Application of thermal piles in thawing a frozen ground

The thawing of the frozen ground below an ice rink facility in Myllypuro, eastern Helsinki, Finland was studied in detail through analytical approach and numerical simulations. A malfunction in the floor-heating system caused freezing conditions in the ground underneath. Over the years, frost heave caused significant deformations to the facility and, in 2012 forced an immediate renovation. During the renovation, the existing old foundation was replaced with a new foundation system comprised of a well-insulated concrete floor-slab and a group of around 240 steel thermal piles. Thermal piles were preferred in this case because the frozen ground surrounding individual piles needed to be thawed, hence a direct contact between the soil and the pile shaft can be prevented. This reduces the possible risk of additional load from the frozen ground being applied on the piles.
As part of the ongoing monitoring programme, a thermal modelling of the thawing process was carried out with commercially available SoilVision Heat (version 2.4.10) software programme. The primary objective of this study was to model the thawing process numerically and therefore, the time needed to thaw the frozen ground can be estimated. Upon complete thawing, establishing the required constant temperature that needs to be maintained in the piles in order to prevent the ground from re-freezing was another objective. The ground temperature profiles obtained from the model simulations were compared with in situ temperature measurements as the thawing progresses. The time needed for complete thawing of the frozen ground from the simulations is in good agreement with analytical estimations and in situ observations. The thermal modelling shows that once the frozen ground is completely thawed, a heat injection at around +7°C by the piles is sufficient to prevent the ground from re-freezing.