no gain in effective stress. Such deformation behaviour does not comply with Terzaghi’s effective stress gain theory. The e-log sv￠ \sigma_{v}^{\prime } relationship of ultra-soft soil is non-linear with large compression index in the first order of log cycle. This paper proposes three compression indices (Cc1* C_{c1}^{*} , Cc2* C_{c2}^{*} and Cc3* C_{c3}^{*} ) for stresses covering three log cycles. Good prediction of settlement magnitude is possible with these newly proposed compression parameters for ultra-soft soil. In addition, implicit finite difference model applying the large strain theory is also proposed and validated with results from laboratory measurements. The time factor curves for ultra-soft soil with large strain compression are also proposed and validated.  相似文献   

15.
On the basis of a double hardening model for clays and available experimental results, a new thermo-elasto-plastic constitutive model for saturated clays is proposed to describe the effects of temperature and overconsolidation ratio on the mechanical properties of saturated clays. Two hardening parameters are introduced: sc￠ {\sigma}_{{\rm c}}^{\prime} and α. The proposed model is then applied to simulate the relevant important features of saturated clays with different overconsolidation ratios under different temperature and loading conditions. The model predictions are compared with available experimental results to demonstrate its accuracy and usefulness.  相似文献   

16.
Dense sands, overconsolidated clays and soft rocks exhibit strain-hardening and strain-softening behaviour in a certain range of confining pressure. The aim of the present paper is to construct a constitutive model of soft rock that can describe not only the strain-hardening behaviour, but also the strain-softening behaviour. An elasto-plastic constitutive model for soft rock is derived by introducing a stress history tensor
  • 1 The preliminary idea was first reported in the reference, the conference paper16. This paper is a complete version of the theory with interpretations and experimental validations
    • . The model is evaluated through a comparison of the calculated results and the experimental results of tests on sedimentary soft rock (porous tuff). In addition, the applicability of the model to numerical analyses is discussed in relation to the uniqueness of the solution in initial and boundary value problems.  相似文献   

    17.
    This paper briefly reviews several calculation methods to evaluate the bearing capacity expressed in terms of undrained strength (cu) of piles bored in clay for their entire length and of piles whose tip is embedded into weak rock. The scope of the paper is to compare the results obtained with those from full-scale pile tests. These tests were carried out within the city of Matera which is well studied from a geotechnical point of view and for which there are statistically significant data on the geomechanical properties of the Subapennine Blue clays and the underlying Gravina Calcarenites. This represents the first attempt to show, on the basis of laboratory and field data, the influence of variability of the above mentioned soils and rocks on the real behaviour of deep foundations.For piles completely bored into Matera clay, the calculation in terms of total stress are able to interpret sufficiently well the bearing capacity of the piles. For piles having their toe embedded in calcarenite, the variability of the strength of the weak rock presents greater uncertainties in the calculation of base and soft resistance.  相似文献   

    18.
    When a cut slope in a saturated clay is undertaken, a transient water flow occurs and stress transferences from the water to the soil skeleton take place in time (consolidation). Mainly in strongly overconsolidated clays, these stress transferences may determine swelling of soil and therefore reduction of its shear strength in time. However, the lowering of the water level associated to the cut increases effective mean stress, which may therefore counterbalance the above-mentioned effect. In the paper, the behaviour of a cut slope in an overconsolidated clay is analysed by a finite element program that incorporates the Biot consolidation theory (coupled analysis), with constitutive relations simulated by the p–q–θ critical state model. In addition, the variation in time of the overall stability is assessed with a computer program that uses the finite element results and formulations of the critical state soil mechanics. In order to achieve a more complex geotechnical interpretation of the problem, the analysis in time of the excess pore pressures, effective stresses, displacements and stress levels is also presented. Finally, comparisons of stability results are analysed by changing some parameters, namely the problem geometry (weight of excavated soil) and the over-consolidation ratio of the clay.  相似文献   

    19.
    Piezocone soundings are a fast and economical approach for geotechnical site characterization, providing three separate and continuous channels of data with depth, including: tip resistance q T, porewater pressure u 2 and sleeve friction f s. Literally hundreds to thousands of data points are collected by a single sounding. Since these readings are functions of both soil type and soil behaviour, they can be used for the delineation of soil stratigraphy.

    One way to process large amounts of data involves clustering. Cluster analysis is an efficient statistical way to analyse the stratigraphic vertical profiling of geomaterials and means to detect the inherent similarity between data sets and group them together. Clustering in previous geotechnical research was based on only two channels of piezocone data (q T and u2). The method works well for soils that are under the groundwater table and was applied to soundings in clay deposits.

    In the present paper, a new cluster analysis approach is developed based on all three channels of data, thus extending the method to soils above the water table and applicable to sands, silts, and clays. Example soil profiles derived by three-channel cluster analysis are presented herein and compared with conventional soil boring and sampling data.  相似文献   

    20.
    Data from three‐dimensional experiments performed on sand in true triaxial equipment have been reviewed to sort out apparent disarray resulting from their interpretation. This has been done based on analyses made possible by recent developments and understanding of factors influencing sand behaviour: occurrence of shear banding, boundary conditions and/or specimen slenderness ratio, cross‐anisotropy, and stability of experimental technique. These factors are reviewed and test data from the literature are evaluated. Experimental data are divided into three groups in which: (a) homogeneous behaviour controls the sand strength; (b) shear banding affects the shape of the three‐dimensional failure surface in the midrange of values of b=(σ2?σ3)/(σ1?σ3), and (c) the data has been misinterpreted. Appropriate interpretation of three‐dimensional strength data for sand exhibiting isotropic, homogeneous behaviour is represented by a smoothly rounded triangular failure surface expressible in terms of the first and third stress invariants. Shear banding effects will cause the failure surface to be ‘indented’ in the midrange of b‐values in all sectors of the octahedral plane. Effects of cross‐anisotropy will result in lower strengths in sector III than in sector I of the octahedral plane, and the failure surface will appear as rotated around the stress origin in principal stress space. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

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