# Cyclically thermally-activated pile under combined axial/horizontal loads in saturated clay

## Author 1

* Aylin Nouri, Ph.D. candidate, MSc

* Faculty of Civil, Water and Environmental Engineering, Shahid Beheshti University (SBU), Tehran, Iran

* Navier, Ecole des Ponts, Univ Gustave Eiffel, CNRS, Marne-la-Vallée, France

* i_noori@sbu.ac.ir; aylin.nouri@enpc.fr

* https://orcid.org/0000-0001-8854-0578

## Author 2

* Ali Noorzad, Associate Professor, Ph.D., P. Eng.

* Faculty of Civil, Water and Environmental Engineering, Shahid Beheshti University (SBU), Tehran, Iran

* a_noorzad@sbu.ac.ir

* https://orcid.org/0000-0002-3785-7679

## Author 3

* Jean-Michel Pereira, Professor, Ph.D.

* Navier, Ecole des Ponts, Univ Gustave Eiffel, CNRS, Marne-la-Vallée, France

* jean-michel.pereira@enpc.fr

* https://orcid.org/0000-0002-0290-5191

## Author 4

* Anh Minh Tang, Research Director, Ph.D.

* Navier, Ecole des Ponts, Univ Gustave Eiffel, CNRS, Marne-la-Vallée, France

* anh-minh.tang@enpc.fr

* https://orcid.org/0000-0002-7149-8497

## Full contact details of corresponding author.

Dr. Ali Noorzad

Faculty of Civil, Water and Environmental Engineering, Shahid Beheshti University (SBU)

Hakimieh, East Vafadar Boulevard, Tehran, Iran

Email: a_noorzad@sbu.ac.ir

Phone: +98-21-7731-3062

Fax: +98-21-7700-6660

Number of words (excluding abstract and references): 2013

Number of figures: 6
# 1 Abstract

In the present study, the influence of cyclic thermal loading on the mechanical response of an energy pile installed in

saturated clay subjected to sustained combined axial/horizontal mechanical loading was investigated using physical

modelling. After completion of the soil saturation process and prior to starting the thermo-mechanical loading, the

pile fixation was released inducing pile head heave. When the rate of pile head heave became small, the model pile

was subjected to compression axial loading of 100 N (i.e. 20% of the estimated ultimate axial load) followed by

horizontal loading of 35, 71, and 109 N (corresponding to accumulative pile head horizontal displacements of 4%,

7%, and 12% of pile diameter) before subsequent 15 heating/cooling cycles. The results indicated that imposing

heating/cooling cycles to the pile resulted in irreversible horizontal displacement, accumulating at a reducing rate as

the number of thermal cycles increased. By the end of the 15th cycle, the accumulated irreversible horizontal

displacement was estimated at 0.57%, 1.15%, and 1.84% of pile diameter under the horizontal loads of 35, 71, and

109 N, respectively. Reversely, thermally induced pile head axial displacement at the end of the 15th cycle is

practically insignificant (smaller than 0.1% of pile diameter).

# 14 Keywords

Repeated loading; Soil/structure interaction; Temperature effects

# 16 List of notations

$w_L$ Liquid limit, %

$w_p$ Plastic limit, %

$S_r$ Degree of saturation, %

$e$ Void ratio

$\rho_s$ Particle density, Mg/m³

$d_o$ Outer diameter of model pile, mm

$d_i$ Inner diameter of model pile, mm

$L$ Embedment length of model pile, mm

$D_i$ Inner diameter of soil tank, mm

$h$ Soil tank depth, mm

$D_{50}$ Median grain-size, µm

$(d_h)_p$ Projected pile head horizontal displacement, mm
$(d_a)_p$ Projected pile head axial displacement, mm

$(d_h)_m$ Measured pile head horizontal displacement, mm

$(d_a)_m$ Measured pile head axial displacement, mm

t Elapsed time, day

$Q_v$ Ultimate axial load (estimated by Nguyen et al. 2020), N
17 **Introduction**

The dual functionality of energy piles for supporting the structural loads and meeting the heating/cooling needs of the building imposes additional stresses/strains on the soil-pile system, which could affect the mechanical behaviour of the energy piles. Previous studies revealed that cyclic thermally-induced volume changes are much more predominant in clayey soils compared to sandy soils. Nevertheless, only a limited number of studies focused on the influence of thermal loading on the mechanical response of axially-loaded energy piles embedded in clayey soils by performing full-scale tests (Laloui et al. 2006; Bourne-Webb et al. 2009; Akrouch et al. 2014), centrifuge model tests (Stewart & McCartney 2014; Ng et al. 2014, 2021a, b) and 1-g small-scale physical model tests (Yavari et al. 2016; Wu et al. 2020; Yazdani et al. 2019; Nguyen et al. 2020; Tang et al., 2021). To the authors knowledge, the works of Vitali et al. (2022) and Zhao et al. (2022) are the only available studies using centrifuge model tests to investigate the influence of monotonic heating and heating-cooling cycles, respectively, on the flexural behaviour of horizontally-loaded energy piles in sands. These studies indicate that (1) the heating operation reduces pile bending moment while causing no additional flexural stress, (2) imposing heating/cooling cycles results in accumulating irreversible horizontal displacement. Although such ratcheting has already been observed in axially loaded energy piles, its occurrence for energy piles subjected to inclined load has not yet been observed. This study thus aims to explore the influence mechanism of cyclic thermal loading on the mechanical response of a single model energy pile installed in saturated clay subjected to sustained combined axial/horizontal mechanical loading through physical modelling under normal gravity.

2. **Experimental set-up**

In order to use the results obtained from 1-g small-scale model tests to assess the thermo-mechanical response of full-scale energy piles, the test program was designed by considering the geometry difference between the small-scale pile and the full-scale pile. The experimental setup consists of a cylindrical steel soil tank (with $D_i$ = 548 mm ($D_i$ ≈ 27 $d_o$), and $h$ = 980 mm), whose internal surface was covered with a two-layer geotextile filter, wrapped in an expanded polystyrene sheet (Fig. 1).
<figure><img src="image_1.png" /><figcaption>Top view of the pile head</figcaption></figure>

<figure><img src="image_2.png" /><figcaption>42 Fig. 1. Schematic view of the experimental setup.</figcaption></figure>
In order to minimize the heat dissipation rate, and maximize the roughness of the model pile surface, the outside surface of a closed-end aluminium tube (with $d_o = 20$ mm, $d_i = 18$ mm, and $L = 600$ mm) was coated with Fontainebleau sand. Given that the soil tank dimensions satisfy the following conditions: $D_i \ge 10d_o$, and soil cushion below the pile toe $\ge (3\sim4)d_o$, boundary effects could be considered negligible (Boominathan & Ayothiraman, 2007; LeBlanc et al., 2010). Kaolin clay powder (with $w_L = 57\%$, $w_P = 33\%$, and $\rho_s = 2.6$ Mg/m³) was mixed with de-aired water to achieve an optimum water content of 29% (Murillo et al. 2013). The grain-size scaling effect could be considered negligible in this study since the $d_o/D_{50}$ ratio is much larger than the proposed limit by Fioravante (2002). The model pile was first fixed to the centre of the tank and the soil was compacted by 50 mm thick layers around the pile to achieve a maximum dry density of 1.45 Mg/m³ (corresponding to $s_r = 95\%$ and $e = 0.79$). The compacted clay is expected to show a double porosity structure with micropores (0.2 - 2 µm) and macropores (5 - 20 µm) (Thom et al. 2007). Although this installation method differs from a full-scale bored pile, it would still provide good soil-pile contact. However, a softer displacement response could be expected in the model pile due to some lesser degree of compaction of soil close to the pile. Finally, the soil mass was saturated for two months by slowly injecting de-aired water from the bottom of the soil tank. Using similar procedure, Nguyen et al. (2020) showed that two months was long enough a period for the soil mass to be fully saturated. During this saturation, the soil mass globally swelled, thus generating uplift forces on both pile shaft and toe. Afterwards, the pile fixation system was removed and a pile head heave was observed. When the rate of pile head heave became small (after 15 days), a 100 N dead-weight (i.e. 20% $Q_v$) was placed on the pile head within seconds and was maintained for two hours to reach equilibrium without waiting for the soil to consolidate. Afterwards, horizontal load was applied in three steps by allowing water flow in the plastic bucket linked to the pile head through a piece of rope over a pulley. In the first, second, and third load steps, horizontal load was increased within 10 minutes from zero to 35 N, 35 to 71 N, and 71 to 109 N, respectively, corresponding to accumulative horizontal displacements of 4, 7, and 12%D, and maintained for more than 10 days at each loading level to reach equilibrium and to consolidate the soil prior to the cyclic thermal loading. At the end of each load step, 15 heating-cooling cycles with temperature variation of $\pm 5$ °C around an ambient temperature of 20 °C (consisting of a 3-hour heating phase followed by a 3-hour cooling phase and eventually, an 18-hour recovery phase) were imposed to the pile by circulating water in fitted aluminium U-shaped tube linked to a temperature-controlled water bath and a peristaltic pump. The heating and cooling phases of each cycle in the model pile simulate 6 months of temperature changes of $\pm 10$ °C in a full-scale
concrete pile of 0.8 m in diameter. This temperature variation is within the range of 3-37 °C of field-scale energy piles (Jiang et al. 2021). It should be noted that the model test could recreate the thermal and mechanical strains similar to field ones.

## 3. Results

In Fig. 2, the continuous line represents the measured pile head heave immediately after removing the fixation system for the first 15-days before starting thermo-mechanical testing, while the dashed line shows the pile head heave extrapolated for the subsequent 90 days during thermo-mechanical testing. The measurements indicate that the pile head heaves rapidly with time in the first days and continues to heave at a reducing rate reaching 6.8% D after 15 days. The reason for this heave could be the pre-mobilized uplift forces on both pile shaft and toe, arising from the saturation-induced soil swelling in the vicinity of the fixed-headed pile (Lalicata et al. 2019; Yunlong & Vanapalli 2021).

<figure><img src="image_3.png" /><figcaption>Fig. 2 Time dependent heave of the pile head after removing the fixation system, measured and extrapolated curves</figcaption></figure>

As illustrated in Fig. 3, prior to the application of horizontal mechanical and cyclic thermal loads, the pile head moves axially by about -0.015%D due to the compressive axial loading of 100 N. This negligible pile head settlement could be attributed to the counteracting effect of the swelling-induced uplift forces.
<figure><img src="image_4.png" /><figcaption>Fig. 3 Changes in pile head axial displacement with time during the compressive axial loading period</figcaption></figure>

As can be seen in Fig. 4, during the application of horizontal loading at each load step, the pile head horizontal displacement increases rapidly with the increase of horizontal load, then keeps on increasing at a reducing rate under the constant horizontal load and reaching 4%D, 7%D, and 12%D at the end of the first, second and third load steps, respectively. Meanwhile, the pile head heaves immediately after increasing the horizontal load, which might be due to further mobilization of the uplift force on the pile shaft due to the tendency of soil located in front of the pile close to the surface to heave. Afterwards, under the constant horizontal loads of 35 and 109 N, the pile head heave initially fluctuates around a steady value of 0.27%D and 0.12%D for about ten days, then begins to decrease gradually and reaches 0.22%D and 0.09%D at the end of the 19-day and 14-day period, respectively. However, under the constant horizontal load of 71 N, pile head heave fluctuates around a steady value of 0.12%D within a period of 11 days. The
gradual reduction of pile head heave could be attributed to the pile being pulled downwards by the mechanically-

induced consolidation and creep of the surrounding clay (Fellenius, 2006).

<figure><img src="image_5.png" /><figcaption>Fig. 4 Changes in horizontal load, pile temperature, and pile head axial and horizontal displacements with time during thermo-mechanical loading stage (Displacement readings at the pile head are zeroed before starting thermo-mechanical testing. The swelling-induced pile head heave has been subtracted from the measured total pile head heave. The dashed line demonstrates displacements extrapolated for the subsequent 15 days at the end of each loading step).</figcaption></figure>
As shown in Fig. 5 (a)-(c), at any level of horizontal loading, the pile head heaves during the heating phase then settles quickly during the subsequent cooling phase, and heaves again during the recovery phase. Meanwhile, it also moves horizontally throughout the entire thermal cycle, except when the horizontal load equals 35 N, the recovery phase could not lead to the pile head horizontal displacement. By the end of each thermal cycle, the model pile keeps accumulating axial and horizontal movements instead of returning to its initial position before the thermal cycle, which means that irreversible displacements occur in both horizontal and axial directions.

<figure><img src="image_6.png" /><figcaption>Fig. 5 Magnified view of the thermal-loading period at the end of (a) the first load step, (b) the second load step, and (c) the third load step.</figcaption></figure>

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<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 5 Continued (the second load step)</figcaption></figure>

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120

121
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>122
123 Fig. 5 Continued (the third load step)</figcaption></figure>
As illustrated in Fig. 6, under the horizontal loads of 35, 71, and 109 N, the irreversible horizontal displacement rapidly increases to 0.09%D, 0.17%D, and 0.25%D during the first thermal cycle, then continues to increase at a decreasing rate with the increasing number of cycles, reaching 0.57%D, 1.15%D, 1.84%D at the end of the 15th cycle, respectively. Thermally-induced irreversible horizontal displacements could be explained by the combined effect of both soil and pile contraction and build-up of excess pore water pressure, contributing to the reduction of horizontal effective stress acting on the pile shaft and subsequently, the reduction of passive soil resistance. Additionally, thermally-induced soil consolidation and creep could contribute in turn to a time-dependent increase in horizontal displacement during the recovery phase. Evidence of these phenomena are available in previous works (Akrouch et al. 2014; Wu et al. 2020; Ng et al. 2014, 2021a, b). The gradual accumulation of thermally-induced irreversible horizontal displacements during thermal cycles could be caused by thermally-induced passive soil resistance mobilization. This kind of ratcheting response was similarly observed for axially-loaded energy piles embedded in clay reported by Nguyen et al. (2020), Wu et al. (2020) and Ng et al. (2014, 2021a, b), and for horizontally-loaded energy pile embedded in dry sand reported by Zhao et al. (2022).

<figure><img src="image_13.png" /><figcaption>Fig. 6 Pile head irreversible axial and horizontal displacements versus the number of heating-cooling cycles (mechanically-induced pile head axial and horizontal displacements due to consolidation and creep of the surrounding clay have been subtracted from the measured total displacements during the thermal-loading period of each respective load step).</figcaption></figure>
It is also inferred from Fig. 6 that under the horizontal loads of 35 and 71 N, the first cycles induce irreversible axial displacement up to -0.03%D and -0.05%D, respectively. This negligible settlement is fully compensated by the pile head heave during subsequent cycles. However, under the horizontal load of 109 N, the first cycle induces irreversible heave up to 0.05%D, which almost remains constant during subsequent cycles and gains an increment of only 0.02%D. This reverse trend could be attributed to the uplift force on the pile shaft effectively mobilized in previous loading steps. It is further worth noting that the accumulated thermally-induced irreversible axial and horizontal displacements under the horizontal load of 71 and 109 N might be influenced by the accumulated irreversible displacements induced by previous loading steps. Overall, with increase in horizontal load, thermal cycles tend to cause a higher irreversible horizontal displacement, resulting in a negligible irreversible heave at the pile head.

## 153 Conclusions

The main conclusion of this study is that under the combined axial/horizontal mechanical loading, imposing heating-cooling cycles to the pile results in irreversible horizontal displacement at the pile head, accumulating at a reducing rate as the number of thermal cycles increases. After 15 heating-cooling cycles, the amount of accumulated thermally-induced irreversible horizontal displacement is estimated at 0.57%D, 1.15%D, and 1.84%D under the horizontal loads of 35, 71, and 109 N, respectively. Reversely, thermally-induced pile head axial displacement at the end of the 15th cycle is practically insignificant (smaller than 0.1%). At higher horizontal loads, thermal cycles induce a higher irreversible horizontal displacement, resulting in a negligible irreversible heave. It is recommended that thermo-mechanical tests are performed with a more significant number of heating/cooling cycles to determine a stabilization threshold of the thermally-induced irreversible pile head horizontal displacement. This study is subject to some limitations, including (1) the absence of pore-water pressure measurement, (2) partial rotational restraint at the pile head, (3) uncertainty about the consolidation degree of the soil mass, (4) the existence of swelling-induced heave, (5) mismatch of the thermal properties of aluminium and concrete, (6) inability in simultaneous scaling of both strength and stiffness of the prototype pile, and (7) inability to reproduce the soil stiffness, stress level and excess pore water pressure within the soil mass at the prototype scale. However, the obtained qualitative results could provide insight into the thermally-induced ratcheting behaviour of mechanically-loaded piles.
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