Thermo-mechanical performance of energy pile groups

Employing geostructures as structural supports and geomaterials as reservoirs for the extraction and storage of heat represent effective means to meet human activity needs since ancient times. This doctoral thesis focuses on the thermo-mechanical behaviour and performance of an innovative, multifunctional technology that couples the aforementioned roles for the structural support and energy supply of any type of built environment, i.e., energy piles. The multifunctional role of energy piles involves mechanical and thermal loads applied to such geostructures. These loads pose unprecedented challenges to engineers because they cause variations in the temperature, stress, deformation and displacement in the subsurface that need to be considered during analysis and design. Prior to this work, a substantial amount of research had been made available to address the thermo-mechanical performance of single energy piles. Design guidance has also been proposed to advise in the geotechnical and structural design of such geostructures. However, energy pile foundations do not consist of a single energy pile but of a group of energy piles. In this framework, (i) limited knowledge, if available, was present to address the thermo-mechanical behaviour and performance of energy pile groups subjected to thermal and mechanical loads; (ii) no simplified models and methods were accessible to perform the analysis and design of energy pile groups against the action of such loads; and (iii) no comprehensive framework for the effect of thermal (and mechanical) loads on the performance and the related design of both single and groups of energy piles was avail-able. To address such challenges, this doctoral research was performed to (i) investigate the thermo-mechanical behaviour and performance of energy pile groups over typical time-scales of practical applications via the first available in situ tests and coupled numerical analyses of such geostructures; (ii) provide the only simplified analytical models and methods for predicting the vertical deformation of energy pile groups subjected to thermal and mechanical loads; and (iii) propose a comprehensive framework for the effect of thermal and mechanical loads on the performance and related performance-based design (e.g., geotechnical and structural) of single and groups of energy piles. The results presented in this thesis suggest the conclusion that (a) the thermo-mechanical behaviour and performance of energy pile groups are critically different from those of single energy piles; (b) thermal loads, applied alone or in conjunction to mechanical loads, represent a serviceability and not an ultimate limit state problem; and (c) no energy pile analysis and design can be considered complete without addressing the behaviour of piles as both isolated elements and in a group.

Cyclic thermomechanical behaviour of pile-soil interface and design methods for energy pile foundations

Elena Ravera

Energy piles are shallow geothermal systems integrated into structures already required for structural functions of civil works that provide both geothermal energy supply and structural support to the built environments. This research work is driven by the need to convey scientifically based answers that meet the needs of practice for the thermomechanical design of energy piles. Two main challenges are part of this thesis: (i) the understanding of the mechanism that govern the behaviour of energy pile-soil interface and (ii) the development of theoretical methods for design purposes. The multifunctional roles of this technology require to investigate new thermomechanical effects related to daily and seasonal temperature variations during their lifetime, thermally induced effects on pile and soil behaviour, as well as structure-pile-soil interaction. The failure mechanism at the pile-soil interface subjected to cyclic thermal loads is investigated from experimental and constitutive perspectives. The effects of thermal cycling and negative temperatures on pile-soil interface behaviour are presented. Experimental procedures and constitutive models for more advanced design analyses are proposed to help develop reliable long-term predictions of the behaviour and performance of energy pile groups. The results suggest that (i) there is no reduction of the shear strength of the fine-grained soil-concrete interface when subjected to cyclic thermal loading in saturated condition, and (ii) no analysis can be considered complete for a robust design of the system without addressing the risk of freezing. The utility of developing theoretical methods enables the examination of complex practical problems in a systematic, albeit approximate, way for routine design. The majority of theoretical procedures has been originally presented for conventional pile groups with some of them modified to address the different type of loads characterising energy pile groups. However, some gaps are still present in the literature especially with regard to the analysis of pile-soil-slab interactions under thermal loads and the analysis of stress-strain behaviour of energy piles in a group. The development of theoretical methods aims to provide analysis tools for the design of energy piles with respect to the previously mentioned shortcomings. Two methods, the interaction factor method and the load transfer method are mathematically formulated to expand and enrich the framework of available analytical and semi-analytical methods. (i) The interaction factor method integrates the available formulation of the method and allows the analysis of the displacement interaction under a broad range of design conditions addressing the pile-soil-slab interactions. (ii) The load transfer method provides a semi-analytical solution to study the thermomechanical response of an energy pile in a group.

EPFL2021

Les circulations d'eau dans les massifs cristallins alpins et leurs relations avec les ouvrages souterrains

The present study has been achieved within the framework of the scientific program AQUALP, supported by the EVIAN Foundation. Its theme is the study of the hydrogeology of the Alpine crystalline. The AQUALP programme consists of two co-coordinated parts : while the "source" part, lead at the Savoy University, studies natural circulation towards springs, the "work" part, which concerns this study, has focused on the interaction between civil engineering works and underground waters. The approach adopted is that of a field study during which water inflows encountered in various Alpine underground works and replaced in their geological contexts were followed up as from the discharge, the physico-chemical and the isotopic chemistry point of view. A tracer test and numerical simulations of underground flows were also undertaken. Main results The summary of the hydrogeological data in the numerous existing underground works allowed to calculate, on the basis of simple analytical solutions, the distribution of the hydraulically conductivity of the massifs crossed. These results show that the deeper one goes the more the hydraulical conductivity decreases. They bring us to specify the thickness of the decompressed zone, i.e. about 500 m. This zone strongly influences the underground flow in these massifs. Presence of mica inhibits the hydraulical conductivity in conferring a more plastic comportment to the rocks during past tectonic phases. The temperature of the water inflows encountered is a complex function of the thickness of the overlying rock and the quantity of waters circulating in the massif. Therefore it constitutes a precious indicator, during the excavating of underground works, of the presence of possible more permeable sections in formations to come. A numerical simulation of such a case has been undertaken and shows the possibility of using this tool to improve the expectation of water inflow in the work. The temperature study as well as the application of the chalcedony geothermometer to the entire of the inflow studied has shown the existence of ascending flow systems, fed by precipitation. Waters circulating in the Alpine massifs have a chemical composition strongly influenced by the reservoir rock. Chemical analyses of major and trace elements have allowed to distinguish waters issued from the various petrographies encountered. One must first of all underline the complementarity of major and trace elements analyses. The discriminating power of these two types of analyses does not apply to the same type of rocks. The major structures hydraulically conductive in crystalline Alpine massifs have been defined. One can mainly quote isolated fractures, strongly fractured and tectonised zones, kakirite zones, the decompressed zone, the down-bending zone, contacts between distinctive hydrogeological units and large metasedimentary structures. Hydraulical and physico-chemical characteristics of waters associated to each type of defined structures have been determined. These structures are straightened and induce water circulation close to the vertical, from the surface towards the work. The infiltration zones of waters determined on the basis of the orientation of major structures generally coincides with the altitudes calculated on the basis of isotopic analyses of waters (0-18). Some anomalies have nevertheless been put into evidence. They are mainly relative to circulation in the decompressed zone or in overlying Quatemary beds. Relationship between surface waters (barrier lakes or rivers) and underground works have also been put in evidence on the basis of the interpretation of isotopic results. Numerous water inflows are poor in tritium and attest of the time of transit through to the work in the order of a number of decades, in relation with the weak permeability observed in depth. These waters are often rich in sodium, because of the alteration of silicates contained in rocks. More recent waters have been encountered in shallow depths, in the decompressed zone or in association with a great geological fault. The examples of the interaction between underground works and springs are systematically linked to important water inflows in the works either through a decompressed zone of the massif, or through a particularly transmissive zone connected with the surface. These examples show that in this fissured environment, extremely divided into compartments and anisotropic, the drying up of springs is not linked with the progressive lowering of a water table along side the work but at punctual and localized inflows. A conceptual model of the hydrogeology of the Alpine crystalline is proposed in this study. It is accompanied with an interaction model between a work and the natural middle. It must help the project manager to foresee the water inflows in underground works and to adopt the optimal solution to minimize the impact of the work on water resources.

EPFL1998

En quête d'habitat

Marie-Paule Thomas

This doctoral thesis is based on the observation that the individualization of society that is taking place today has led to new differentiations within the domains of lifestyle and residential choice. Our main hypothesis was that lifestyle influences residential choice. What kinds of differentiation with regard to lifestyles are we seeing currently? What connections are there between lifestyles, residential preferences and localization? Theoretically, the typical methods for describing residential choice and environment rely on epistemological, methodological and empirical corpuses that are quite varied and, at times, contradictory. On one hand do we find economic and demographical approaches based on the theory of the "rational" actor who attempts to optimize his localization according to "functional and economic" parameters (i.e. proximity to the workplace, the price and size of the residence depending on income and background); on the other we find sociological and anthropological approaches based on theories of social action where choice as a general rule is "socially" determined based on the individual's family ties, tradition, culture, way of living or even social position (Authier & al., 2010). Above and beyond such rational or social perspectives, other factors also influence the choices we make, including the sensitive dimension related to an individual's physical and emotional experience of space. In order to grasp all three logics of action in a single analytical frame we based our analysis on the postulate that residential choice is "plural." Thus do multiple criteria – accessibility, social rooting of families, social status of the residential residence, appreciation of the morphological qualities of the environment, etc. – figure into the decision. We assumed that based on their lifestyles that households blended one or the other of these logics of action, thus resulting in their ranking their criteria for choice in different ways. We proposed a conceptualization of lifestyle that offers a compromise between classical structuralist perspectives (Bourdieu, 1979) and individualist perspectives (Beck, 1983). In our opinion residential choices are influenced by a three-fold dimension of lifestyle: urban practices and daily life (stylization), values and urban representations (evaluation) and resources/constraints (stratification variables). Thus did we couple classic vertical forms (training, income, gender, etc.) with new horizontal forms (attitudes, opinions, values, practices, etc.) that engender social inequalities. Residential choice therefore results in a compromise (resulting from lifestyle) between the different functional, social and sensitive criteria that are or are not present in the structural, socio-historical, environmental context (potential receptiveness). Methodology The cities of Bern and Lausanne in Switzerland – two cities of comparable size but whose socio-historical backgrounds and urban structures vary significantly, particularly with regard to the quality of public transportation service – served as the field for our study. As families often use long-term residential choices to realize plans for the future, we chose to look at them. We used a combination of quantitative and qualitative methods for our empirical work. At the macro level we describe the territory using a series of functional, social and sensitive indicators that allowed us to characterize "potential receptiveness" at different levels, from citywide to neighborhood. At the micro-sociological level we conducted: a quantitative telephone survey of 500 households in each agglomeration, a qualitative survey comprised of 40 semi-directive interviews in eight neighborhoods that distinguished themselves by their geographical location (central, suburban, peri-urban) and architectural diversity. Principal findings In general our findings show that income inequalities have a discriminatory effect on the size and price of the residence as well as on the probability of finding a living space that is entirely adapted. These inequalities however only play a secondary role when it comes to explaining differences in terms of residential preferences. Whether or not the households had a weak or strong income, certain areas of the city were never considered if they did not correspond to the individual's lifestyle. To begin we showed that the immediate environment plays an important role in quality of life for families, especially with regard to the presence or absence of elements favoring children's autonomy. Access to public transportation for instance is a decisive factor in terms of localization for most families. Above and beyond these commonalities we highlighted six axes of differentiation for residential preference that indicate the qualities most sought after by families: do they want to live in an environment that first and foremost is safe, densely-populated, convivial, traditional, calm or elite? To identify how families classify choice criteria and in so doing position themselves on the different axes we did a typological analysis, thus allowing us to categorize households into seven major "residential lifestyles," each of which valued the environment's social, sensitive and functional qualities in varying degrees. Thus we found there was not a quality of life that held true for all groups but rather qualities of life. Two main groups stood out, highlighting as such the contrast between ways of living typical of (modern) industrial society, based on classic models of upward social mobility, work and material success, and those originating from (post-modern) post-industrial society, based on self-enrichment, post-materialism and criticism of so-called "bourgeois" values. The groups identified are products of the historical sedimentation of the different ways of relating to the environment that have appeared over time. By comparing our analysis of residential lifestyles and the structural/cultural differences between the cities of Lausanne and Bern we were able to highlight the impact of context on the spatial distribution of lifestyles at different scales (from nationwide to the neighborhood level). We were also able to prove that geographical distinctions such as center, suburban and peri-urban become less important; each area has its own individual functional (accessibility, services), sensitive (urban morphology, architecture) and social (social-spatial diversity, segregation) qualities. Based on these characteristics, an environment is more or less receptive to different types of residential lifestyles (can I do everything on foot in this neighborhood? Can my children go to by themselves? Is there a strong social/community dynamic?). With such a detailed qualitative and quantitative description of areas it becomes easier to determine who would like to live in a given neighborhood and who, on the contrary, would not ("describe where you live and I'll tell you who you are?"). To guarantee the success of urban development or regional policies, those responsible for planning and development need to consider urban planning not only in terms of "social mixing" but in terms of the "mixing of lifestyles" as well. Through this notion of potential receptiveness, the urbanism of lifestyles is one of the main functional conclusions of this study, for which we propose avenues of change and recommendations (how to plan and think about urban development based on use?).

EPFL2011