Axial and radial thermal responses of energy pile under six storey residential building

Abstract

The axial and radial thermal responses of a cast-in-place energy pile, 10 m long and 0.6 m in diameter, installed in unsaturated sand under a six storey building are examined during a heating–cooling cycle. The instrumentation in the pile was configured to compare radial and axial thermal responses at the same elevations and to evaluate the temperature and axial thermal stress distribution across the cross-sectional area of the pile. The magnitudes of the axial thermal strains were more constrained than the radial thermal strains at all depths, leading to the development of axial and radial thermal stresses of up to –4.5 and –0.015 MPa, respectively, for a change in average pile temperature of 24.1 °C. The magnitudes of the radial thermal stresses with changes in pile temperature were significantly lower than the axial thermal stresses at all depths of the pile, indicating that the radial thermal expansion had negligible effects on the development of axial thermal strains and stresses. The temperature distribution over the cross section of the pile showed low variations at all depths, indicating that it would be justified to consider a uniform temperature distribution at least in piles of similar dimensions and with even heat exchanger layouts.

Effects of Cyclic Temperature Variations on Thermal Response of an Energy Pile under a Residential Building

“…The results shown in Figure 6 indicate that the load of the building does not have any significant impact on the reversibility of the axial thermal strains for frequent cyclic temperature changes of the pile. The calculated ultimate capacities of compressive and tension mechanical loads are 2701 kN (9.6 MPa) and 2157 kN (7.6 MPa), respectively (Faizal et al, 2018b). The design compressive and tension axial working mechanical loads of the 10 m long, 0.6 m diameter energy pile with a factor of safety of 1.9 are 1404 kN (4.96 MPa) and 1122 kN (3.97 MPa), respectively (Faizal et al, 2018b).…”

“…The calculated ultimate capacities of compressive and tension mechanical loads are 2701 kN (9.6 MPa) and 2157 kN (7.6 MPa), respectively (Faizal et al, 2018b). The design compressive and tension axial working mechanical loads of the 10 m long, 0.6 m diameter energy pile with a factor of safety of 1.9 are 1404 kN (4.96 MPa) and 1122 kN (3.97 MPa), respectively (Faizal et al, 2018b). The design pile head load is thus approximately 52%…”

“…One of the two energy piles was instrumented with vibrating wire strain gauges (VWSGs) and thermocouples as shown in the schematic in Figure 1 and was subjected to thermal cycles in this study. Faizal et al (2018b) described in details the instrumentation and installation of these energy piles. In summary, the instrumented energy pile contained 30 VWSGs (model Geokon 4200) installed at five depths of the pile to monitor both axial and radial strains.…”

“…The outer axial VWSGs were attached to the reinforcement bars, the central axial VWSGs were attached to the outer side of the tremie guides that housed removable tremies used to pour concrete, and the radial VWSGs were attached to steel bars welded across the diameter of the pile. Further details about the sensors, instrumentation and installation process are documented elsewhere (Faizal 2018;Faizal et al 2018b). The concrete temperatures were recorded from day two of casting and were observed to reduce with curing time to the magnitudes of the surrounding undisturbed soil temperatures (Bouazza et al 2015;Singh et al, 2015, Faizal 2018 and were evenly distributed with depth, indicating that the shaft geometry remained uniform with depth without any defects in the concrete.…”

“…The concrete temperatures were recorded from day two of casting and were observed to reduce with curing time to the magnitudes of the surrounding undisturbed soil temperatures (Bouazza et al 2015;Singh et al, 2015, Faizal 2018 and were evenly distributed with depth, indicating that the shaft geometry remained uniform with depth without any defects in the concrete. Technology's USB-TC08 data loggers and Campbell Scientific CR1000 data loggers, respectively (Faizal et al, 2018b).…”