A modelling study of the variation of thermal conductivity of the English Chalk

Thermal conductivity is required when designing ground heating and cooling schemes, electrical cable conduits and tunnel ventilation. In England these infrastructures are often emplaced within the Chalk. To improve knowledge on chalk thermal conductivity, over the few scattered measured values, estimates have been made from multi-component mixture models based on the mineral composition, porosity and the structure of the Chalk. The range in mid values for the thermal conductivities is 1.78–2.57 W m−1 K−1 where the lowest values are for the Upper Chalk. Variations in porosity are the main factor for the variation in thermal conductivity. The effect of fracturing is to reduce the bulk thermal conductivity, but the reduction is small for fractures that are saturated. For an averagely fractured chalk with 60% fracture saturation, the reduction in thermal conductivity is around 22% for a thermal conductivity of 2.15 W m−1 K−1. In the near-surface zone, where fracture apertures will be at their greatest and unsaturated conditions may prevail for part of the year, the seasonal variation in thermal conductivity may be significant for infrastructure design.