Rafiei, A; (2017) Understanding the thermo-mechanical behaviour of thermal piles in sand. Doctoral thesis , UCL (University College London).

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Abstract

Thermal piles are piled foundations that can be used both to extract heat at shallow depth from the ground and to transfer load from the structure to the ground. Despite an increased number of applications of thermal piles in recent years, knowledge of the thermo-mechanical behaviour of thermal piles is still limited. The literature reveals that additional thermal loading results in considerable induced axial load and stress along the pile, that can lead to a reduction in safety factor down to 1. Also, there are inconsistencies in the literature regarding the thermo-elastic/plastic, reversible/irreversible response of thermal piles and also on the effects of cyclic thermal loading on the side shear friction at the soil–pile interface. Moreover, the framework proposed in the Thermal Pile Standard (Ground Source Heat Pump Association, 2012) has not been tested for various soils conditions. In this study, the effect of thermo-mechanical loading on the mechanical performance of thermal piles and the soil–pile interface is investigated. A 1g laboratory model was developed using a stainless steel model pile embedded in medium-dense, dry sand. Strain and temperature along the pile were monitored using multiplexed fibre Bragg grating sensors. A 2D finite difference heat transfer model was developed in Matlab, predicting the temperature profiles within the soil. Findings from the numerical model were used to design the location of the temperature sensors in the soil. Laboratory tests were divided into five scenarios, involving both shaft resisting and shaft and base resisting piles. It was found that under thermo-mechanical loading, up to 68.4% of the maximum induced load was transferred to the pile toe for the shaft resisting pile, compared to virtually none under mechanical loading. It was further found that the level of restraint caused by medium-dense sand with a relative density of 57% was rather limited in the absence of surcharge load and the degree of freedom varied between 0.97 and 1.0. Moreover, it was found that the location of the null point shifts during each heating/cooling period. For a shaft and base resisting pile heated up to 50°C, the maximum induced thermal load was found to be 90% of the ultimate capacity of the pile. The maximum induced stress remained below the BS 8004:1986 (British Standards Institution, 1986) recommendations. Irreversible settlements were observed for both types of pile. The load threshold, where the limit to thermo-elastic behaviour was observed, was found to be up to 18% of the ultimate pile capacity, while this value was up to 31% in the case of shaft and base resisting pile. Despite an increase in the side friction during heating periods (up to 32% compared to the friction under ultimate state mechanical loading), the subsequent cooling periods seemed to reduce the friction level, and cyclic skin friction degradation and accumulation of pile settlement were observed in the heating and cooling cycles. The results also show deviations from the proposed framework for a model pile in sand mainly due to a variable friction angle at the soil–pile interface.

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