Volume 1, Issue 6 (June 2004)
Freeze-Thaw Effects and Gas Permeability of Utility Line Backfill
Backfill materials used in utility trenches must maintain physical and mechanical integrity when subjected to the seasonal effects of freezing and thawing. Materials used over gas utility lines must also have adequate permeability to allow any leaking gas to flow upward and out. To help determine how soils and flowable fills might perform as backfill over utility lines, we conducted laboratory tests to measure the permeability of backfill materials before freezing, during freezing, and after thawing. The two materials investigated in this study were a silty sand, and a flowable fill made with Type F fly ash. Our work also examined the susceptibility of these materials to frost heave and thaw weakening. An apparatus and standard test method for performing permeability during freezing and after subsequent thawing did not exist. We developed a method by adapting the ASTM Standard Test Method for Frost Heave and Thaw Weakening Susceptibility of Soils (D 5918) and the ASTM Standard Test Method for Measurement of Pneumatic Permeability of Partially Saturated Porous Materials by Flowing Air (D 6539). Although more data are needed to confirm specific conclusions determined from this study, the test method developed here appears to be useful for evaluating the effects of freeze-thaw on backfill materials for utility trenches. Additional work is needed to demonstrate whether these laboratory results correspond to actual field conditions.