Reconstruction of a U.S. Department of Agriculture Forest Service campground facility in the North Dakota National Grasslands required redesign and substantial construction change because of an unstable clay subgrade. The original proposal provided for removing the old asphalt and adding additional base and a new asphalt surface. When the asphalt cement was removed, it was found that the base course had migrated into the clay subgrade, leaving the subgrade unstable. Options explored for redesign were thickened gravel base sections, lime stabilization, and geosynthetic reinforcement. The geogrid-reinforced base was selected. Design analysis consisted of two phases: (a) bearing capacity analysis for construction traffic and (b) flexible pavement analysis and design to support long-term recreation traffic. The first involved primarily Tensar design methods; the second, an empirical and mechanistic approach. Empirical methods, based on 1993 AASHTO design procedures, included Tensar methods and the recent Perkins―Michigan Department of Transportation model. The mechanistic approach used the EVERSTRESS and KENLAYER elastic layered programs. All design methods used were found beneficial and are recommended. The final flexible pavement sections constructed were dictated by the construction traffic and consisted of 2 in. of asphalt concrete on a reinforced base course ranging in thickness from 4 to 12 in. The project had to be completed in 3 weeks, so investigation and testing were limited, and the design parameters were based primarily on field dynamic cone penetrometer testing and correlations. Enforcement of the limited wheel loads became a continuous inspection problem. In addition, because of the fineness of the base aggregate produced, a separation geotextile had to be added to preclude migration of the base aggregate through the geogrid into the subgrade.
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