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Design and Construction of Rock Cap Roadways: Case Study in Northeast Washington State

In recent years, the Washington State Department of Transportation (WSDOT) has deviated from its normal policy of correcting frost heaving and thawing problems on state highways. WSDOT’s traditional approach for frost design rehabilitation has been to place a crushed stone base at least half the depth of the frost penetration. This approach has served WSDOT well on the majority of its highway system; however, other measures were sought to mitigate extensive frost-related problems in northeast Washington. To isolate the flow of water from the pavement structure, a capillary break using a free-draining aggregate or “rock cap: layer was used on projects during the last 5 years. The rock cap material was a maximum 75-mm (3-in.) material with 0% to 15% passing a 12.5-mm (1/2-in.) sieve. The open-graded nature of the rock cap provides a positive drainage blanket so that excess water can be eliminated from the roadway structure, and thus eliminates frost-heaving and thaw-weakening problems. However, constructing with a large stone material presents special construction considerations, particularly pertaining to the stability of the material. WSDOT’s construction experience is detailed, and rock cap performance in Washington State is summarized.