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

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.

SOI mixed-mode design techniques and case studies

Marija Blagojevic

Since the emergence of Silicon On Insulator (SOI) technology the research activities have been concentrated on a detailed analysis of SOI devices and their use in different application fields (low-voltage, low-power circuits, high temperature electronics, digital circuits, etc.). The present thesis deals with low-voltage, low-power mixed-mode design using a SOI technology. The goal is to establish the concepts that allow one to design SOI mixed-mode circuits, and to apply these concepts to the design of fully depleted (FD) and partially depleted (PD) SOI circuits and systems. Two studies have been realized in the scope of this work: a Hall sensor based microsystem design using an experimental FD SOI technology a DRAM design using capacitor-less 1T floating-body PD SOI memory cells. At the beginning of this thesis, two major types of SOI devices are introduced, namely FD and PD devices. The most important properties of these devices are then presented and compared to those of CMOS bulk devices. A design methodology based on design retargeting from bulk to SOI technology is further proposed. This methodology is developed with the aim of being implemented for the design of FD SOI circuits. It consists in using the same device dimensions and bias currents for the bulk and the corresponding FD SOI design. The analysis performed proves that such an approach permits one to obtain better circuit performances using the FD SOI technology. The EKV model is chosen for Spice simulations. For that purpose the extraction of the EKV intrinsic parameters has been performed for the experimental 0.5 μm FD SOI technology. The EKV model card obtained from transistor measurements is implemented straightforwardly for circuit simulations. The first study presented in this thesis is an FD SOI Hall sensor front-end for energy measurement. The mixed-mode microsystem design is completely based on the proposed design methodology. The system level solutions are also included, such as the spinning-current method that serves for reduction of the offset and low frequency noise of the analog front-end, and a high resolution analog-to-digital conversion technique. The integrated microsystem is entirely functional. Furthermore, according to the existing literature, this integrated circuit is the first microsystem completely realized using FD SOI technology. The overall system error that is obtained is less than 1.5 %. During the second part of this work, a novel sensing scheme that exploits an automatic reference generation based on successive approximations has been developed for the PD SOI capacitor-less 1T DRAM. The 1T DRAM reference current is generated by an adjustable current source. The electrical calibration of the reference current is performed using a digital-to-analog converter and successive approximations algorithm. The proposed scheme is evaluated in a 2 kb test chip. The circuit integrated using PD SOI technology contains a 2 kb 1T memory array, as well as automatic reference generators and peripheral circuits. The automatic reference generator comprises current mode sense amplifier, M/3M converter, and successive approximations register. All blocks implemented in the test chip are described in detail, and PD SOI design issues are discussed. A number of experimental results demonstrate the potential of the PD SOI 1T DRAM for future embedded DRAM applications. The measured retention time under holding conditions is higher than 1s. In the continuous read mode, a few hundreds of the read cycles can be performed without a refresh operation. The test chip has a measured access time of 25 ns with a read cycle time of 70 ns.

EPFL2005

Thermo-mechanical performance of energy pile groups

Alessandro Francesco Rotta Loria

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.

EPFL2018

Evaluation of a Web-Based Training Environment for Hands-On Experimentation

Denis Gillet, Anh Vu Nguyen Ngoc, Stéphane Sire

The role of evaluation in software development has been well established. It is of first importance in usability engineering. Over the last years, many interactive systems have benefited from user evaluation, including e- learning applications. As with other interactive systems, designers of Web- based training environments can benefit from the results of an evaluation to detect the strengths and weaknesses of their application, and to prepare the next version. However, it is not obvious to evaluate educational software. It requires the transposition of the usability concepts of effectiveness, efficiency and satisfaction to a domain that is at the crossroads of multiple sub-domains: interactive systems, collaborative work and learning theories. In this article, we present a use-case of the evaluation of a flexible learning environment for hands on experiments. The first part of the article presents the flexible environment proposed to engineering students at the Swiss federal institute of Technology. It also defines the objectives of a user evaluation according to some of the design choices we made. In this evaluation, we have two types of objectives. First, we want to check if the flexibility permitted by a Web-based training environment is really put into practice by the students, if it has some effects on the place and the time at which they choose to work. Second, we want to check if a new component that we have introduced in our environment for sharing and commenting experimental results, the eJournal, really supports the flexible learning scenario and enhances collaborative learning. The second part of the article examines some of the most common evaluation methods used in interactive system design and in Computer Supported Cooperative Work. For each of these methods, such as checklists, questionnaires, log analysis and interviews, we describe how they could contribute to meet our evaluation objectives as defined in the first part. Finally, the third part of the article reports on our user evaluation of the flexible environment described in the first part. This reports describes first the evaluation methodology that we have selected. It describes the evaluation tools selected among the tools introduced in the second part, and the adaptations we made to fit our purposes. In particular, it describes some of the constraints imposed by experimenting with a panel of students pursuing their academic year at the same time. The reports then give some of the results of the evaluation we made with 30 students in automatic control using our prototype during the winter semester 2002/2003. The contribution of the article is to expose, through a use-case, the challenges of the evaluation of a Web-based training environment for sharing laboratory experiments. We think this is a necessary step towards the development of standard practices for evaluating e-learning applications.

2003