Predicting Temperature Profiles in Soil and the Effect of Heat Conduction on Buried Thermoplastic Pipes During a Wildfire

Abstract

A new calculation method is developed to predict the temperature profile in soil exposed to the heat from a wildfire. The calculations explicitly account for the effect of moisture on conduction heat transfer through the soil. The method is used to generate a fragility curve, which shows the probability that the temperature on the exterior surface of a buried thermoplastic plastic pipe will exceed the maximum service temperature of 60°C as a function of soil cover thickness. Contrary to a previously published fragility curve, the new curve indicates that the temperature of a plastic pipe with a typical soil cover thickness of 0.3 m is not expected to exceed the maximum service temperature of 60°C when heated by conduction through the soil, even under the most intense wildfire heating conditions. The discrepancy between the new and the existing curve is attributed to the fact that the previous study did not account for the energy required to evaporate the moisture. The new fragility curve was developed based on temperature-dependent thermal conductivity data for three common types of soil with water content ranging from 5% to 35%. These are the only data that could be found in the literature. Future work to confirm the validity of the fragility curve for other soil types could involve the use of existing models to predict the thermal conductivity of the soil as a function of its texture, porosity, and water content.