INSULATION PERFORMANCE BENEATH ROADS AND AIRFIELDS IN ALASKA

In 1968, the Alaska Department of Highways constructed its first experimental installation of expanded plastic foam for frost heave control at a site 11 mi south of Anchorage. This was followed, in 1969, by construction of both the first insulated roadway section over permafrost in North America at a site near Chitina, and the first insulated airfield runway at Kotzebue. Since that time, six additional roadway sections on permafrost, totalling 3.6 lane-mi, have been insulated by the Alaska Department of Transportation, along with four additional airport installations. Applications of insulation for frost heave control have been numerous, totalling 11 lane-mi. Materials used for subgrade insulations have been primarily extruded-expanded polystyrene foam (Dow's Styrofoam HI and UCI Foamular) with one installation of polyurethane foam and three of molded polystyrene beadboard. Evaluations of the long-term thermal performance of these installations have included sampling and testing of the insulations to determine the retained thickness, thermal conductivity, and compressive strength properties. Based on these observations, foamed-in-place polyurethane insulation is not accepted for use as a subgrade insulation by the Department of Transportation, whereas extruded polystyrene insulation has demonstrated superior performance and longevity. Molded polystyrene beadboard insulation layers have given acceptable performance, but must be installed at a thickness 30 percent to 50 percent greater than the extruded polystyrenes to provide comparable thermal performance. Comparisons were made between measured late summer permafrost thaw depths for insulated airfields, and calculated thaw depths using the Modified Berggren calculation method and actual site soil and insulation properties. These commparisons demonstrated that this method of calculation results in calculated thaw depths slightly greater than the actual values, but provides reasonable values for a conservative design.