Ninth International Conference on Permafrost ... - IARC Research

Maximizing Construction Season in a Subarctic Environment,Fort Wainwright, AlaskaIntroductionQuentin GehringShannon &Wilson, Inc.Frank J. WuttigShannon &Wilson, Inc.The Denali Village housing project involves the constructionof approximately 200 housing units (50 buildings) on apartially-developed site along the banks of the Chena River,encompassing approximately 150 acres on Fort Wainwrightnear Fairbanks, Alaska. Common construction practiceon Fort Wainwright has been to replace frost-susceptiblefoundation soils with nonfrost-susceptible fills, mitigatingthe risk of undesirable frost-related movements. However,the design build team proposed founding the housing unitson frost-protected shallow foundations to reduce bothconstruction and costs.Construction of the project provided several designchallenges from a frozen-ground standpoint. Thispresentation describes the design of frost-protectedshallow foundations outside standard design criteria, andlists strategies we considered to provide temporary frostprotection for overwintered unfinished buildings and buildingpads. The desired outcome of temporary frost protectionwas to lengthen the construction season and accelerate theconstruction schedule.ClimateASCE 32-01 has developed a standard for the designand construction of frost-protected shallow foundations.The standard applies to areas free of permafrost, to areaswhere the mean annual air temperature is greater than 0°C,and to areas where the design air-freezing index is less than2,500°C-days.Interior Alaska, including Fort Wainwright, has acontinental climate characterized by large daily and annualtemperature ranges. Mean annual temperatures are slightlybelow freezing. The mean annual air freezing index exceeds3,000°C-days. The recorded freezing index for Fairbanks isplotted in Figure 1. The 100-year mean return period designfreezing index developed by the U.S. National Oceanic andAtmospheric Administration (NOAA) is 3,900°C-days.The mean daily temperatures typically rise abovefreezing in mid-April and fall below freezing in mid-October, providing about a six-month construction season.Foundation construction in frost-susceptible soils generallybegins after the seasonal frost has thawed, delaying the startof construction until late spring. Seasonal frost depths canextend several meters into the ground in cold, low-snowyears. In areas kept clear of insulating snow cover, frostdepths can exceed 4 m.Figure 1. Climate information.Site ConditionsThe site is on the Chena River floodplain. Prior toconstruction, the site was primarily wooded with birch andspruce with an under-story of alder and willow. A portion ofthe site was developed as a campground, including gravelroads, parking areas, and trails.Exploratory drilling by the U.S. Army Corp of Engineersand follow-up drilling by our firm showed that the site isunderlain by a surficial layer of silty frost-susceptiblesoils overlying generally nonfrost-susceptible (NFS) sandsand gravels. Localized areas of both shallow and deeppermafrost were encountered across the site, which affectedthe foundation design. Groundwater levels were 3 m ormore below the planned finished floor elevation, but pastexperience in the area indicates the water table can fluctuateseasonally 1 m to 2 m.Foundation DesignThe housing units are being founded on shallow,thickened-slab foundations with in-slab radiant floor heatbearing on controlled nonfrost-susceptible fills. A typicalsection of a frost-protected shallow foundation is shown inFigure 2. Frost protection is provided with either perimeterinsulation designed to keep the soils below the foundations ina permanently thawed state, or by excavating and replacingfrost-susceptible soils beneath the foundation system with aNFS fill. For this project, given the variation in subsurfaceconditions, four distinct foundation design solutions wereproposed to address permafrost and frost-susceptibilityissues.83