Analysis of unsaturated materials hydration incorporating the effect of thermo-osmotic flow

The geological disposal of a high level radioactive waste relies in a system composed of engineered and geological barriers. The soils and rocks involved in the design of this type of solution are generally initially unsaturated and subject to complex thermal, hydraulic and mechanical (THM) coupled phenomena triggered by the simultaneous heating and hydration of the barrier materials under confined conditions. Mathematical THM formulations are typically used to analyze the behavior and long term performance of the barriers system. These types of formulations generally do not include some coupled processes, for example thermo-osmosis (i.e. the movement of liquid water induced by gradient of temperature), because they are considered not significant when compared against the main or direct processes (e.g., Darcy’s, Fourier’s and Fick’s laws). In this work, the potential effects of thermo-osmotic phenomenon is studied in detail. Typical flow equations are modified to include thermo-osmotic flows and then they are implemented in numerical simulators. Two case studies are analyzed. The first one focuses on a simple and already proposed model to study the behavior of a geological barrier for nuclear waste when subjected to heating and hydration. The other case corresponds to the study of an engineered clay barrier material in the laboratory subjected to hydraulic and thermal gradients similar to the ones expected in real repository conditions. In both cases the analyses with and without thermo-osmotic flows are compared. From these comparisons it is observed that the effect of thermo-osmosis can be quite significant. Thermo-osmotic effects also assisted to explain the apparent low wetting observed in the hydration of a clayey barrier material

Back-calculation of soil parameters from displacement-controlled cavity expansion under geostatic stress by FEM and machine learning

Estimating soil properties from the mechanical reaction to a displacement is a common strategy, used not only in in situ soil characterization (e.g., pressuremeter and dilatometer tests) but also by biological organisms (e.g., roots, earthworms, razor clams), which sense stresses to explore the subsurface. Still, the absence of analytical solutions to predict the stress and deformation fields around cavities subject to geostatic stress, has prevented the development of characterization methods that resemble the strategies adopted by nature. We use the finite element method (FEM) to model the displacement-controlled expansion of cavities under a wide range of stress conditions and soil properties. The radial stress distribution at the cavity wall during expansion is extracted. Then, methods are proposed to prepare, transform and use such stress distributions to back-calculate the far field stresses and the mechanical parameters of the material around the cavity (Mohr-Coulomb friction angle ϕ, Young’s modulus E). Results show that: (i) The initial stress distribution around the cavity can be fitted to a sum of cosines to estimate the far field stresses; (ii) By encoding the stress distribution as intensity images, in addition to certain scalar parameters, convolutional neural networks can consistently and accurately back-calculate the friction angle and Young’s modulus of the soil

Modeling root system growth around obstacles

State-of-the-Art models of Root System Architecture (RSA) do not allow simulating root growth around rigid obstacles. Yet, the presence of obstacles can be highly disruptive to the root system. We grew wheat seedlings in sealed petri dishes without obstacle and in custom 3D-printed rhizoboxes containing obstacles. Time-lapse photography was used to reconstruct the wheat root morphology network. We used the reconstructed wheat root network without obstacle to calibrate an RSA model implemented in the R-SWMS software. The root network with obstacles allowed calibrating the parameters of a new function that models the influence of rigid obstacles on wheat root growth. Experimental results show that the presence of a rigid obstacle does not affect the growth rate of the wheat root axes, but that it does influence the root trajectory after the main axis has passed the obstacle. The growth recovery time, i.e. the time for the main root axis to recover its geotropism-driven growth, is proportional to the time during which the main axis grows along the obstacle. Qualitative and quantitative comparisons between experimental and numerical results show that the proposed model successfully simulates wheat RSA growth around obstacles. Our results suggest that wheat roots follow patterns that could inspire the design of adaptive engineering flow networks

Dystopia as Liberation: Disturbing Femininities in Contemporary Thailand

Despite the stereotypical, outsider view of Thailand as a thriving hub of international sex tourism, traditional and local constructions of Thainess instead privilege the position of the ‘good’ Thai woman—a model of sexual propriety, demure physicality and aesthetic perfection. This is the image of femininity that is heralded by Thailand's Tourist Authority and by government agencies alike as a marketable symbol of cultural refinement and national pride. But this disturbing ‘utopian’ construction of femininity might for some be considered a dystopia shaped by forms of power centred on elite urban rule. In mainstream definitions of Thainess, the monstrous and grotesque inverses of ‘good’ womanhood are located in the ‘dystopian’ visions of rural-based folk traditions that abound with malevolent female spirits and demons, and in the contemporary Thai horror films that draw on these tropes. Adopted by Thai feminists and by street protestors in Bangkok at times of recent political unrest, portrayals of a ‘monstrous-feminine’ have been adopted as central to a carnivalesque strategy of response and resistance to elite discourses of control. Such forces serve to symbolically disturb and destabilise middle-class constructions of a Utopian vision of Thainess with Bangkok as its cultural core. This paper examines instances of how and why the counter-strategy of primitivism and monstrosity has developed, and the extent to which it translates ‘dystopian’ expressions of female sexuality in new imaginaries of ‘dystopia’ as a space of liberation from stultifying cultural and political norms

Analysis of friction induced thermo-mechanical stresses on a heat exchanger pile in isothermal soil

Copyright © 2014 SpringerIn most analytical and numerical models of heat exchanger piles, strain incompatibilities between the soil and the pile are neglected, and axial stresses imposed by temperature changes within the pile are attributed to the thermal elongation and shortening of the pile. These models incorporate thermo-hydro-mechanical couplings in the soil and within the pile foundation, but usually neglect thermo-mechanical couplings between the two media. Previous studies assume that the stress changes imposed by temperature variations in a heat exchanger pile are mainly due to the constrained thermal elongation and shortening of the pile. Also, several recent approaches utilize spring models that focus only on the soil-pile interface in modeling temperature-induced stresses in a heat exchanger pile and implicitly ignore the effect of the full displacement field on soil-pile interaction. By contrast, in this paper, interface elements are introduced in a numerical model of a heat exchanger pile, analyzed in axisymmetric and stationary conditions. The pile is subjected to a uniform temperature increase, with free top and fixed top conditions in elastic and elasto-plastic soil profiles. Simulation results show that the constrained vertical elongation is the most detrimental factor for pile foundation performance. However it is worth noticing that while mechanical constraints (e.g., fixed top and/or fixed bottom) impose maximum stress increases at the ends of the pile , interface effects result in maximum stresses around the mid-length of the pile. This preliminary study indicates that soil-pile friction does not increase pile internal stresses to the point where it would be necessary to over-dimension the foundation pile for heat exchanger use. Furthermore, one cannot expect a significant gain in foundation performance due to the improvement of soil-pile frictional resistance as a result of increased lateral stresses at soil-pile contact. Additional numerical analyses are ongoing, in order to investigate the role of the degree of fixity induced by the building on the heat exchanger pile, and to extend these preliminary analyses to transient operational modes and cyclic thermo-mechanical loading of the heat exchanger pile

Using Microstructure Descriptors to Model Thermo-mechanical Damage and Healing in Salt Rock

Presented at the 48th US Rock Mechanics/Geomechanics Symposium of the American Rock Mechanics Association (ARMA), Minneapolis, MN, 1-4 June 2014.Copyright © 2014 by the American Rock Mechanics AssociationCreep processes in halite (salt rock) include glide, cross-slip, diffusion and dynamic recrystallization. Diffusive Mass Transfer (DMT) can result in crack rebonding, and mechanical stiffness recovery. On the one hand, viscoplastic laws relating creep microscopic processes to microstructure changes are empirical. On the other hand, theoretical models of damage and healing disconnect thermodynamic variables from their physical meaning. The proposed model enriches the framework of Continuum Damage Mechanics (CDM) with fabric descriptors. In order to infer the form of fabric tensors from microstructure observation, creep tests were carried out on granular salt under constant stress and humidity conditions. A stress path comprising a tensile loading, a compressive unloading, a creep-healing stage and a reloading was simulated. Macroscopic and microscopic model predictions highlight the increased efficiency of healing with time. A preliminary Finite Element model illustrates the impact of healing on the stress distribution in the Excavation Damage Zone (EDZ). The model presented in this paper is expected to improve the fundamental understanding of damage and healing in rocks at both macroscopic and microscopic levels, and the long-term assessment of geological storage facilities

A perspective on Darcy's law across the scales: from physical foundations to particulate mechanics

This paper puts forward a perspective or opinion that we can demonstrate Darcy’s law is valid at any scale where fluid can be modelled/analyzed as a continuum. Darcy’s law describes the flow of a fluid through a porous medium by a linear relationship between the flow rate and the pore pressure gradient through the permeability tensor. We show that such a linear relationship can be established at any scale, so long as the permeability tensor is expressed as a function of adequate parameters that describe the pore space geometry, fluid properties and physical phenomena. Analytical models at pore scale provide essential information on the key variables that permeability depends on under different flow regimes. Upscaling techniques based on the Lippman-Schwinger equation, pore network models orEshelby’s homogenization theory make it possible to predict fluid flow beyond the pore scale. One of the key challenges to validate these techniques is to characterize microstructure and measure transport properties at multiple scales. Recent developments in imaging, multi-scale modeling and advanced computing offer new possibilities to address some of these challenges

Micro-Macro Modeling Approach for the Triggering of Viscous Fatigue Damage in Halite Polycrystals under Cyclic Loading

Presented at the 48th US Rock Mechanics/Geomechanics Symposium of the American Rock Mechanics Association (ARMA), Minneapolis, MN, 1-4 June 2014.Copyright © 2014 by the American Rock Mechanics AssociationUnderground cavities in salt rock formations used for Compressed Air Energy Storage (CAES) undergo cyclic loads and are subject to a fatigue phenomenon that induces a decrease of rock’s strength and stiffness. A micromechanical analysis of this phenomenon is necessary to understand its mechanisms and elaborate relevant constitutive models. The polycrystalline nature of rock salt has a crucial effect on crack propagation and rock damage and, hence, on fatigue behavior. This behavior was investigated herein on the basis of self consistent upscaling approaches for viscous heterogeneous materials. The internal stresses in the polycrystal were modeled based on experimental data available for halite single crystals, and a monotonic compression test was simulated, which allowed tracking the triggering of fatigue damage. Results show that tensile stresses are developed in the polycrystal under global compressive load, the amplitude of which depends on the macroscopic load rate or frequency. These tensile stresses can exceed in some conditions the tensile strength of grains or of grains interfaces and cause cracking and damage in the polycrystal

Thermo-Mechanical Radial Expansion of Heat Exchanger Piles and Possible Effects on Contact Pressures at Pile-Soil Interface

This letter shows that the increase of heat exchanger pile capacity in response to heating, observed in several small-scale laboratory studies, cannot be directly attributed to the increase of contact pressure at the soil-pile interface. The main thermo-hydro-mechanical processes that influence the capacity and behaviour of heat exchanger piles include thermal hardening of the soil, thermally induced water flow, excess pore pressure development and volume changes upon thermal consolidation. Due to the lack of understanding of the behaviour around the soil-pile interface, thermo-mechanical interactions between the heat exchanger pile and the ground are not taken into account appropriately in energy foundation design. However, in situ and reduced-scale experiments provide evidence about temperature-induced changes in pile capacity, presumably as a result of the altered stress state around the test pile. A finite-element analysis was conducted to quantitatively assess the radial stresses and strains undergone by a heated pile embedded in deformable soil. The study indicates that radial contact pressures typically increase less than 15 kPa, which cannot fully explain the increase in shaft resistance observed in heating tests. Further analyses are underway to characterise the mechanisms that govern pile load-displacement behaviour and the limit state