Heat exchanger anchors for thermo-active tunnels

• Forschungsstelle: ASTRA SBT
• Projektnummer: FGU2009/002
• Projekttitel Englisch: Heat exchanger anchors for thermo-active tunnels

Schlüsselwörter (Englisch)

anchors – heat exchanger geostructures – bored tunnel – cut and cover - recommendations

Kurzbeschreibung (Englisch)

Heat exchanger geo-structures consist in using ground thermal energy via fluid pipes cast in concrete foundation structures. The use of tunnel anchors as heat exchanger represents a unique opportunity for a sustainable and cost effective heating/cooling of buildings or underground structures in the neighbourhood of tunnels.

This project aims at validating the feasibility of heat exchanger anchor technique in tunnelling. The work will benefit from the applicant’s expertise in the field of energy geo-structures. Two practical cases will be studied: anchored retaining walls (cut and cover) and tunnel anchors, in soils and soft rocks. Our action will focus on the determination of the optimal energy geo-structure design, considering the thermal properties of the ground, the expectable energy demand (for instance a public building), and the static behaviour of the anchors.

The work will consist of two parts: first, the investigation of the thermo-hydro-mechanical behaviour of a ground / energy anchors system will be studied. Finite element simulations will be conducted. Verification of structural and lateral friction resistances will be carried out using the sizing tool for heat exchanger piles developed by the applicant. In a second time, technical recommendations for the implementation of the technology in tunnelling will be formulated.

Projektbeschreibung (Englisch)

Ground has very good heat storage properties, which can be used through the concept of energy (or heat exchanger) geostructures. An energy geostructure consists in a supporting (or retaining) geostructure, associated with a heat exchange system with the ground. Heat exchange consists in transporting ground thermal energy to the buildings via fluid pipes cast in the concrete structure. This technology has been successfully applied to building piles for 25 years. When a tunnel are built in urban regions, the use of its structural and/or reinforcing elements as heat exchanger represents a unique opportunity for a sustainable and cost effective heating/cooling of buildings or underground structures in the neighbourhood. To adapt heat exchanger geostructures to a tunnel construction can also increase the tunnel social acceptance. So far, heat exchange technology has been successfully used in two large scale tunnelling projects in Austria, heat exchange being carried out directly from the tunnel walls (see the State of the Art section of the proposal).

In the case of bored tunnels and cut and covers, an innovative and promising extension of the energy geostructure technology would be the use of ground anchors as heat exchangers. Ground anchors support retaining walls (cut and cover) or are used in combination with tunnel lining. They transmit the tensile forces applied to it to a competent stratum, generally through the intermediary of grout. The use of ground anchors would considerably increase the energy efficiency of the system in tunnels, by using a much larger volume of soil for heat storage, while it could be advantageously used in some situations when direct heat exchange between the ground and the tunnel walls is not possible or inefficient.

The geotechnical problematic associated with heat exchange in deep foundations or stabilizing element embedded in the ground lies in two aspects. First, temperature changes involve additional load in the foundation and change in the amount of mobilized friction along its side. Second, temperature changes in soil modify some of its essential properties (friction angle and compressibility). In the case of anchors, the situation is rather similar to floating piles, for which there is a long experience of heat exchange effects.

This project aims at validating the feasibility of heat exchanger anchor technique in tunnelling, applying rigorous design method. The work will rely on the applicant’s existing calculation tools and expertise in the field. Two practical cases will be studied: anchored retaining walls (cut and cover) and tunnel anchors, in soils and soft rocks. Our action will focus on the determination of the optimal energy geostructure design, considering the thermal properties of typical grounds and the expectable energy demand (for instance a public building) on the one hand, and the static behaviour of the heat exchange anchors on the other hand.

The work will consist of two parts: first, the investigation of the thermo-hydro-mechanical behaviour of a ground / energy anchors system will be studied. Finite element simulations will be conducted to analyse the temperature field in the ground when the anchors are heated or cooled, as well as the induced mechanical response of the system (additional displacements and stresses), which is highly non-linear. These simulations will also focus on long term cyclic effects (due to the alternation of heating and cooling phases) on the mechanical behaviour. For this purpose, research-oriented finite element codes available at EPFL-LMS will be used. In parallel, ad-hoc verification of structural (tendon and concrete, as well as head and lining) and lateral friction resistances will be carried out using the sizing tool for heat exchanger piles developed by the applicant. Some of the features of this numerical tool will be modified to take into account the specificities of heat exchanger anchors.

In a second time, based on the first part of the research, some technical recommendations for the implementation of the technology to new constructions such as cut and covers and tunnels will be formulated, in terms of allowable temperature within the heat exchanger anchor pipes, as well as geotechnical sizing (number of anchors that can be equipped, modifications of the pullout resistance of the anchors, specific construction measures).

Erwartete Erkenntnisse/ Nutzen, Nutzniesser (Englisch)

The knowledge obtained from this project will be useful for all tunnel engineering practitioners. It will provide with technical recommendations for the implementation of the technology to new constructions such as cut and covers and tunnels. There is a great potential for the use of the technique in forthcoming tunnelling works in Switzerland.

Due to the sustainability of the project, it will be for the benefit of the tunnel and underground construction users, as well as any potential user of the surface buildings that could be heated and cooled by the system.

Methoden (Englisch)

The following approaches will be progressively used to meet the above milestones and to finally propose constructive measures for heat exchanger anchors in tunneling:

1) Bibliographic analysis: the Austrian experience in “thermo-active” tunnels will be focused on and information on the heating/cooling performance of the system, and the constructive measures that has been followed. In addition, the database of the Swiss Geothermal Society will be used to quantify the potential of the technology in Switzerland.

2) Energy calculations: some calculations will be run to determine what could be the expectable heat exchanger anchors disposition for a typical energy demand, using available numerical design tools for heat exchanger piles (based on a calculation of the ground heat storage capacity as a function of the number of heat exchanger structures embedded in the soil).

3) Finite element simulations of the heat exchanger anchor / ground interactions: These calculations will be carried out with the thermo-hydro-mechanical model from the LMS. This model has been developed on the basis of the homogenization theory (averaging theory), and associated to a thermo-plastic constitutive model developed by Laloui (1993) and Modaressi and Laloui (1997). The aim of these calculations is to quantify the additional stresses and displacement caused by the temperature change in the anchors on a rigorous basis, and to conduct an sensibility study with respect to various model parameters, boundary conditions (i.e thermal solicitation, pre-stressing), geometric configuration of the anchors.

4) Geotechnical sizing: in order to propose constructive measures for heat exchanger anchors in tunneling, some simplified calculations (with respect to finite element simulations) will be carried out, based on the classical sizing theories (service and ultimate conditions). The results of the finite element analysis will provide complementary evolution laws for specific modifications of the ground response due to temperature changes.

Spezielle Geräte und Installationen (Englisch)

The existing EPFL-LMS software and hardware will be used for the study.

Allgemeiner Stand der Forschung (Englisch)

Thermal effect in soils have been extensively studied since the last twenty years, and thermo-mechanical models have been developed, which rather finely reproduce them (Hueckel and Baldi 1990, Laloui and Cekerevac 2003, Laloui and François 2009). From the geotechnical practice point of view, it has been demonstrated that the heating of foundations may have an important role in the improvement of the soil characteristics. In addition, thermal pre-treatment of clays may have a very positive effect on their resilience under cyclic loading, which results in a higher resistance of the foundations of the building against earthquakes. (Laloui et al. 2005). The applicability to multi-physic and multi-phase models taking into account the thermo-hydro-mechanical couplings arising in soil has been recently applied to heat exchanger structures (heat exchanger piles). An extensive numerical study has been conducted by Laloui et al. (2006) and Silvani et al. (2009).

The geotechnical problematic associated with heat exchange in deep foundations or stabilizing element embedded in the ground lies in two aspects. Based on our experience, we can summarize them as follows:

1. Temperature effects on the structure (additional displacements, increase of the overall load within the anchor, modification of the friction mobilization along the anchor due to temperature induced displacement).
2. Temperature effects on the soil which may modify some of its essential properties (friction angle and compressibility), and changes in the interface behaviour between the concrete and the soil.

These features are further affected by the cyclic (seasonal) temperature changes.

Various in situ and lab tests as well as theoretical works have allowed quantifying these effects in the case of heat exchanger piles (Laloui et al. 2003, Bourne-Webb et al. 2009). An experimental and numerical analysis of a full scale energy pile on the EPFL site has been conducted by the LMS. This test pile is unique in the world and allows the measurement of the stresses, the displacements, and the temperature variations of the energy pile. Such a full scale characterisation had been so far unattainable. This pile has provided some practical answers to the concerns of engineers and contractors involved in energy geostructures about temperature influence. (Laloui et al. 2003, Fromentin et al. 1999). This work has been coupled to an extensive laboratory test campaign to determine the ground response to heating/cooling, using specially designed research class prototype equipment.

The use of heat exchanger anchors in tunnels is a novel application. It is an extension of the existing applications of heat exchanger geostructures in tunnelling. The first thermo-active traffic tunnel has recently been built in Austria (Lainzer tunnel). The primary side wall lining of the tunnel (cut and cover method) consists of bored piles, each third pile is used as heat exchanger pile (Brandl 2006). The annual heating output is 214 MWh. Since autumn 2004, the energy system has run permanently for a school near the tunnel. In another lot of the tunnel, an “energy geotextile” has been installed, placed between the primary and the secondary lining of the bored tunnel. Similar technologies (energy diaphragm walls, energy tunnel inverts) are used in the four new metro stations that are currently being built for the Viennese metro line extension (Adam 2009).

Projektziele (Englisch)

The purpose of this project is to study an innovative technique relying on the heat storage capacity of the ground around tunnels, by using stabilizing anchors as heat exchangers, and to propose some recommendations for the construction of such installations. When tunnels are built in urban regions, the use of their structural and/or reinforcing elements as heat exchanger represents a unique opportunity for a sustainable and cost effective heating/cooling of buildings or underground structures in the neighbourhood.

Forschungsplan (Englisch)

The work will be carried out over a period of 1 year. A research engineer will be in charge of the work, under the supervision of experienced research staff of the Laboratory of Soil Mechanics at EPFL.

Month 1:

Milestone 1: Synthesis of « thermo-active » existing tunnel realizations in the world.

Month 1-Month 2:

Milestone 2: Study and synthesis of the capabilities of heat exchanger anchors in the frame of tunnels in urban areas.

Month 3-Month 7:

Milestone 3: Finite element simulation of heat exchanger anchors placed in a tunnel with concrete lining

Month 3-Month 7:

Milestone 4: Finite element simulation of heat exchanger anchors supporting a retaining wall (diaphragm wall for cut and cover)

Month 6-Month 8:

Milestone 5: Study of the effect of temperature in service conditions on the stability of anchors

Month 9-Month 11:

Milestone 6: Propositions and recommendations for the adaptation of existing anchor designs to the heat exchanger anchor design.

Month 12:

Writing of the final report

Umsetzung und Anwendungen (Englisch)

The practical interest of the research directly stems from its main objective, which is to propose constructive measures for heat exchanger anchors in tunneling, which could be used by tunneling practitioners

Berichtsnummer (Deutsch)

1417

Berichtsnummer (Englisch)

1417

Literatur (Englisch)

Adam D. 2009. “Tunnels and foundations as energy sources – Practical applications in Austria”, Deep foundations on Bored and Auger Piles – Van Impe and Van Impe Eds., Taylor and Francis, pp. 337-342.

Bourne-Webb P. J., Amatya B., Soga K., Amis T., Davidson C. and Payne P. 2009. “Energy pile test at Lambeth College, London: geotechnical and thermodynamic aspects of pile response to heat cycles”, Géotechnique, Vol. 59, No. 3, pp. 237–248.

Brandl H. 2006. “Energy foundations and other thermo-active ground structures”, Géotechnique, vol. 56, No. 2, pp. 81–122.

Fromentin A., Pahud, D., Laloui, L. and Moreni, M. 1999. “Pieux échangeurs: conception et règles de pré-dimensionnement“, Revue Française de Génie Civil, ed. Hermès Science Publications, Vol. 3, No 6, pp. 387-421.

Hueckel T. and Baldi, G. 1990. “Thermoplastic behavior of saturated clays: an experimental constitutive study”, Journal of Geotechnical Engineering, ASCE, vol. 116, pp. 1778-1796.

Laloui L. and Cekerevac C. 2003. “Thermo-plasticity of clays: an isotropic yield mechanism”, Computers and Geotechnics, vol. 30, num. 8, pp. 649-660.

Laloui L. Charlier R., Pijaudier-Cabot. G. 2005. “Coupled Multi-Physics Processes in Geomechanics”, Special issue, European Journal of Civil Engineering, vol. 9, num. 5-6, 280 p.

Laloui L. and François B. 2009. “ACMEG-T: A soil thermo-plasticity model”, Journal of Engineering Mechanics, doi: 10.1061/(ASCE)EM.1943-7889.0000011.

Laloui L., Nuth M. and Vulliet L. 2006. “Experimental and numerical investigation of the behaviour of heat exchanger pile”, International Journal for Analytical and Numerical Methods in Geomechanics, vol. 30, pp. 763-781.

Laloui L., Nuth M. and Vulliet L. 2006. “Experimental and Numerical investigation of the behaviour of heat exchanger pile”, International Journal for Analytical and Numerical Methods in Geomechanics, vol. 30, pp. 763-781.

Laloui L., Moreni M. and Vulliet L. 2003. “Comportement d’un pieu bi-fonction, fondation et échangeur de chaleur“, Canadian Geotechnical Journal, vol. 40, pp. 388-402.

Mattsson N.; Steinmann G.; Laloui L. 2008. “Advanced compact device for the In-situ determination of geothermal characteristics of soils“, Energy and Buildings, vol. 40, pp. 1344-1352.

Silvani C., Nuth M., Laloui L., and Peron H. 2009. “Understanding the thermo-mechanical response of heat exchanger piles“, First International Symposium on Computational Geomechanics (ComGeo I) - Proceedings, State of the Art Books and Monographs in Engineering Sciences, Pietruszczak et al. eds., International Centre for Computational Engineering, pp. 589-596.