The situation is inside a 10-year old addition to a municipal building in central PA. A large conference room slab-on-grade consisting of nine 20'x20' "quadrants" separated by 2 construction joints and 2 saw-cut joints. No expansion material was observed anywhere within the placement area and no dowels, reinforcing steel or WWF was found during initial investigation. The center 3 quadrants have apparently "heaved" shortly after placement, because some flash-patch material was used along the construction joints prior to the original flooring installation. Subsequently, additional "heaving" of the center sections has created a 1/4 to 1/2 inch difference in elevation between sections. All 3 center sections have raised the same amount, and the 6 outside sections are all at the same relative elevation. The 4 to 5 inch thick slab is on a vapor barrier of plastic above minimum 6-inches of #57 stone on compacted clayey subgrade. Investigation of in-situ subgrade conditions is on-going but... what could have cause the center sections to be "jacked up" above the outside sections?