New insights in the thermomechanical modelling of soils

info:eu-repo/semantics/nonPublishe

New basis for the constitutive modelling of aggregated soils

Natural and compacted soils are usually characterized by aggregation of particles. The mechanical behaviour of these materials depends on soil structure. The oedometric compression tests performed on aggregated samples presented here showed that these materials exhibit a yield limit depending not only on stress history and stress state but also on soil structure. Evidence is provided using the neutron tomography technique. These results revealed that soil structure modification occurs together with plastic deformations. The experimental results are used to propose a new state parameter to quantify the soil structure. Based on pore-scale experimental observations, an evolution law for this parameter is proposed as a function of associated plastic strains. Considering both soil fabric and inter-particle bonding effects, a new yield limit depending on stress state, stress history and soil structure is introduced for the aggregated soils. Accordingly, a new constitutive framework consistent with strain hardening plasticity is proposed to consider soil structure effects in the modelling of aggregated soil

An innovative device for determining the soil water retention curve under high suction at different temperatures

To characterise the water retention behaviour of fine soils, high suction values are applied. In this range of values, the vapour equilibrium technique is usually used. This paper presents an innovative device, a sorption bench that permits the determination of the water retention curve of soil. With this new testing method, the time required for testing is significantly reduced. In addition, this apparatus enables the thermal conditions of a test to be controlled; thus, the applied suction can be better controlled, and the water retention curve for different temperatures can be determined. Another valuable aspect of the device is the adopted technical solution that permits weighing of the samples inside the desiccators at any time. Consequently, the water content kinetics can be defined without disturbing the drying or wetting processe

Cyclic thermomechanical response of fine-grained soil-concrete interface for energy piles applications

Understanding the behaviour of soil-structure interfaces is critical for addressing the analysis and design of energy geostructures. In this study, the interface failure mechanism of energy piles (where a shear band is detached from the surrounding soil that behaves under oedometric conditions) is experimentally analysed in laboratory for saturated conditions. The choice of material (clayey soil and concrete), temperature range, and stress level is based on conditions that are likely to be encountered in practice. Specifically, cyclic thermal tests under constant vertical effective stress in oedometric conditions as well as constant normal stiffness (CNS) interface direct shear tests (in which samples have been subjected to thermal cycles between 10 and 40 °C) are presented. From a practical perspective, the results show very low volumetric strain variations and negligible effects on shear strength. The volumetric aspects do not appear to have significant impact on the shear resistance of the interfaces against cyclic thermal loads. Fundamental insight on the effects of thermal cycles on the concrete-soil interface behaviour which are relevant to energy piles are presented. In addition, the proposed interpretation procedure provides a basis for the standardisation of thermomechanical testing in geotechnical engineering

Load transfer approach for the geotechnical analysis of energy piles in a group with slab

Thermally induced group effects characterise closely spaced energy piles. It has been observed experimentally that the behaviour of energy piles subjected to mechanical and thermal loads, in which the piles are located sufficiently close to each other, is different from the behaviour of single isolated piles. Therefore, civil engineers encounter new challenges in the geotechnical design of such foundations. This leads to the necessity to develop practical tools to address their analysis and design. The conventional load transfer method is one of the commonly used methods for the analysis of axially loaded conventional piles. Thus, the purpose of this study has been to propose a formulation of the load transfer method to consider the thermally induced effects among energy piles in groups. The soil response is characterized in a lumped form by ascribing the behavioural features of the soil to interface elements. The individual response, in terms of strain and stress of an energy pile in a group, can be addressed for the first time through the application of the displacement factor in the load displacement curve of the single isolated energy pile. A validation through a full-scale field test reveals the capability of the approach to provide the necessary information in the analysis and design phases of the foundation for one-way thermal loads

A coupled model of mechanical behaviour and water retention for unsaturated soils with double porosity

Many natural soils and engineering geomaterials, such as aggregated soils and compacted clay pallets, exhibit two levels of porosity corresponding to the inter- and intraaggregate pores within their hierarchical structure. Mechanical behavior of these materials, in particular when unsaturated, is an issue of added complexity which should be described an appropriate constitutive framework. A coupled water retention–mechanical constitutive model for unsaturated soils with double porosity is presented here. Based on the multi-scale experimental results, the model incorporates the inter-particle bonding, fabric and partial saturation effects in a single framework. It is formulated within the framework of hardening elasto-plasticity and is based on the critical state concept. The mechanical model is coupled with the water retention law which itself takes into account the two levels of porosity. The coupling is made through the expression of the effective stress and the evolution of the preconsolidation pressure with suction. On the other hand, the mechanical model at the macro-scale is also coupled with the pore-scale behavior of the materials through an internal variable which accounts for the evolution of the soil structure. The model is used for numerical simulation of the behavior of aggregated and bonded soils. Comparison of numerical simulations and the experimental results show that the model can successfully address the main features in the behavior of unsaturated soils with double porosity

Towards a secure basis for the design of geothermal piles

Using pile foundations as heat exchangers with the ground provides an efficient and reliable energy source for the heating and cooling of buildings. However, thermal expansion or contraction of the concrete brings new challenges to the design of such structures. The present study investigates the impact of temperature variation on the mobilised bearing capacities of geothermal piles. The mechanisms driving the variations and redistribution of mobilised bearing forces along geothermal piles are identified using Thermo-Pile software. The EPFL and Lambeth College test piles are modelled and analysed as real-scale experiments. Three simple representative cases are used to investigate the impact of over-sizing geothermal piles on their serviceability. It is found that the mechanisms responsible for the variations and redistribution of mobilised bearing forces along the piles are unlikely to cause geotechnical failure, even if the ultimate bearing force of a pile is reached. Furthermore, over-sizing geothermal piles compared to conventional piles can have a negative impact on their serviceability

Numerical simulation of the non-isothermal mechanical behaviour of soils

Research interest in the thermo-mechanical behaviour of soils is growing as a result of an increasing number of geomechanical problems involving thermal effects. This paper concerns the constitutive modelling of non- isothermal mechanical behaviour of clayey soils. An elastoplastic model is extended to non-isothermal conditions to account for non-linearities and hardening that occurs as a result of heating of soils. This constitutive model includes evolution of the yield limit with temperature through a dependency law for preconsolidation pressure, with respect to heating. Numerical simulations support the theoretical aspects of the paper by demonstrating the ability of this non- isothermal constitutive law to represent the complete behaviour of clayey soils at different temperatures and stress states. Therefore, the model can be used for quantitative predictions of thermo-mechanical behaviour of clayey soils