High School Slab on Grade Settlement Evaluation and Permanent Structural Slab Repairs

A low-rise high school building constructed on the site of an existing construction debris landfill included initial site preparation using Deep Dynamic Compaction (DDC) to mitigate potential future settlement of the building's first-floor slab on grade. Within the first six years of building occupancy, the first-floor slab on grade settled up to 6 in. because of consolidation of the landfill. Differential settlement of the first-floor slab relative to the pile-supported building frame caused severe distress to the interior partition walls, buried storm and sanitary drainage systems, interior piping, mechanical and electrical systems, and other finish work. This paper presents our evaluation of the landfill consolidation effects using precision settlement surveys as well as measurements from vibrating-wire borehole extensometers to model the settlement rates throughout the entire building footprint. The paper also summarizes the final repair design and construction procedures used to mitigate future settlement damage. The cost for repairs completed to date is approximately $10MM.