Advances in Materials Science and Engineering

Volume 2017 (2017), Article ID 7121785, 9 pages

https://doi.org/10.1155/2017/7121785

## Thermal Effect on Structural Interaction between Energy Pile and Its Host Soil

^{1}Department of Civil Engineering, University of Science and Technology Beijing, Beijing 100083, China^{2}State Key Laboratory of Building Safety and Built Environment, China Academy of Building Research, Beijing 100013, China

Correspondence should be addressed to Qingwen Li

Received 6 April 2017; Revised 23 June 2017; Accepted 4 July 2017; Published 15 August 2017

Academic Editor: Shuo Yin

Copyright © 2017 Qingwen Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Energy pile is one of the promising areas in the burgeoning green power technology; it is gradually gaining attention and will have wide applications in the future. Because of its specific structure, the energy pile has the functions of both a structural element and a heat exchanger. However, most researchers have been paying attention to only the heat transfer process and its efficiency. Very few studies have been done on the structural interaction between the energy pile and its host soil. As the behavior of the host soil is complicated and uncertain, thermal stresses appear with inhomogeneous distribution along the pile, and the peak value and distribution of stress will be affected by the thermal and physical properties and thermal conductivities of the structure and the host soil. In view of the above, it is important to determine thermal-mechanical coupled behavior under these conditions. In this study, a comprehensive method using theoretical derivations and numerical simulation was adopted to analyze the structural interaction between the energy pile and its host soil. The results of this study could provide technical guidance for the construction of energy piles.

#### 1. Introduction

In the 1980s, geotechnical engineers in Austria and Switzerland began to use the building foundation as a heat exchanger for the ground-source heat pump (GSHP). The GSHP is a device that can better utilize the energy stored in the soil to transfer the stored heat energy to the structure using pipes laid underground and realize energy balance during both winter and summer. In summer, the host soil acts as a heat sink by transferring the heat from the buildings into the host soil. In winter, the host soil acts as a heat source and transports the heat from the host soil to the buildings. Energy pile is one of the promising areas in the burgeoning green power technology; it is gradually gaining attention and will have wide applications in the future. By taking advantage of the good thermal conductivity of concrete in the energy pile and the large heat exchange area between the pile and the host soil, the performance of the heat exchanger could be improved. Moreover, the energy pile can save the cost of drilling holes and preserve the underground space resources. Compared to the conventional GSHP that has been in use in the past 20 years, the energy pile system (bored pile, precast concrete pile, and underground diaphragm wall) has witnessed rapid development globally, especially in Canada, Japan, and some European countries.

Because of its specific structure, the energy pile has the functions of both a structural element and a heat exchanger. It must withstand not only forces such as the frictional force and tip resistance, and the stresses as in the case of normal piles, but also the thermal stresses caused by the temperature changes during heat transfer. However, most researchers have been paying attention only to the heat transfer process and efficiency. In connection with heat transfer in an energy pile, Gao et al. (2008) studied the thermal performance and ground temperature of vertical pile-foundation heat exchangers and aimed at providing guidelines for improving the design of large-scale ground-coupled heat pumps in a district heating and cooling system [1]; Moon and Choi (2015) studied the heating performance characteristics of a GSHP system with energy piles and energy slabs [2]; Faizal et al. (2016) analyzed the heat transfer enhancement mechanism of geothermal energy piles [3]; Caulk et al. (2016) reported the parameterization of a calibrated geothermal energy pile model [4]; Ghasemi-Fare and Basu (2016) presented a predictive assessment of heat exchange performance of geothermal piles [5]. Regarding studies on laying of piles, Cui et al. (2011) analyzed the heat transfer performance of pile geothermal heat exchangers with spiral coils [6]; Go et al. (2014) designed an energy pile with a spiral coil by considering the effective thermal resistance of the borehole and the effects of groundwater advection [7]; Xiang et al. (2015) developed a new practical numerical model for the energy pile with spiral coils [8]; Fadejev and Kurnitski (2015) used a whole building simulation software to simulate the geothermal energy piles and borehole design with heat pump [9]; Park et al. (2015) studied the coil-type ground heat exchanger by considering the relative constructability and thermal performance of a cast-in-place concrete energy pile [10]; Park et al. (2016) calculated the influence of coil pitch on the thermal performance of coil-type cast-in-place energy piles [11]; Yang et al. (2016) conducted laboratory investigations to analyze the thermal performance of an energy pile with spiral coil ground heat exchanger [12]. Several scholars had conducted research on the heat exchange efficiency of energy piles. Bozis et al. (2011) evaluated the effects of design parameters on the efficiency of heat transfer in energy piles [13]; Park et al. (2015) estimated the constructability and heat exchange efficiency of large diameter cast-in-place energy piles with various configurations of heat exchange pipes [14]; Yoon et al. (2015) reported the thermal efficiency and cost analysis of different types of ground heat exchangers in energy piles [15]; Cecinato and Loveridge (2015) analyzed the factors influencing the thermal efficiency of energy piles [16]; Astrain et al. (2016) performed a comparative study of different heat exchanger systems in a thermoelectric refrigerator and their influence on efficiency [17]; Akrouch et al. (2016) conducted experimental, analytical, and numerical studies on the thermal efficiency of energy piles in unsaturated soils [18]. On energy piles, there are some more research papers which provide technical guidelines for the construction of heat exchanger [19–21].

Numerical simulation is an important prediction method in engineering because of its high accuracy and low cost and the rapid development of computer techniques. Hence, many scholars use analytical tools such as finite element analysis software and finite difference software to solve problems on energy piles. Bezyan et al. (2015) built a 3D model to simulate the heat transfer in geothermal pile-foundation heat exchangers with a spiral pipe configuration [22]; Pu et al. (2015) developed a new practical numerical model for the energy pile with vertical U-tube heat exchangers [23]. Further, several scholars had conducted research on energy piles using numerical simulation methods [24–26]. Most of the above research work covers theoretical analysis, laying of piles, heat exchange efficiency, field test, and numerical simulation of energy piles. However, studies on the structural interaction between the energy pile and its host soil are scarce. As the behavior of the host soil is complicated and uncertain, thermal stresses appear with inhomogeneous distribution along the pile, and the peak value and distribution of stress would be influenced by the thermal and physical properties and thermal conductivities of the structure and the host soil. In view of the above, it is important to determine the thermal-mechanical coupled behavior under these conditions. In this study, a comprehensive method using theoretical derivations and numerical simulation was adopted to analyze the structural response between the energy pile and its host soil. The results of this study can provide technical guidance for the construction of energy piles engineering.

#### 2. Theoretical Analysis

As the foundation of the structure, the energy pile should be able to withstand forces such as the frictional force and tip resistance and the stresses as in the case of normal piles. The lateral friction force of the energy pile can be calculated by the method.where , is the soil pressure coefficient, is internal friction angle, and is the vertical effective stress.

The tip resistance force can be obtained using the rigid-plastic body theory; the tip resistance force is given bywhere is the effect factor of cohesion, is the loading factor for the weight of the soil, is the overload factor, is the diameter of the tip of the pile, is the depth of the buried soil, is the specific gravity of the soil, and is the average specific gravity of the soil.

The thermal stress due to temperature variations resulting from heat transfer should be considered. According to Fourier’s law, the equation for heat conduction could be expressed aswhere is the temperature, is the specific heat, and is the density; and are the thermal conductivities in the - and -directions, respectively.

For deformable materials, the stress increment caused by a change in temperature is given by where is the stress increment. is the Kronecker delta; when , its value is 1, and when , its value is 0; is the bulk modulus, is the coefficient of thermal expansion, and is the temperature increment.

According to the generalized Hooke’s law,where is the stress, is the total strain, is the normal strain, , and .

Lewis and Schrefler (1987) proposed the effective stress [27] caused by change in temperature as follows.

#### 3. Numerical Simulation of Normal Pile

##### 3.1. Simulation Model and Parameters

To predict the structural response between the energy pile and its host soil, a 3D model was built in finite difference software FLAC3D, as shown in Figure 1.