THM-coupled finite element analysis of frozen soil: formulation and application

A fully coupled thermo-hydro-mechanical (THM) finite element (FE) formulation is presented that considers freezing and thawing in water-saturated soils. The formulation considers each thermal, hydraulic and mechanical process, and their various interactions, through fundamental physical laws and models. By employing a combination of ice pressure, liquid pressure and total stress as state variables, a new mechanical model has been developed that encompasses frozen and unfrozen behaviour within a unified effective-stress-based framework. Important frozen soil features such as temperature and porosity dependence of shear strength are captured inherently by the model. Potential applications to geotechnics include analysis of frost heave, foundation stability or mass movements in cold regions. The model's performance is demonstrated with reference to the in situ pipeline frost heave tests conducted by Slusarchuk et al. Detailed consideration is given to FE mesh design, the influence of hydraulic parameters, and the treatment of air/ground interface boundary conditions. The THM simulation is shown to reproduce, with fair accuracy, the observed pipeline heave and the porosity growth driven by water migration

Coupled thermal-hydrological-mechanical analyses of the Yucca Mountain Drift Scale Test-Comparison of field measurements to predictions of four different numerical models

The Yucca Mountain Drift Scale Test (DST) is a multiyear, large-scale underground heating test designed to study coupled thermal–hydrological–mechanical–chemical behavior in unsaturated fractured and welded tuff. As part of the international cooperative code-comparison project DEvelopment of COupled models and their VALidation against EXperiments, four research teams used four different numerical models to simulate and predict coupled thermal–hydrological–mechanical (THM) processes at the DST. The simulated processes included heat transfer, liquid and vapor water movements, rock-mass stress and displacement, and stress-induced changes in fracture permeability. Model predictions were evaluated by comparison to measurements of temperature, water saturation, displacement, and air permeability. The generally good agreement between simulated and measured THM data shows that adopted continuum model approaches are adequate for simulating relevant coupled THM processes at the DST. Moreover, thermal-mechanically induced rock-mass deformations were reasonably well predicted using elastic models, although some individual displacements appeared to be better captured using an elasto-plastic model. It is concluded that fracture closure/opening caused by change in normal stress across fractures is the dominant mechanism for thermal-stress-induced changes in intrinsic fracture permeability at the DST, whereas fracture shear dilation appears to be less significant. This indicates that such changes in intrinsic permeability at the DST, which are within one order of magnitude, are likely to be mostly reversible

A simplified procedure to assess the dynamic stability of a caisson breakwater

The paper describes a simplified method of analysis used to evaluate the stability of a caisson breakwater to sea wave actions. An intensive laboratory program was performed in order to evaluate the static and dynamic characteristics of the foundation soil. Anisotropic and isotropic consolidated cyclic triaxial tests and cyclic simple shear tests were used to define the cyclic interaction diagram for the foundation soil. The possibility of foundation cyclic mobility due to wave loading and their effect on the breakwater stability was examined combining the cyclic interaction diagram with the results of finite element analysis. The potential reduction in soil strength is then incorporated into a conventional stability analysis. The procedure is illustrated by a specific application to a caisson breakwater that is part of the extension works of the Barcelona Harbour.Postprint (published version

Nitrate reducing bacterial activity in concrete cells of nuclear waste disposal

Leaching experiments of solid matrices (bitumen and cement pastes) have been first implemented to define the physicochemical conditions that microorganisms are likely to meet at the bitumen-concrete interface (see the paper of Bertron et al.). Of course, as might be suspected, the cement matrix imposes highly alkaline pH conditions (10 < pH < 11). The screening of a range of anaerobic denitrifying bacterial strains led us to select Halomonas desiderata as a model bacterium capable of catalyzing the reaction of nitrate reduction in these extreme conditions of pH. The denitrifying activity of Halomonas desiderata was quantified in batch bioreactor in the presence of solid matrices and / or leachate from bitumen and cement matrices. Denitrification was relatively fast in the presence of cement matrix (< 100 hours) and 2 to 3 times slower in the presence of bituminous matrix. Overall, the presence of solid cement promoted the kinetics of denitrification. The observation of solid surfaces at the end of the experiment revealed the presence of a biofilm of Halomonas desiderata on the cement paste surface. These attached bacteria showed a denitrifying activity comparable to planktonic bacterial culture. On the other side, no colonization of bitumen could be highlighted as either by SEM or epifluorescence microscopy. Now, we are currently developing a continuous experimental bioreactor which should allow us a more rational understanding of the bitumen-cement-microbe interactions

Development of a predictive framework for geothermal and geotechnical responses in cold regions experiencing climate change

Cold regions, which are expected to suffer particularly severe future climate effects, will pose very challenging geotechnical conditions in the 21st century involving ground freezing and thawing. Given the uncertainty of future environmental changes and the vast expanses of the cold regions, it is appropriate to address problems such as pipeline or road construction with analytical methods that have multiple scales and layers. High- and middle-level predictive tools are described that integrate climatic predictions from AOGCMs and their down-scaling schemes, geological and topographical (DEM) information, remotely-sensed vegetation data and non-linear finite element analysis for soil freezing and thawing. These tools output broad scale predictions of geothermal responses, at a regional scale, that offer hazard zoning schemes related to permafrost thawing. A more intensive localscale predictive tool is then outlined that considers fully-coupled thermo-hydro-mechanical processes occurring at the soil-element level and outputs detailed predictions for temperature changes, pore water behaviour, ground stresses and deformation in and around geotechnical structures. Applications of these tools to specific problems set in Eastern Siberia and pipeline heave tests are illustrated.Postprint (published version

3D zero-thickness coupled interface finite element:Formulation and application

In many fields of geotechnical engineering, the modelling of interfaces requires special numerical tools. This paper presents the formulation of a 3D fully coupled hydro-mechanical finite element of interface. The element belongs to the zero-thickness family and the contact constraint is enforced by the penalty method. Fluid flow is discretised through a three-node scheme, discretising the inner flow by additional nodes. The element is able to reproduce the contact/loss of contact between two solids as well as shearing/sliding of the interface. Fluid flow through and across the interface can be modelled. Opening of a gap within the interface influences the longitudinal transmissivity as well as the storage of water inside the interface. Moreover the computation of an effective pressure within the interface, according to the Terzaghi’s principle creates an additional hydro-mechanical coupling. The uplifting simulation of a suction caisson embedded in a soil layer illustrates the main features of the element. Friction is progressively mobilised along the shaft of the caisson and sliding finally takes place. A gap is created below the top of the caisson and filled with water. It illustrates the storage capacity within the interface and the transversal flow. Longitudinal fluid flow is highlighted between the shaft of the caisson and the soil. The fluid flow depends on the opening of the gap and is related to the cubic law