Behaviour Of Natural Soft Clays Research Articles

Describing inherently anisotropic behaviours of natural clay by a hypoplastic model

The purpose of this paper is to model the loading direction‐dependent behaviours of inherently anisotropic intact clay by means of the hypoplastic framework. The basic hypoplastic model for clays proposed by Mašín (2013) is enhanced by incorporating a predefined anisotropic asymptotic state boundary surface formulation which is based on an anisotropic variable by projecting the microstructure tensor onto the normal of the spatially mobilized plane. The capability of the proposed hypoplastic model is first illustrated by simulations of intact Wenzhou soft clay under drained torsional shear tests using the hollow cylinder apparatus. The model is then used to predict undrained shear results on San Francisco Bay mud. Comparisons between the predicted and measured results demonstrate that the proposed hypoplastic model is capable of modelling the combined effect of the principal stress direction and intermediate principal stress on the stress‐strain behaviour of natural soft clay with inherent anisotropy.

1-D compressibility behaviour of cement-lime stabilized soft clays

This paper addresses the effect of chemical stabilisation on the one-dimensional compression behaviour of natural soft clay, based on incremental loading oedometer tests. The effect of cement-lime mixture with curing period from 1 to 28 days is evaluated for various dosages ranging from 1 to 13% by dry mass of clay. An important effect of cement-lime stabilisation is the development of cementation-induced yielding stress, associated with the initial void ratio, position and shape of compression curves as well as the ability of sustaining higher effective consolidation stress. The result provides the effect of cement-lime treatment on the compression index, coefficient of consolidation and hydraulic conductivity. The role played by cement-lime dosage is investigated in the evolution of compression index, creep coefficient and ratio of compression index to creep coefficient. The time-dependency of creep coefficient is studied on stabilised clays with various cement-lime dosages. The compression results are complemented by the stress-dependent analysis of creep coefficient and compression modulus, which provides insight into the influence of effective stress levels on treated clays with different dosages of cement-lime. This study provides a reference to analyse the effect of cement-lime stabilisation on natural soft clays in the practice of soft ground improvement.

A procedure for determining long-term creep rates of soft clays by triaxial testing

The paper presents a procedure for determining a significantly larger range of creep rates (isotaches) of soft clays than found from standard incremental loading oedometer tests. The test is performed in a triaxial apparatus by first loading up to the virgin compression line at a relevant effective stress level and then determine the creep rates at different stress states along an unloading branch keeping the horizontal to vertical effective stress ratio constant. The test procedure is tested on a soft Norwegian plastic marine clay. Challenges due to the non-linear behaviour of natural soft clays where the standard creep and compression indexes are not constant values are demonstrated, and recommendations for how to account for this effect are given. The obtained results from the test are also compared with results from a parallel long-term creep phase in a standard oedometer apparatus taken from the same block sample.

An isotach elastoplastic constitutive model for natural soft clays

The time and strain rate dependency observed in natural soft clays is formulated within the framework of conventional elastoplasticity. Creep of soft clays is essentially like the response of an overdamped oscillatory system, i.e., the strain rate decays in an exponential manner. A characteristic strain rate and time relationship is presented based on data from creep tests on a large number of different soft clays. The evolutionary change of strain rate is found to affect the mechanical response of soft clays in an isotach manner. Taking strain rate as another stress-like variable, a loading–isotach (LI) yield curve is proposed, which describes the combined hardening mechanisms of loading and variation of strain rate. Incorporation of this LI yield curve into critical state soil mechanics results in an isotach elastoplastic (IEP) model in triaxial stress–strain–strain rate space, which has been dubbed ‘Hunter Clay’. The effects of fabric anisotropy and inter-particle cementation, which are typical features of natural soft clays, are also introduced to produce an advanced hierarchical constitutive model for soft clay. Qualitative predictions are first described and compared with the characteristic behaviour of natural soft clays. Experimental validations using test data for two soft clays are then carried out, and comparisons of the model predictions and experimental data demonstrate the capability of the model in reproducing realistic behaviour of natural soft clays. This work confirms that the complex mechanical behaviour of natural soft clays can be reproduced satisfactorily within the general framework of classical plasticity theory.

Constitutive modelling of Otaniemi soft clay in both natural and reconstituted states

A rate-independent anisotropic elastoplastic constitutive model for natural soft clays is proposed, with emphasis on the effects of cementation or structure and fabric anisotropy. The characteristic non-linear normal compression line for structured soils is described by defining a variable compressibility as a function of plastic straining. No reference to the ‘intrinsic’ reconstituted state is required. The effect of plastic anisotropy is investigated from virgin constant stress ratio consolidation tests, which provide an accurate description of the equilibrium states of soil fabric. The change of fabric anisotropy is correlated with the change of inclination of the yield and plastic potential surfaces via a pair of improved rotational hardening laws. Experiments on Otaniemi clay samples in both natural and reconstituted states are simulated. Comparisons of numerical predictions with experimental data illustrate the feasibility and advantage of the proposed methodology to describe the mechanical behaviour of natural soft clays.

Modelling strain-rate-dependency of natural soft clays combined with anisotropy and destructuration

The paper aims to investigate modelling the strain-rate-dependency of natural soft clays combined with anisotropy and destructuration using an elasto-viscoplastic model. The model is based on Perzyna's overstress theory and the elastoplastic model S-CLAY1S. Tests at constant strain-rate and creep tests under both one-dimensional and triaxial conditions on several clays are simulated. Simulations highlight the loading scenarios in which it is necessary to account for anisotropy and/or destructuration in order to get accurate predictions. Comparisons between the predicted and measured results demonstrate that the proposed model can successfully reproduce the time-dependent behaviour of natural soft clays under different loading conditions.

An anisotropic elastic–viscoplastic model for soft clays

Experimental evidences have shown deficiencies of the existing overstress and creep models for viscous behaviour of natural soft clay. The purpose of this paper is to develop a modelling method for viscous behaviour of soft clays without these deficiencies. A new anisotropic elastic–viscoplastic model is extended from overstress theory of Perzyna. A scaling function based on the experimental results of constant strain-rate oedometer tests is adopted, which allows viscoplastic strain-rate occurring whether the stress state is inside or outside of the yielding surface. The inherent and induced anisotropy is modelled using the formulations of yield surface with kinematic hardening and rotation (S-CLAY1). The parameter determination is straightforward and no additional experimental test is needed, compared to the Modified Cam Clay model. Parameters determined from two types of tests (i.e., the constant strain-rate oedometer test and the 24 h standard oedometer test) are examined. Experimental verifications are carried out using the constant strain-rate and creep tests on St. Herblain clay. All comparisons between predicted and measured results demonstrate that the proposed model can successfully reproduce the anisotropic and viscous behaviours of natural soft clays under different loading conditions.

Numerical modelling of the behaviour of natural soft clay

Numerical modelling of the behaviour of natural soft clay

Time effects on the stress-strain behaviour of natural soft clays

Laboratory tests on a wide variety of lightly overconsoli-dated natural clays show that important engineering properties such as undrained strength and preconsolidation pressure are time-dependent. Relaxation and step-changing procedures suggest that these properties decrease by 10–20% with a tenfold decrease in the speed of testing. The influence of strain rate on undrained shear strength appears to be independent of soil plasticity, test type or stress history during laboratory reconsolidation. Its influence on stress-compressibility curves from oedo-meter tests is independent of soil plasticity and test conditions. Ageing and delayed compression in clays produce a stiffer arrangement of the soil particles, and a quasi-preconsolidation pressure that is in part available for foundation design. The results from a series of conventional tests and stress-probe tests on two different clays show that yield envelopes in lightly overconsoli-dated natural clays are time-dependent, and contract in p&rime, q-space...

Time effects on the stress-strain behaviour of natural soft clays : Graham, J; Crooks, J H A; Bell, A L Geotechnique, V33, N3, Sept 1983, P327–340

Time effects on the stress-strain behaviour of natural soft clays : Graham, J; Crooks, J H A; Bell, A L Geotechnique, V33, N3, Sept 1983, P327–340

Time effects on the stress-strain behaviour of natural soft clays

Laboratory tests on a wide variety of lightly overconsoli-dated natural clays show that important engineering properties such as undrained strength and preconsolidation pressure are time-dependent. Relaxation and step-changing procedures suggest that these properties decrease by 10–20% with a tenfold decrease in the speed of testing. The influence of strain rate on undrained shear strength appears to be independent of soil plasticity, test type or stress history during laboratory reconsolidation. Its influence on stress-compressibility curves from oedo-meter tests is independent of soil plasticity and test conditions. Ageing and delayed compression in clays produce a stiffer arrangement of the soil particles, and a quasi-preconsolidation pressure that is in part available for foundation design. The results from a series of conventional tests and stress-probe tests on two different clays show that yield envelopes in lightly overconsoli-dated natural clays are time-dependent, and contract in p&rime, q-space with increasing time. Des essais faits en laboratoire sur une grande variété d'argiles naturelles légèrement surconsolidées indiquent que des caractéristiques importantes, telles que la résistance non-drainée et la pression de préconsolidation dépendent du temps. Des méthodes de relaxation et de changement d'étapes donnent l'impression que ces propriétés décroissent de 10% à 20% lorsque la vitesse de l'essai chutede 90%. L'influence de la vitesse de déformation sur la résistance à la rupture au cisaillement dans l'état non-drainé semble être indépendante de la plsticité du sol, du type d'essai et de la déformation précédente pendant la reconsolidation en laboratoire. Son influence sur les courbes de déformation/compressibilité obtenues par des essais à l'oedomètre est indépendante de la plasticité du sol et des conditions de l'essai. La compression retardée et de vieillissement dans les argiles produit un arrangement plus raide des particules du sol et une pression de quasi-préconsolidation qui est en partie disponible pour le calcul des fondations. Les résultats d'une série d'essais conventionnels et d'essais pour étudier la contrainte faits sur deux argiles différentes démontrent que dans les argiles naturelles légèrement surconsolidées les enveloppes de fluage dépendent du temps et se contractent dans l'espace p', qà mesure que le temps s'augmente.