Settlement of Building Floor Slab-on-Grade Constructed on Unsuitable Urban Fill in Chicago

In developed urban areas, many sites have near surface materials described as urban fill (brick-bat, concrete rubble, and debris/garbage) which are hidden from the design team. Urban fill materials are variable in composition and density and therefore often result in slab settlement/movement and cause millions of dollars in damages/repair costs and impacts to operations. A solution is to both properly identify urban fill and properly remedy the subgrade. For loose urban fill, permeation grouting is one of the solutions that has exhibited success. In one case, the project team selected compaction grouting which in fact made slab settlement worse, due to the presence of compressible soft soils below, insufficient overburden pressure and extra weight of grout led to new settlement of soft soils below the urban fill. Two case studies are presented. An existing apartment building experienced wall and slab distress issues. The distress included floor slab settlement/cracking/movement, sloping of the floor slab, buckling of the metal framing in the interior walls and ceiling due to forces applied to the interior walls, and damages to below slab utility conduits. The apartment building floor slab-on-grade settlement/cracking/movement and interior wall distress were related to the long term consolidation/compression of loose urban fill that was likely placed as backfill during demolition of old structures at the subject property or used to raise the grade from historic ravines. Permeation grouting of the loose urban fill and reconstruction of the concrete floor slab-on-grade were performed to remediate the problems. A school floor slab had settled several years after construction as a result of consolidation/settlement of loose urban fill. A preliminary recommendation from a structural engineer recommended compaction grouting. The settlement recurred a few years later after compaction grouting.