Thermal performance of trench backfills used for frost protection of water service lines

Abstract

This paper describes the thermal performance of different configurations of trenches using various trench geometries and backfill and insulation materials. A limited number of laboratory tests were conducted to confirm the thermal properties of the backfills reported in the literature. The frost protection of water service is measured in terms of the number of days it takes for the frost front to reach the depth of water service lines under sustained subzero surface temperatures. The study specifically included typical granular materials used within the City of Ottawa (formerly the Region of Ottawa-Carleton), but the findings can be equally applied to other urban areas in Canada where the surficial terrain is rocky. The thermal analyses of the trenches were conducted using the finite element method that appropriately represents phase changes when the temperature falls below the freezing point of water. The analyses indicate that the use of sandwich insulation is ineffective, and that latent heat of backfill and native soils has a great impact on the frost protection of water service lines. A list of possible trench depths with different backfills and their thermal performance is provided.Key words: backfill, thermal performance, frost penetration, water service, finite element analysis.