On thermal interaction between slab-on-ground structures and subsoil in Finland

Abstract

This research focusses on the part of the building envelope that forms the interface between the indoor environment and the subsoil. The attempts to control indoor air conditions and the interaction between the interior and the environment for comfort, health and economic reasons have emphasised the significance of slab-on-ground structures as a borderline between the indoors and the soil. In order to study the thermal and moisture behaviour of a slab-on-ground structure one must know the ambient boundary conditions. The main objective of this study is to determine the thermal conditions along the fill layer underneath a slab-on-ground structure of a heated building.

Thermal interaction between the slab-on-ground structure of a heated building and the subsoil under varying temperature conditions is a complicated three-dimensional heat transport problem. Many state-of-the-art methods consider slab-on-ground­subsoil interaction to occur only between two boundary temperatures ­ the internal and external temperatures ­ ignoring the thermal effect of the subsoil itself and treating it merely as a conduction body in between. However, any building on subsoil or bedrock is built on a vast thermal storage reservoir with a relatively constant balance of heat: the earth's crust. Seasonal variations occur only close to the soil surface, and the thermal disturbance caused by a heated building on soil surface is marginal compared to the massiveness of the soil and rock volumes beneath it and the heat they can store.

According to the field measurements and numerical simulations of this study, the temperature distribution in the fill layer underneath a slab-on-ground structure is influenced by three ambient boundary temperatures: the internal, the external and the subsoil temperature. The influence of internal boundary temperature and the thermal resistance of the slab structure are the most significant factors affecting the temperature distribution in the fill layer in the central part of the slab. The impact of external temperature increases near the external wall line. According to the performed numerical simulations, the DSA analysis and their comparison with field measurements, the periodic external boundary temperature and the fill layer underneath the slab structure interact directly through the footing wall as well as laterally along the fill layer underneath the slab. The influence of constant subsoil boundary temperature is lesser: according to static-state weighting factor analysis, its impact on the fill temperature is 10 to 20 per cent. However, constant subsoil boundary temperature acts as a balancing force by warming the fill layers under the slab structure during winter and by cooling them in summer. Based on previous observations, a semi-analytical method for the estimation of temperature distribution along the fill layer underneath a slab-on-ground structure in periodic conditions was developed. The method applies the superposition principles, where the annual mean boundary temperatures form a basic steady-state temperature distribution and the amplitudes of external and internal temperatures contribute periodic seasonal alterations.

Keywords: slab-on-ground structure, temperature distribution, heat flow, subsoil, fill layer