Abstract
This paper presents a new constitutive model on the poroplastic behaviour of earthen materials accounting for stiffness degradation, using the approach of continuum damage mechanics. The poroplastic behaviour is modelled based on the bounding surface plasticity (BSP) theory and the concept of effective stress while isotropic damage is modelled using a scalar variable. Plastic flow and damage evolution occur simultaneously in a coupled process which take into account the impact of suction. The model was successfully validated against results of triaxial compression tests performed at different relative humidities and confining pressures. Despite the relatively small number of material parameters, this model can reproduce the essential features of earthen materials behaviour observed experimentally: suction-induced hardening and stiffening, post-peak softening, as well as the progressive transition from contractant to dilatant volumetric behaviour. Use of the BSP theory allows to reproduce a smooth stress–strain relation as experimentally observed, instead of an abrupt change upon plastic yielding predicted by classic elastoplastic models. Furthermore, the present model also furnishes a quantitative description on the degradation of elastic properties hitherto not accounted for, thanks to the additional scalar damage variable.
Highlights
Unfired crude earth is a hygroscopic porous material that contains generally a small quantity of active clay minerals
The main conclusion drawn from all these experiments is that the mechanical properties of earthen materials, whatever their origin and their implementation technic, are strongly influenced by their water content: even a small increase of the latter leads to a significant reduction of both the failure strength and Young’s modulus E, while the irreversible plastic strain is increased
(2) Seven constants on the plastic behaviour: k0 and C define the position of critical state line (CSL) in the e À ln p0 plane while M defines the slope of CSL in p0 À q plane; n and r specify the shape of bounding surface; h controls the hardening modulus; and m controls the stress-dilatancy
Summary
Unfired crude (untreated) earth is a hygroscopic porous material that contains generally a small quantity of active clay minerals. Apart from plastic deformations and suction effects, a progressive but non-negligible degradation of Young’s modulus E with increasing stress level were observed in recent study [6, 7] This stiffness reduction is conjectured to be a consequence of damage induced by initiation and development of micro-cracks. The above experimental observations motivate the development of a new constitutive model which can simulate the poroplastic behaviour of earthen materials while accounting for damage effects. After a summary of experimental investigations showing the main trends on the mechanical behaviour of earthen materials, a new constitutive model is presented for unsaturated earthen materials It is based on the BSP theory and the effective stress concept. The performance of this model is investigated by comparing the numerical simulation with a series of triaxial tests results on compacted earth samples at different hydraulic conditions and confining pressures
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