3363
Analysis of the freeze thaw performance of geothermal heat exchanger borehole
grout materials
Étude de la résistance au gel et dégel des sondes géothermiques verticales
Katzenbach R., Clauss F., Rochée S.
Technische Universität Darmstadt, Institute and Laboratory of Geotechnics, Germany
ABSTRACT: If the exposure of vertical borehole heat exchangers even to few freeze-thaw cycles may possibly lead to damage either
of the body of backfill grout filling the borehole or it may lead to damage of the structure of the surrounding subsoil is subject of an
ongoing discussion. Previous research has been focusing on the damage of the backfill grout body solely. The purpose of the research
presented with this paper is to study the performance of the entire system subsoil-grouting body in terms of the freeze-thaw resistance
by means of experimental analysis. As no standard procedure is available in codes or literature for these particular applications, a
defined test setup and a reproducible procedure for the test conduction have been developed. Experiments were conducted in a
cylindrical encasement with a sample of grout and saturated sand. Parallel, experiments with blank samples of each material were
carried out. The samples were exposed to a temperature range varying between -20 °C and +20 °C. Analysis and interpretation of the
results bases on grain size distributions before and after freeze thaw cycles, evaluation of weathering, temperature and strain
measurements.
RÉSUMÉ : L’exposition des sondes géothermiques verticales à un nombre (même faible) de cycles de gel-dégel peut amener à une
détérioration du mortier de remblayage assurant la jonction entre la sonde et le sol ou à une modification de la structure du sous-sol
environnant. Les recherches antérieures se sont uniquement concentrées sur le mortier de remblayage. Il s’agit ici d’étudier
expérimentalement la résistance aux cycles de gel-dégel du système complet constitué à la fois du coulis d’injection et du sol. Aucun
protocole standardisé pour ce type d’applications n’est disponible actuellement dans les normes ou dans la littérature, un banc d’essai
et un protocole reproductible ont donc été conçus pour réaliser l’expérience. Les tests ont été menés dans une gaine cylindrique sur
des échantillons constitués de sable saturé en eau et de matériau de remplissage thermique. Des tests avec des échantillons témoins ont
été menés en parallèle pour chaque matériau. Les échantillons ont été exposés à des températures variant entre -20 °C et +20 °C.
L’analyse et interprétation des résultats est basée sur l’observation des courbes granulométriques avant et après les cycles de gel et
dégel, l’évaluation de l’altération des échantillons et les mesures de températures et déformations.
KEYWORDS: geothermal energy, borehole heat exchanger, freeze thaw, grout material, subsoil, experimental.
1 INTRODUCTION
If the exposure of a borehole heat exchanger to freeze thaw
cycles may lead to damage either the grouting or the
circumfluent subsoil is subject of an ongoing discussion.
Freeze-thaw-cycles occur because of an inaccurate
dimensioning of the installation, of temporary high abstraction
capacities or of modified terms of use, for example added
apartments or buildings (Müller 2009).
Vertical borehole exchangers are composed of pipes in
which flows a heat-transfer medium and of a backfill grout body
between those cables and the surrounding subsoil. Grouting
materials serve to stabilize the borehole during the setting up
and to obtain a good thermal connection between heat transfer
medium and subsoil. Moreover, those materials have to
guarantee a sufficient sealing on target to avoid connections
between groundwater aquifers.
Sealing of the borehole is related with the freeze-thaw-
resistance of the grouting materials and more widely of the
freeze thaw resistance of the whole system composed of
grouting materials and circumfluent subsoil. Damages are
principally due to the properties of water: volume of water
contained in the pores of the material expands of 9 % by
freezing, which may lead to deteriorate permanently the
structure, even after thawing. Hence are connections between
groundwater storeys possible which presents an environmental
risk.
2 SET OF PROBLEMS
2.1
Research significance
Some cases of damage are documented in line with a project
of the engineering companies GEOWATT AG and Ingénieurs-
Conseils SA (Bassetti 2006). Various experimental analyses
have been in addition carried out on the frost resistance of the
backfill grout body in recent decades (Herrmann 2008, Müller
2009, Niederbrucker et al. 2008), but few investigations have
been performed to evaluate the resistance of the whole system
grouting-subsoil. Purpose is to design and develop a new test
stand to determine the effects of the subsoil on the freeze thaw
resistance of the whole system, focusing on the damages inside
the materials as well as the alteration of contact surface, in
accordance with conditions related to borehole heat exchangers.
2.2
Influence parameters of frost in soils
According experimental and theoretical analyses referenced
in literature (Rückli 1950, Reutel et al. 1992), freeze penetration
in the soil is influenced by following parameters:
Grain structure
Existing and mobilized water
Temperature distribution during freeze and thaw periods
Stress distribution
Presence of water in soil is given as main cause of
degradations. Types of frost effects are different between fine
