A rate-independent constitutive model for natural clays is presented, formulated within the framework of kinematic hardening with elements of bounding surface plasticity. The modelling framework is intended to include effects of damage to structure caused by irrecoverable plastic strains caused by sampling, laboratory testing, or geotechnical loading. The incorporation of a structure measure allows the size of the bounding surface to decay with plastic deformations. This model can be seen as a logical extension from the Cam-clay model. The steady fall of stiffness with strain towards the Cam-clay value is controlled by a particular interpolation function. This ensures a smooth degree of approach between a kinematically hardening bubble (which is the boundary of the elastic region) and the bounding surface during their relative translation with stress history. The model describes the essential phenomena of pre-failure behaviour of natural clays: stiffness variation with strain, volumetric change accompanying distortion, peak strength at small strains. Illustrative numerical results for common geotechnical experiments on a low-sensitivity Swedish clay demonstrate the potential of the proposed model.

h i g h l i g h t sThe thermal conductivity of a fine-grained sand has been investigated. The measurements were carried out using a steady state divided bar method. The degree of saturation has a significant effect on the sand thermal conductivity. A selected prediction models were validated against the experimental results. A new empirical model based on water content and porosity has been proposed. a b s t r a c tThe thermal properties of soils are of great importance in many thermo-active ground structures such as energy piles and borehole heat exchangers. In this paper the effect of the porosity and degree of saturation on the thermal conductivity of a sandy soil that has not been previously thermally tested is investigated using steady state experimental tests. The steady state apparatus used in these tests was designed to provide high performance in controlling all boundary conditions. Twenty thermal conductivity experimental tests have been carried out at different porosity and saturation values. The performance of selected prediction methods have been validated against the experimental results. The validation shows that none of the selected models can be used effectively in predicting the thermal conductivity of Tripoli sand at all porosity and saturation values. However, some can provide good agreement at dry or nearly dry condition while others perform well at high saturations. The performance of most of the selected models also increases as the soil approaches a two phase state where conduction plays the dominant role in controlling heat transfer. An empirical equation of thermal conductivity expressed as a function of water content and porosity has been developed based on the experimental results obtained.

SUMMARYIn this paper, two complex critical-state models are implemented in a displacement finite element code. The two models are used for structured clays and sands, and are characterized by multiple yield surfaces, plastic yielding within the yield surface, and complex kinematic and isotropic hardening laws. The consistent tangent operators}which lead to a quadratic convergence when used in a fully implicit algorithm}are difficult to derive or may even not exist. The stress integration scheme used in this paper is based on the explicit Euler method with automatic substepping and error control. This scheme employs the classical elastoplastic stiffness matrix and requires only the first derivatives of the yield function and plastic potential. This explicit scheme is used to integrate the two complex critical-state models}the sub/superloading surfaces model (SSLSM) and the kinematic hardening structure model (KHSM). Various boundary-value problems are then analysed. The results for the two models are compared with each other, as well with those from standard Cam-clay models. Accuracy and efficiency of the scheme used for the complex models are also investigated.

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This paper reviews a method of accurately predicting changes in pore water pressure within a slope as a response to a given climate. Observations are made on the importance of correctly determining the hydraulic parameters and correctly assessing local climate conditions. The mechanical response of the slope to pore water pressure changes is modelled and it is shown how closely deformation magnitude and rate relates to climate. A diagnostic railway embankment is subjected to a simulated future climate scenario in which rain events are of higher intensity but are less persistent and average temperatures increase significantly. These climate changes are shown to reduce infiltration and increase evaporation, resulting in negative pore water pressures persisting throughout the winter months and providing improved stability.