# Impact of Temperature Cycles at Soil – Concrete Interface for Energy Piles

Andreea Roxana Vasilescu, Anne-Laure Fauchille, Christophe Dano,
Panagiotis Kotronis, Richard Manirakiza, Philippe Gotteland

► To cite this version:

Andreea Roxana Vasilescu, Anne-Laure Fauchille, Christophe Dano, Panagiotis Kotronis, Richard Manirakiza, et al.. Impact of Temperature Cycles at Soil – Concrete Interface for Energy Piles. International Symposium on Energy Geotechnics (SEG-2018), Sep 2018, Lausanne, Switzerland. pp.35-42, 10.1007/978-3-319-99670-7_5 . hal-05480631

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# Impact of Temperature Cycles at Soil – Concrete Interface for Energy Piles

Andreea Roxana Vasilescu<fnref n="1" />,<fnref n="2" />✉, Anne-Laure Fauchille<fnref n="1" />,
Christophe Dano<fnref n="3" />, Panagiotis Kotronis<fnref n="1" />, Richard Manirakiza<fnref n="2" />,
and Philippe Gotteland⁴

**Abstract.** Energy piles are double purpose foundation elements used both for transferring loads to the soil and temperature regulation in buildings. The response of the pile soil interface is influenced by daily and seasonal temperature variations. In order to assess the impact of the thermal cycles on the mobilization of shear strength, in situ and laboratory experiments were performed. Three energy piles were instrumented with vibrating cord sensors in the foundation of a waste water treatment plant in the North East (NE) part of Paris (at Sept Sorts, Seine et Marne), France. The deformation of these sensors has been recorded for more than one year and the impact of temperature cycles was assessed. In addition the impact of cyclic thermal loading on soil deformation at the soil pile interface was also studied in the laboratory using an interface direct shear device equipped with a temperature control system. The preliminary results show that the undisturbed soil temperature below 5 m deep remains constant, around 13 °C and that the response of sand concrete interface at different temperatures and temperature cycles consists in the overall low contraction of the sand.

## 1 Introduction

The global energy demand increase has stimulated the research for new sustainable and cost effective energy sources. Geothermal energy pile foundations are such an alternative solution for heating and/or cooling needs. The technique consists in encasing heat exchanging pipes in pile foundation, connected to a ground source heat pump in order to extract/inject heat from/into the ground (Laloui et al. 2003; Brandl 2006; Bourne-Webb et al. 2009; McCartney and Murphy 2012; Sutman et al. 2015; You et al. 2016). Although this solution has been used for some time in Europe, the information concerning the long term thermo-mechanical behaviour of the foundation and of the surrounding soil is still limited (Amatya et al. 2012). During the operation of the ground source system energy piles experience temperature changes which can have an impact on the pile deformation (expansion during heating, contraction during cooling)

<footnote n="1">¹ Ecole Centrale de Nantes, Université de Nantes, CNRS, Institut de Recherche en Génie Civil et Mécanique (GéM), 1 rue de la Noë, 44000 Nantes, France
andreea.roxana.vasilescu@ec.nantes.fr</footnote>

<footnote n="2">² PINTO SAS, 48 Rue Jules Verne, 35300 Fougères, France</footnote>

<footnote n="3">³ Univ. Grenoble Alpes, CNRS, Grenoble INP*, 3SR, 38000 Grenoble, France ⁴ Fédération Nationale des Travaux Publics, 3 rue de Berri, 75008 Paris, France</footnote>
as well as on the soil-pile interface (Bourne-Webb et al. 2009; Amatya et al. 2012; Bourne-Webb et al. 2015; Di Donna et al. 2015; Yavari et al. 2016).

To better understand the behaviour of energy piles during building operation, three piles (two energy piles and a conventional pile) were instrumented during the construction of a building in a water treatment plant at Sept Sorts, in the NE of Paris. Additionally, a second experimental campaign was performed in laboratory, to focus on the effect of temperature changes on the soil-concrete interface. An interface direct shear device adapted for thermo-mechanical loading was used.

In a first step, experimental instrumentation and preliminary results are presented for the in-situ study at Sept-Sorts, and in a second step for the laboratory investigation.

# 2 Sept Sorts Full Scale Test

## 2.1 Experimental Setup

Two energy piles (P18 and P29) and a conventional pile (P15) were instrumented during the construction of the pretreatment building of the Sept Sorts water treatment plant in Seine-et-Marne department, in France (Fig. 1) and their behaviour under exploitation conditions was recorded.

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Location of the instrumented conventional pile P15 and energy piles P18 and P29 on the 3D plot of the pretreatment building in the water treatment plant.</figcaption></figure>

This building is supported by 100 continuous flight auger piles (0.420 m diameter, 9 m length) out of which 45 are energy piles. The mechanical properties of soils under the pretreatment building are presented in Table 1 (Sémofi 2013). For this project, the energy extracted by the abovementioned foundation is used for heating/cooling of the water treatment plant's administrative building located next to the pretreatment building.

The energy piles P29 and P18 are located under the slab and a wall, respectively while P18 is located under a wall (Fig. 1). These locations were chosen in order to assess the effect of the pile head constraint. The conventional instrumented pile, P15, is located at the equal distance of 4 m to the two energy piles.

Seven vibrating wire sensors were incorporated inside the energy piles P18 and P29 (Fig. 2a) in order to monitor the distribution of axial strain as a function of depth. The sensors were oriented longitudinally and attached to the transversal reinforcement bars
<table><caption>**Table 1.** Soil properties.</caption><thead><tr><th>Soil</th><th>E<sub>M</sub>(MPa)*</th><th>c' (kPa)*</th><th>φ' (°)*</th></tr></thead><tbody><tr><td>Modern colluvial soil</td><td>5</td><td>0</td><td>30</td></tr><tr><td>Marls and altered gravels</td><td>11</td><td>5</td><td>25</td></tr><tr><td>Marls and gravels</td><td>30</td><td>10</td><td>25</td></tr><tr><td>Coarse limestone</td><td>100</td><td>50</td><td>30</td></tr></tbody></table>

*E<sub>M</sub> Menard pressuremeter modulus, c' effective cohesion and φ' effective friction angle

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** (a) Temperature and deformation sensors positions (b) Seasonal evolution of temperature in pile P29 and (c) Measured strain change in pile P29.</figcaption></figure>

then cast in fresh concrete during pile installation. Each vibrating wire sensor is equipped with a thermistor to monitor the temperature variations at each sensor location (Fig. 2a).

Five vibrating wire sensors were installed in the conventional pile P15 to quantify the effect of geothermal activation of the foundation on non-geothermal piles. The conventional pile P15 is therefore considered as the reference pile.

The atmospheric temperature variations are recorded with a temperature sensor installed in the data acquisition box, next to the station. From September 2016 to September 2017 temperatures and strains were recorded every 60 min with a precision of 2 °C and 10 µm/m, respectively.

## 2.2 Preliminary In-situ Results

Seasonal temperature variations recorded in October 2016, January 2016, April 2017 and July 2017 (representing autumn, winter, spring and summer respectively) in each sensor installed in P29 are depicted in Fig. 2b.
Below 5 m deep, the pile temperature is around 13 °C and remains constant along the acquisition time. This temperature is higher than the recorded winter and spring temperatures and lower than the summer and autumn temperatures, thus confirming that ground can be used as a source of heat during winter and conversely it can be used as a heat sink during summer.

The ground temperature measured in our test is similar to previous observations found in the literature (Laloui et al. 2003; Brandl 2006; McCartney and Murphy 2012; Loveridge et al. 2016; Minh Tang et al. 2017). Note here that a seasonal cyclic thermal loading is observed even before the geothermal activation of the foundation.

The mechanical load (the weight of the superstructure) for this study is considered constant and only the effect of the thermal loads (temperature changes) is presented. Between 0 and 5 m deep, temperature variations in P29 are accompanied by small but not negligible strain changes ($\epsilon_{T-obs}$) in the range of 40 $\mu$m/m (Fig. 2c). From autumn 2016 to winter 2016, the temperatures are decreasing and the pile is contracting.

# 3 Laboratory Soil-Concrete Interface Tests

Temperature changes encountered in energy foundations also have an influence on the soil-structure interface. Heating-cooling cycles result in expansion-contraction of the pile (Laloui et al. 2003; Brandl 2006; Bourne-Webb et al. 2009) thus changing the normal pressure at the soil-pile interface which may lead to changes in the soil-pile shear resistance (Xiao et al. 2014; Di Donna et al. 2015; Yavari et al. 2016; Xiao et al. 2017). The in-situ instrumentation described above is only suitable for assessing the temperature and deformations evolution in piles but does not provide information of the soil changes at the soil-pile interface.

For a better understanding of the interactions between temperature and deformations of soil at the soil-concrete interface, a new direct shear device adapted for thermomechanical loading was employed at Centrale Nantes.

## 3.1 Experimental Setup

The new direct shear device is composed of a square shear box divided in two parts (upper part and bottom part). The box is installed in a container filled with water to consider saturated conditions. The upper part accommodates samples of 100 mm × 100 mm soil specimens with an initial height of 25 mm. The bottom part accommodates a 140 mm × 100 mm × 11 mm concrete plate.

A temperature sensor (precision of ±0.2 °C) is situated in the lower part. The thermal loading is applied through a closed loop circuit, that passes through a system installed under the container accommodating the shear box and that is connected to a refrigerated heating circulator bath with air-cooled cooling machine. The thermoregulation system is equipped with one Pt100 internal temperature sensor and one Pt100 temperature sensor external connection, in which the bottom temperature sensor is plugged-in. The temperature in the tested specimen is measured through the above mentioned Pt100 temperature sensor installed in the bottom of the lower shear box which is also used for running the temperature tests.
A load cell is installed on each actuator to measure both the vertical and horizontal loads applied on the sample. Two linear variable differential transducers (LVDTs) are used to measure the horizontal and vertical displacements with a precision of ±10 µm.

In the present work direct shear tests on Fontainebleau sand NE34 (Sibelco company, 99% quartz) in contact with a concrete structural element are presented. The literature review shows that the temperature effect on this sand is negligible (Di Donna et al. 2016; Yavari et al. 2016), this choice of soil was made in order to evaluate the performance of the testing device and the repeatability of the testing procedure. The physical properties of this sand are presented in Table 2.

<table><caption>**Table 2.** Fontainebleau sand properties (Pra ai 2013).</caption><thead><tr><th>d<sub>50</sub> * (mm)</th><th>Gs* (g/cm<sup>3</sup>)</th><th>e<sub>max</sub> * ( )</th><th>e<sub>min</sub> * ( )</th><th>C<sub>u</sub> = d<sub>60</sub>/d<sub>10</sub> * ( )</th></tr></thead><tbody><tr><td>0.23</td><td>2.65</td><td>0.866</td><td>0.545</td><td>1.72</td></tr></tbody></table>

*d<sub>50</sub> mean diameter, G<sub>s</sub> particle density, e<sub>max</sub> maximum void ratio, e<sub>min</sub> minimum void ratio, C<sub>u</sub> coefficient of uniformity

The same concrete plate was used for all experiments. Roughness measurements performed with a superior surface roughness tester were acquired before and after the tests. The value of the arithmetic mean roughness Ra was 10 to 12 µm before and after the tests. Therefore no significant changes of Ra after the direct shear tests were recorded in the study conditions.

Dry Fontainebleau sand was poured directly into the box and compacted by tamping. The sample was then placed in the loading frame and a vertical load was applied (consolidation) and kept constant during the entire test (constant normal load CNL test). Distilled water was added in the container in order to perform the test in almost saturated conditions. Temperature was then set to 13 °C.

A series of monotonic displacement-controlled direct shear tests were performed at 13 °C (in-situ soil temperature below 5 m deep, Fig. 2(b)), under constant normal stress equal to 50 kPa, 100 kPa and 150 kPa (corresponding to the effective lateral pressure at 5 m, 10 m and 15 m deep, respectively). The rate of applied horizontal displacement was 0.5 mm/min. Tests at 13 °C were used as references. A series of monotonic displacement controlled direct shear tests were also performed at 8 °C and 18 °C. In that case, the temperature was imposed at 13 °C in a first step and then imposed to 8 °C or 18 °C in a second step. Then the sample was subjected to shearing conditions.

A series of displacement controlled direct shear tests after 10 thermal cycles was done for the three temperatures and the three vertical loading cited above. The procedure for a cycle between 8 and 18 °C is given as example: (1) the temperature was decreased from 13 °C to 8 °C; (2) 10 temperatures cycles between 8 °C and 18 °C cycles were imposed, (3) shearing.

## 3.2 Preliminary Laboratory Results

The interface friction angle was calculated from the three monotonic tests at 8 °C, 13 °C and 18 °C and for the cyclic test 8 18 °C (Fig. 3a). The calculated interface friction
angle for all of the tests is equal to 24.5° ± 0.7°. This value is lower than the soil internal friction angle (36°) which confirms that the shearing occurs at the interface rather than in the soil. A standard variation of 0.7° of the interface friction angle indicates that there is no influence of temperature on the shear strength mobilization for the sand concrete interface.

During the thermal cyclic test (8 18 °C), the volumetric strain ε<sub>v</sub> shows cycles with an amplitude in the range of 0.25% which qualitatively highlight a thermo-elastic behavior. However, an accumulation of negative strains is registered (Fig. 3b). This result was interpreted by small grain rearrangement and gradual volume reduction (a dilating phase during heating is followed by a contracting phase during cooling for each cycle) leading to an overall contraction of the sample. It may explain the slightly higher interface friction angle obtained for this series of tests.

<figure><img src="image_4.png" /><figcaption>Fig. 3. (a) Interface friction angle; (b) Volumetric deformation during cyclic thermal loading for a normal stress of 50 kPa.</figcaption></figure>

**4 Conclusions and Perspectives**

Energy piles represent an economic and sustainable energy source for space heating/cooling. The preliminary experimental results available for the 3 instrumented piles from Sept Sorts show that seasonal temperature variation is limited to the first 5 m of soil after which the temperature remains constant, around 13 °C. This temperature is higher than the air temperature in winter and lower than the air temperature in summer and the temperature difference can be used as a heat source/sink (depending on the season) thus providing proof of concept. Additionally the, effect of cyclic temperature changes on the soil-pile (sand-concrete) interface was studied in the laboratory using an interface direct shear device adapted for thermo-mechanical loading. The effect of temperature on shear strength is low and the temperature cycles result in an overall low contraction of the sand.

In perspectives, data acquisition in Sept-Sorts is still running and the heat pump has been activated in the water treatment plan in December 2017. The evolution of
temperatures and strains, in the 3 instrumented piles in Sept Sorts, are now being monitored in geothermal conditions. Moreover, interface direct shear experiments are programmed on various clayey and sandy soils, and especially soils from Sept-Sorts, at different temperatures and loading conditions in the laboratory. This new series of experiments will help to understand the relationships between the temperature and shearing behaviour of a large panel of natural soils. Future in-situ and laboratory tests will participate to improve the design of more efficient energy piles.

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