The coupling effect of initial shear stress and thermal cycles on the thermomechanical behaviour of clay concrete and sand-concrete interfaces has been studied. A set of drained monotonic direct shear tests was conducted at the soil-concrete interface level. Samples were initially sheared to half of the material's shear strength and then they were subjected to five heating/cooling cycles before being sheared to failure. The test results showed that the effect of thermal cycles on the shear strength of the materials was negligible, yet shear displacement occurred during application of thermal cycles without an increase in shear stress, confirming the coupling between the shear stress and temperature. In addition, a slight increase of stiffness due to the coupling was observed which diminished with further shearing.

The effect of temperature on the monotonic and cyclic shearing response of a soil–structure interface is of critical importance for the application of thermal-active geo-structures. To investigate this, soils and soil–concrete interfaces were comprehensively tested with a temperature-controlled direct shear device under both fixed temperatures and thermal/mechanical cycles within the range of 2–38 °C. Monotonic and cyclic shearing with various boundary conditions, including constant normal load (CNL), constant normal stiffness (CNS) and constant volume (CV), were conducted to resemble the conditions that thermal-active-geo-structures may experience. The strength properties of the sand, clay, and sand–concrete and clay–concrete interfaces were partially influenced by heating and cooling under all boundary conditions. However, several effects were observed which could affect the performance of thermo-active structures. Heating cycles caused the clay–concrete interface to be overconsolidated, implying a lower excess pore pressure would be generated during shearing. The cyclic CNS tests suggested that the interface strength could degrade due to (thermally induced) cyclic shear displacements, with this effect strongly related to the state of the soil rather than the temperature directly. In these tests, the medium-dense sand–concrete interface degraded to almost zero shear strength after 5 cycles, whereas the clay–concrete interface asymptotically degraded to around 60% of its strength after 10 cycles.

The formulation of a two surface/bubble thermo-mechanical constitutive model consistent with the principles of thermodynamics is presented. This allows plastic deformations inside the outer yield surface, resulting in a smooth stress–strain prediction and progressive cyclic deformations. This is achieved by the translation of the inner yield surface (also known as the bubble surface) with the stress state of the soil, inside the outer yield surface, by using a kinematic rule. The constitutive equations, including the hardening rules, are derived by specifying a Gibbs-type energy potential and a rate of dissipation potential function, ensuring thermodynamic consistency. The kinematic rule is divided into isothermal and non-isothermal parts. With the isothermal component, the model is capable of capturing the hysteresis behaviour of soils during cyclic mechanical loading. With the non-isothermal part, the model is able to predict the shakedown behaviour of soils observed when they are subjected to heating–cooling cycles. The performance of the model is compared with various experimental data for isothermal and non-isothermal conditions, and is shown to be in good agreement.

The formulation of a new thermo-mechanical constitutive model consistent with the principles of thermodynamics is presented. The model is capable of predicting the rate-independent thermo-mechanical behavior of fine-grained soils. The constitutive equations are derived by defining only a Gibbs-type free energy and a dissipation potential, in accordance with the hyperplasticity method. The addition of thermo-elasticity to the energy potential, and the embedding of the identified thermo-mechanical mechanisms into a newly proposed dissipation potential, enables the model to describe the thermo-mechanical behavior. The proposed dissipation potential eliminates the application of shift stress, which results in a simpler formulation in the context of hyperplasticity. The step-by-step procedure of deriving the equations, as well as a detailed analysis of the parameters of the model, is presented. The performance of the model is shown to be in good agreement with experimental data. A qualitative description of the possible micro-scale mechanisms for fine-grained soils, when subjected to temperature variation, is presented, as a step towards including the mechanisms in the formulation.

This paper presents a new Modified Cam Clay (MCC) type yield function, that is designed for robust and efficient use with implicit stress integration algorithms. The proposed yield function attains non-elliptical (e.g. tear and bullet) shapes, as well as the typical elliptical shape of the MCC model. Like that of MCC, and unlike most other yield functions with non-elliptical shapes available in literature, it is non-singular and unique throughout stress space. The experimental yielding stresses of a wide range of geomaterials have been accurately simulated using the yield surface. The yield function can be used in constitutive models based on classical elasto-plasticity theory.

The numerical implementation of a recently developed thermomechanical constitutive model for fine-grained soils based on hyperelasticity-hyperplasticity theory (Golchin et al. 2020), is presented. A new unconventional implicit stress return mapping algorithm, compatible with elasticity derived from Gibbs (complementary) energy potential, in strain invariant space, is designed and the consistent tangent operator for use in boundary value problems (such as in the finite element method) is derived. It is shown that the rate of convergence of the stress integration algorithm is quadratic. The numerical results are in good agreement with available data from thermomechanical element tests found in literature.