Introduction

Ground source heat pump is a type of energy efficient and environmentally friendly air-conditioning system that utilizes underground shallow geothermal resources for both heating and cooling. The traditional ground source heat pump system is mostly in the form of drilling and burying heat exchange pipes, which will occupy a large amount of underground space. The energy pile can support the load from superstructures as well as realize the utilization of shallow geothermal energy through the heat exchange pipes arranged in pile shaft1. Compared with the traditional installation method of heat exchange pipes, the energy pile technology can reduce the installation costs as well as the occupation of underground space. The energy pile technology has been successfully applied in engineering practice all around the world2. Many researchers have investigated the heat transfer performance and behavior of energy piles3,4,5. Moreover, the thermo-mechanical properties of different types of energy piles were also analyzed in detail6,7,8. The bearing capacity and thermo-mechanical properties of energy piles were analyzed through field tests9,10, and several types of geothermal heat exchangers were adopted to determine the most efficient type11,12,13. The field test results indicated that the induced radial thermal stresses were much smaller than the axial thermal stresses, and the radial thermal stress had little effect on the development of thermo-mechanical loads. The pre-stressed high strength concrete (PHC) pile14,15, steel pile16 and cast-in-place pile17,18 were normally adopted to equip the heat exchange pipe of geothermal heat exchanger. The thermo-mechanical behavior of cement fly-ash gravel (CFG) pile was also investigated through full-scale tests19. Centrifuge and 1 g model tests were also carried out to investigate the effect of cyclic heating and cooling on the thermo-mechanical behavior of energy piles, and the end restraint effects of energy piles were also analyzed based on the test results20,21,22. Moreover, the influence of heat exchange pipe configurations and numbers23,24, pile diameter and length, fluid mixture compositions and flow rates25,26,27 on the pile energy efficiency and thermo-mechanical behavior of energy pile were also investigated through numerical researches.

With the urbanization advancement in China, the slurry discharge problem caused by the bored pile construction as well as the soil compaction effect caused by driven/hammered PHC pile installation limit the application of these two piles in urban areas. The pre-bored grouted planted (PGP) pile is a type of non-displacement pile, and the PHC pile is inserted into the pile hole which is drilled and grouted in advance, and the PGP pile installation process will not discharge slurry28,29. The pile shaft of PGP pile consists of a high-strength PHC pile and cement soil layer30, and the behavior of PGP pile is proved to be much better than cast-in-situ pile31,32,33. The soil disturbance in PGP pile installation is very limited34,35, and the heat exchange pipes of the ground source heat pump system can be arranged on the external side of PHC pile. The heat exchange pipes will be placed into the ground with the PHC pile to form a new type of geothermal energy pile: pre-bored PHC energy pile. For conventional driven/hammered PHC pile, the heat exchange pipes have to be equipped on the internal side of PHC pile, otherwise the heat exchange pipes would be damaged in PHC pile installation process. The survival rates of heat exchange pipes for pre-bored PHC energy pile could be effectively increased as the resistance from liquid cemented soil on PHC pile was very small in the PHC insertion process. The pre-bored PHC energy pile is a relatively new type of energy pile. The research on heat transfer performance and thermo-mechanical properties of pre-bored PHC energy pile is very limited, and there is a lack of design methods for engineering practice of pre-bored PHC energy pile.

In this paper, the heat transfer performance and thermo-mechanical properties of pre-bored PHC energy pile were investigated through a group of full-scale tests, and the distributed fiber optic sensors (DFOS) were arranged in the test piles. The field tests for pre-bored PHC energy pile under summer and winter conditions were conducted, and the heat transfer efficiency and pile shaft temperature and strain of test piles were measured in the test process. The research results can be applied for the engineering practice of pre-bored PHC energy pile.

Field test

Geological conditions of test site

The soil layer distribution and soil properties of the test site were measured before the thermo-mechanical tests as shown in Table 1, in which H is the thickness of soil layer, w is the water content of soil, γ is the unit weight of soil, e is the void ratio of soil, LL and PL are the liquid limit and plastic limit, respectively, c is the cohesion of soil and φ is internal friction angle. The c and φ are measured by consolidated undrained triaxial tests in laboratory. Table 1 shows that the test site was distributed with deep muddy clay and silty clay soils. The length of test pre-bored PHC energy pile was 45 m, and the fine sand layer was selected as the bearing stratum of test piles.

Test pile

Two full-scale pre-bored PHC energy piles were designed at the test site to investigate their heat transfer performance and thermo-mechanical properties. The DFOS were arranged in pile shaft to measure the temperature and strain changes in the test process. The range of DFOS was ± 2000 µε, the resolution was 0.1 µε, and the accuracy was 0.3%FS. The operating temperature of DFOS was − 40℃ – +80 ℃, which indicated that the DFOS could be used to test the temperature and strain changes of pile shaft in energy pile operation process. The stress in the cemented soil was very small in the load transfer process of pre-bored PHC pile,29 and the DFOS were not equipped in the cemented soil layer. The DFOS were calibrated in the laboratory before being equipped along the test pile. The arrangement of DFOS along PHC pile is presented in Fig. 1.

Table 1 Soil profiles and properties of test site.
Full size table
Fig. 1
Fig. 1
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Arrangement of DFOS along PHC pile. (a) PHC nodular pile, (b) PHC pipe pile.

The DFOS were embedded into the pre-drilled groove at the surface of PHC pile shaft. The epoxy resin adhesive was then poured into the gap between the DFOS and groove. The epoxy resin would bond the DFOS to the PHC pile as a single unit and serve as a protective layer for the DFOS after it was hardened. The DFOS were arranged in U-shaped form along the pile shaft, and the distance of neighboring sensor was 1 m. It was assumed that the strain measured by the DFOS was identical to the strain of PHC pile shaft at the identical cross section.

The test pre-bored PHC energy pile installation process is presented in Fig. 2. The heat exchange pipes were directly attached on the external surface of PHC pile by gluing, and the heat exchange pipes would be surrounded by cemented soil when the construction process of test pile was completed. The diameter of cemented soil column of pre-bored PHC pile was designed to be 100–200 mm larger than the diameter of PHC pile. The drilling rig was firstly used to drill and grout a 560 mm diameter pile hole, and the depth of pile hole was 45 m. The 400 mm PHC pipe pile with a wall thickness of 95 mm was adopted as the upper part of the high-strength core pile, and the length of PHC pipe pile segment was 33 m. The 400(500) mm nodular pile with a length of 12 m was used for the bottom part of test pile. The nodular pile end with a diameter of 400 mm was welded to the 400 mm PHC pile, to ensure the integrity of the PHC pile.

Fig. 2
Fig. 2
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Photograph and schematic of pre-bored energy PHC pile. (a) Photograph of pre-bored PHC energy pile installation process, (b) Layout of heat exchange pipes along pile shaft.

The advantages of the combined use of PHC pipe pile and PHC nodular pile have been introduced in the previous research28. The nodules along the nodular pile can largely increase the shaft resistance of PHC pile – cemented soil interface. Moreover, the frictional capacity of cemented soil – soil interface was also better than that of concrete – soil interface for bored pile30. The heat exchange pipe was made of high-density polyethylene pipe HDPE (PE100) with a diameter of 32 mm and a wall thickness of 3 mm. Rafai et al. introduced a type of displacement cast in situ energy pile, and the heat exchanger loops were equipped at the reinforcement cage36. The heat exchanger loops were finally filled with concrete. The field test results indicated that the displacement cast in situ energy pile showed only a minor reduction in shaft resistance when subjected to multiple thermal cycles. It can be seen in Fig. 2 that the heat exchange pipes were fixed on the external surface of PHC pile, and the heat exchange pipes could be placed into the ground along with the PHC pile. The heat exchange pipes were very close to the surrounding soil, which indicated that the heat exchange efficiency of pre-bored PHC energy pile would be improved. The PHC pile could be inserted into the pile hole by their own weight29, and the pre-bored PHC energy pile installation would bring little disturbance to the ambient environment. Moreover, the cemented soil in test pile hole was in liquid state as shown in Fig. 2a, and the resistance from the liquid cemented soil applied on the heat exchange pipes in pre-bored PHC energy pile installation process was small, which could effectively protect the heat exchange pipes.

The cemented soil in the pile hole would become the protective layer for the heat exchange pipes as the cemented soil strength would increase with curing time. The cemented soil samples were taken from the test pile hole, and the cemented soil specimens were manufactured for unconfined compressive strength tests. The 70.7 mm × 70.7 mm × 70.7 mm cubic cemented soil specimens were cured in the standard curing room for 28 days. The unconfined compressive strength tests were then carried out, and the strength of cemented soil was 1.32 MPa. The PHC pile – cemented soil interface shear tests were also carried out which has been introduced in the previous research,30 and the peak shear strength of PHC pile – cemented soil interface reached 159.7 kPa. The shear strength of PHC pile – cemented soil interface was much larger than the shear strength of cemented soil – soil interface. The PHC pile, heat exchange pipe and cemented soil layer were an integral in the load transfer process, and the heat exchange pipe was protected by the cemented soil layer. Moreover, the pure cement grout was injected into the enlarged base to form the enlarged grout base, which could improve the base capacity.

Totally 46 pre-bored PHC energy piles were installed for the practical engineering project, and the distance of neighboring energy piles was around 4–5D (D is pile diameter). The spacing of two test piles were about 10 m. The hydraulic pressure in heat exchange pipe was measured in the pre-bored PHC energy pile installation process. The initial hydraulic pressure in heat exchange pipe was set to be 1 MPa, and the hydraulic pressure in heat exchange pipe of 46 pre-bored PHC energy piles were all around 1 MPa after pile installation. Hence, it can be considered that the survival rate of heat exchange pipe of pre-bored PHC energy pile reached 100%, which proved that the unique installation method of pre-bored PHC energy pile effectively increased the survival rate of heat exchange pipe. Moreover, the heat exchange pipes were arranged on the external surface of PHC piles, which could probably improve the heat exchange efficiency of energy pile.

Initial temperature of soil layers in test site

The thermal response tests were firstly conducted to measure the initial average temperature of soil layers in the test site. The field thermal response test equipment was adopted in the tests, and the test system consisted of heat pump system, electric heater, make-up water tank, circulating pump, circulating pipeline, temperature and flow monitoring components. The flow meter was equipped at the outlet pipe, and temperature monitoring points were arranged at both inlet and outlet pipes to automatically record the circulating water flow and temperature in the heat exchange pipe system. The non-heating cycle test in which the water in heat exchange pipe was not heated or cooled was carried out to obtain the initial average temperature of the soil layers. The average temperature of water from outlet pipe could be considered as the initial average temperature of soil layers when the difference of water temperature from inlet pipe and outlet pipe was less than 0.5 ℃. The variation of water temperature from inlet pipe and outlet pile with test time is arranged as shown in Fig. 3. This figure shows that the water temperature from outlet pipe was stabilized at about 21.4 ℃ after the test times was larger than 10 h, and the difference between the water temperature from inlet pipe and outlet pipe was also less than 0.5 ℃. It could be considered that the initial average temperature of soil layers within the depth of test pile was 21.4 ℃.

Fig. 3
Fig. 3
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Initial average temperature of soil layers in the test site.

Field test of pre-bored PHC energy pile

Constant heat flow test

The constant heat flow test was firstly carried out on test pile 1 to measure the comprehensive thermal conductivity of soil layers within the depth of test pre-bored PHC energy pile, and the entire flowchart of the experimental testing process for test pile 1 is presented in Fig. 4.

Fig. 4
Fig. 4
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Flowchart of experimental testing process for test pile 1.

The constant heat flow test was carried out with a test time of 48 h, and the heating power was set as 3 kW. The water flow rate of heat exchange pipe was 1.013m3/h. The variation of water temperature from inlet pipe and outlet pipe with test time is then arranged as presented in Fig. 5.

Fig. 5
Fig. 5
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Variation of temperature from inlet and outlet pipes with test time. (a) Variation of temperatures from inlet and outlet pipes, (b) Relationship between average water temperature of inlet and outlet pipes and test time ln(t).

It can be seen from Fig. 5a that the water temperature from the inlet pipe and outlet pipe increased rapidly within the first 8 h, and the increasing rate decreased gradually as the test time kept increasing. The test period in the range 24–48 h could be considered as the steady state thermal conductivity process. The heat transfer power of test pile 1 can be calculated according to the water temperature from the inlet pipe and outlet pipe during the test time of 24–48 h,

$$Q=\rho \cdot c \cdot G \cdot \Delta t/3600$$
(1)

in which ρ is the gravity of water, c is the specific heat of water, G is the water flow rate, and Δt is the water temperature difference between the inlet pipe and outlet pipe. The density of water was 1.0 × 103kg/m3, the value of c was 4.186 × 103 J/(kg ℃), G was 1.013m3/h, and the value of Δt = 31.63–29.3 ℃=2.33 ℃. The calculated heating power of test pile Q = 2.745 kW according to Eq. (1).

The integrated thermal conductivity λ of soil layers around test pile could then by calculated based on the calculated heating power by the following equation37,

$$\lambda =\frac{Q}{{4\pi kH}}$$
(2)

in which k is the slope of the relationship curve between the average water temperature of inlet and outlet pipes and test time ln(t) as shown in Fig. 5(b), and H is the length of test energy pile. The value of k was 2.456, and the length of test energy pile H was 45 m. The calculated integrated thermal conductivity of the soil layers within the depth of test energy pile λ = 1.98 W/(m °C) according to Eq. (2).

The temperature of test pile shaft in the constant heat flow test was measured by DFOS. The variation of test pile shaft temperature with test time is then arranged as shown in Fig. 6. It can be seen from Fig. 6 that the initial temperature of pile shaft increased from 15℃ to 20℃ in the depth of 0–15 m before the constant heat flow test. Moreover, the temperature of pile shaft was around 20℃ in the depth of 15–45 m. Figure 5 also shows that the temperature of pile shaft increased obviously with the test time increased, while the increasing rate decreased with test time. When the test time reached 48 h, the temperature of test pile shaft increased from 25℃ to 30℃ at the depth of 0–10 m. This is because that the initial pile shaft temperature at the depth of 0–10 m was affected by the atmospheric temperature, which was smaller than the initial pile shaft temperature at the depth of 10–45 m. The pile shaft temperature at the depth of 10–33 m was around 30℃ after 48 h, which indicated that the pile shaft temperature at the depth of 10–33 m was increased by 10℃. It can also be seen in Fig. 6 that the measured pile shaft temperature was decreased at the depth of 35 m, and the pile shaft temperature at the depth of 35–45 m was around 28℃. This is due to the reason that the water temperature from the inlet pipe decreased along the depth, and the water temperature from the outlet pipe was 2.33 ℃ smaller than that from the inlet pipe as shown in Fig. 5(a). As a result, the increase of pile shaft temperature was about 8℃ at the depth of 35–45 m after 48 h.

Fig. 6
Fig. 6
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Variation of temperatures of test pile shaft in constant heat flow test.

The pile shaft strain measured by DOFS in the constant heat flow test are presented in Fig. 7. It can be seen in Fig. 7 that the pile shaft strain induced by the temperature change of pile shaft also increased with the test time, and the increasing rate of pile shaft strain decreased with the increase of test time. Moreover, the maximum pile head strain reached 90 µε when the test time reached 48 h, and the measured maximum pile base strain was 88.9 µε. Figure 7 also shows that the pile shaft strain reached two peak values at pile head and pile base, respectively, while the strains were relatively small around the middle of pile shaft. This is because that the test pile head was not constrained, and the constraint force on pile base was also small as there was no pre-compressive stress at pile base. On the contrary, the constraint force from the surrounding soil was relatively large around the middle part of pile shaft, which limited the development of pile shaft strain.

The axial free strain εf and axial thermally induced stress σT of pile shaft can be calculated by the following equations,

$${\varepsilon _f}={\alpha _c} \cdot \Delta T$$
(3a)
$${\sigma _T}=E \cdot ({\varepsilon _T} - {\alpha _c} \cdot \Delta T)$$
(3b)

in which αc is the linear expansion coefficient of concrete of pile shaft, and the empirical linear expansion coefficient of concrete was 10 µε/℃ when the temperature was in the range 0–100 ℃; ΔT is the value of temperature change of pile shaft; E is the elasticity modulus of pile shaft, and the value of E was 40 GPa; εT is the measured strain of pile shaft, and the value of compressive stress and compressive strain were set to be positive.

Fig. 7
Fig. 7
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Variation of pile shaft strains with time in constant heat flow test.

The temperature change of pile shaft was about 10℃ when the test time reached 48 h as shown in Fig. 6, and the axial free strain of pile shaft should be around 100µε according to Eq. (3a). Th maximum pile shaft strain at pile head and pile base were 92.7µε and 88.9µε, respectively when the test time reached 48 h as shown in Fig. 7. Hence, the measured maximum pile shaft strain at pile head and pile base were relatively close to the free strain, which demonstrated that the constraint force at pile head and pile base were relatively small. Figure 7 also shows that the minimum pile shaft strain was 54.9µε at depth of 26 m. The calculated thermally induced stress σT at the depth of 26 m was 1.8 MPa according to Eq. (3b).

Constant temperature test under summer conditions

After the increased temperature of soil layers induced by the constant heat flow test of test pile 1 was recovered, the constant temperature test under summer conditions for test pile 1 and test pile 2 were then conducted. The constant temperature test under summer conditions for two test piles were both lasted for 48 h. The water temperature of the inlet pipe was set to be 35 °C which was around the average atmosphere temperature in summer of test site. The relationship between water temperature from inlet pipe and outlet pipe of test pile 1 and test time is presented in Fig. 8. It can be seen from Fig. 8 that the water temperature from inlet pipe and outlet pipe tended to be stable after the test time reached 8 h. The water temperature from inlet pipe and outlet pipe were around 35.2 ℃ and 32.5 ℃, respectively when the test time was in the range 24–48 h. The measured water flow rate of the heat exchange pipe system was 1.11m3/h, and the calculated heat exchange power of test pile 1 under summer conditions was 3.48 kW according to Eq. (1). The length of test pile 1 was 45 m, and the unit heat exchanger power q = 3.48 × 103/45 = 77.3 W/m.

Fig. 8
Fig. 8
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Variation of water temperatures of the inlet pipe and outlet pipe of test pile 1 with test time under summer conditions.

The relationship between the water temperature from inlet pipe and outlet pipe of test pile 2 and test time is then arranged as shown in Fig. 9. This figure shows that the water temperature from inlet pipe and outlet pipe also tended to be stable after the test time reached 8 h. The water temperature from inlet pipe and outlet pipe were 35.2 °C and 32.2 °C, respectively when the test time reached 48 h. The measured water flow rate of heat exchange pipe system was 1.10 m3/h, and the calculated heat exchanger power of test pile 2 was 3.84 kW according to Eq. (1). The unit heat exchange power q = 3.84 × 103/45 = 85.3 W/m. Hence, the unit heat exchange power of test pile 1 and test pile 2 under summer conditions were 77.3 W/m and 85.3 W/m, respectively. The heat exchange efficiency of two test pre-bored PHC energy piles were close to each other. The average unit heat exchange power of two test piles was 81.3 W/m in this research.

Fig. 9
Fig. 9
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Variation of water temperatures of the inlet pipe and outlet pipe of test pile 2 with time under summer conditions.

The unit heat exchange power of conventional energy pile was in the range 15–80 W/m, and the unit heat exchange power of ground source heat pump technology was 40–60 W/m17. Hence, the measured average unit heat exchange power of test pre-bored PHC energy pile was larger than that of conventional energy pile and ground source heat pump technology. This is probably because that the heat exchange pipes of pre-bored PHC energy pile were equipped at the external side of PHC pile and in contact with the surrounding soil.

The pile shaft temperature of test pile 1 and test pile 2 were measured by the DOFS arranged in the test pile. The variation of pile shaft temperature of test pile 1 and test pile 2 with test time are presented in Fig. 10. It can be seen from Fig. 10 that the variation of pile shaft temperature of test pile 1 and test pile 2 in the test process were basically the same. The pile shaft temperature of test pile 1 and test pile 2 both increased with the test time, while the increasing rate decreased gradually with the increase of test time. When the test time reached 48 h, the temperature of test pile 1 shaft increased in the range 8.5–10.9 °C at different depths, and the temperature of test pile 2 shaft increased in the range 9.8–12.8 °C at different depths. The temperature change of test pile 1 and test pile 2 was relatively close to each other, which indicated that the thermal conductivity of soil layers around two test piles was almost the same.

Fig. 10
Fig. 10
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Variation of pile shaft temperature with test time under summer conditions. (a) Test pile 1, (b) Test pile 2.

The pile shaft strain of test pile 1 and test pile 2 measured by the DOFS are presented in Fig. 11. This figure shows that the forms of the distribution of pile shaft strain of test pile 1 and test pile 2 were relatively close to each other. The pile shaft strain reached two peak values at pile head and pile base, respectively, and the minimum pile shaft strain occurred around the middle of pile shaft. Figure 11a shows that the maximum strain of test pile 1 at pile head and pile base reached 99.8 µε and 88.3 µε, respectively, when the test time reached 48 h. The minimum strain of test pile 1 was only 53.9 µε at the depth of 25 m when the test time reached 48 h.

Fig. 11
Fig. 11
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Variation of pile shaft strain with test time under summer conditions. (a) Test pile 1, (b) Test pile 2.

Figure 11b shows that the maximum strain of test pile 2 at pile head and pile base were 118.4 µε and 103.3 µε, respectively, when the test time reached 48 h. Moreover, the minimum strain of test pile 2 was 64.2µε at the depth of 24 m when the test time reached 48 h. The thermal strain of two test piles were close to each other, which also indicated that the shaft resistance provided by the surrounding soil of two test piles were similar.

The thermally induced stress of test pile can be calculated based on the measured pile shaft temperature and strain as shown in Figs. 10 and 11. The distribution of thermally induced stress of two test piles when the test time reached 48 h are presented in Fig. 12. This figure shows that the distribution of thermally induced stress of two test piles were close to each other. The value of thermally induced stress at pile head and pile base were small, and the thermally induced stress gradually increased from pile head and pile base to the middle of test pile shaft. Moreover, the maximum thermally induced stress of test pile 1 was 2.06 MPa at the depth of 25 m, and the maximum thermally induced stress of test pile 2 was 1.83 MPa at the depth of 24 m.

Fig. 12
Fig. 12
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Distribution of thermally induced stress along pile shaft of two test piles.

The measured maximum thermally induced stress of test pile 1 and test pile 2 were both around 2 MPa. The maximum thermally induced stress of test pile 1 and test pile 2 occurred at 25 m and 24 m, respectively, which was both around the middle of test pile shaft. This is because that the pile shaft would expand under summer conditions, and at the null point the displacement is minor and stress is major38. In this research, the null point of test pile 1 and test pile 2 under summer conditions was 25 m and 24 m, respectively. Moreover, the ultimate compressive strength of the concrete of PHC pile shaft was 80 MPa, and it could be assumed that the thermally induced stress in pre-bored PHC energy pile under summer conditions would not affect the structural safety of pile shaft.

Constant temperature test under winter conditions

The constant temperature test of two test piles under winter conditions were then conducted after the pile and soil temperature change induced by the constant temperature test under summer conditions were recovered. The constant temperature test of test pile 2 under winter conditions was not carried out because the thermal response test equipment was unfortunately malfunctioned. Only the test results of test pile 1 under winter conditions were analyzed in the paper. The water temperature of the inlet pipe was set to be 8 °C which was around the average atmosphere temperature in winter of the test site, and the entire test time was also 48 h. The relationship between water temperature of inlet pipe and outlet pipe of test pile 1 and test time under winter conditions is presented in Fig. 13. This figure shows that the water temperature from the inlet pipe and outlet pipe tended to be stable after the test time reached 16 h. Moreover, the water temperature from the inlet pipe and outlet pipe were 8.4 °C and 11.0 °C, respectively when the test times reached 48 h. The measured water flow rate of the heat exchange pipe was 0.98m3/h. The calculated heat exchange power of test pile 1 under winter conditions was 2.96 kW according to Eq. (1). The unit heat exchange power was 65.8 W/m as test pile length was 45 m.

Fig. 13
Fig. 13
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Variation of temperatures of the inlet pipe and outlet pipe of test pile 1 with time under winter conditions.

The temperature of pile shaft of test pile 1 in the constant temperature test under winter conditions was also measured by DOFS. The variation of pile shaft temperature in the test process is then arranged as presented in Fig. 14. This figure shows that the pile shaft temperature decreased gradually with the increase of test time. The initial pile shaft temperature was in the range 18.0–20.2 °C, and the pile shaft temperature decreased to 8.20–11.0 °C after the test time reached 48 h. The pile shaft temperature was decreased by 9.2–10.3 °C at different depths.

The variation of pile shaft strain of test pile 1 measured by DOFS in the test process is presented in Fig. 15. It can be seen from Fig. 15 that obviously tensile strain occurred in the test pile shaft, and the tensile strain was caused by the decrease of pile shaft temperature. Moreover, the tensile strain also reached two peak values at pile head and pile base, respectively. The maximum tensile strain at pile head and pile base were − 97.5 µε and − 90.6 µε, respectively when the test time reached 48 h. The minimum tensile strain was − 61.3 µε at the depth of 21 m when the test time reached 48. This is also because that the constraint force at pile head and pile base were relatively small, while the constraint force increased from pile head and pile base to the middle part of pile shaft. The pre-bored PHC pile was a non-displacement pile, and there was no pressure at the pile base after installation of test pile. The test pile base was located at the fine sand layer, and the constraint force at pile base was small in pile contracts process. Hence, the measured pile shaft strain was large at the depth of 40–45 m.

Fig. 14
Fig. 14
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Variation of temperature of test pile 1 with test time under winter conditions.

Fig. 15
Fig. 15
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Variation of pile shaft strain of test pile 1 with test time under winter conditions.

The thermally induced stress of test pile 1 occurred in the constant temperature test under winter conditions can also be calculated according to Eq. (3b). The change of pile shaft temperature was in the range − 10.3 – -9.2 °C when the test time reached 48 h, and the corresponding pile shaft strain change was − 97.5 – -61.3µε. The distribution of thermally induced stress of test pile 1 after the test time reached 48 h is then presented in Fig. 16.

Fig. 16
Fig. 16
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Distribution of thermally induced stress of test pile 1 under winter conditions.

It can be seen from Fig. 16 that the thermally induced stress at pile head and pile base were also relatively small due to the small constraint force. Moreover, the maximum thermally induced stress was − 1.6 MPa at the depth of 24 m. It should be noted that the tensile strength of concrete of PHC pile shaft was much smaller than the compressive strength. The ratio of the ultimate tensile strength and compressive strength of concrete was about 0.1, and the ultimate tensile strength of PHC pile shaft was only 8 MPa in this research. The thermally induced stress occurred in the pile shaft was smaller than the ultimate tensile strength of pile shaft. Nevertheless, it is necessary to pay special attention to the tensile stress occurred in the pile shaft of pre-bored PHC energy pile under winter conditions, as the value of tensile stress reached 0.2 times of the ultimate tensile stress of pile shaft. In the engineering practice of pre-bored PHC energy pile, the long-term cyclic temperature loading process may increase the tensile stress under winter conditions, and the ultimate tensile strength of pile shaft will probably decrease under cyclic temperature loads.

Conclusion

In this paper, the heat transfer performance and thermo-mechanical properties of two pre-bored PHC energy piles are studied based on the full-scale test results, and the following conclusions can be obtained:

  1. (1)

    The unique installation method of pre-bored PHC energy pile could effectively increase the survival rate of heat exchange pipe in energy pile installation process. The survival rate of heat exchange pipe of 46 pre-bored PHC energy piles reached 100% in this practical project.

  2. (2)

    The unit heat exchange power of two test piles under summer conditions were 77.3 W/m and 85.3 W/m, respectively, and the average value was 81.3 W/m, which was larger than that of conventional energy piles; the maximum thermally induced stresses of test pile 1 and test pile 2 were 2.06 MPa and 1.83 MPa, respectively.

  3. (3)

    The unit heat exchange power of test pile 1 under winter conditions was 65.8 W/m; obvious shrinkage occurred in the pile shaft in the test process, and the maximum thermally induced stress was − 1.6 MPa at the depth of 24 m; it was necessary to pay attention to the effect of tensile stress on structural safety of pile shaft for the pre-bored PHC energy pile under long-term cyclic temperature loads.

  4. (4)

    The two test piles in this research were engineering piles, and the test time for summer and winter conditions were both 48 h due to the tight schedule of the practical engineering project. The effect of long-term cyclic temperature loads on structural safety of pre-bored PHC energy pile was not investigated. The research on heat transfer performance and thermo-mechanical properties of pre-bored PHC energy pile under long-term cyclic temperature loads should be carried out in the following study.