The Criss Library at the University of Nebraska - Omaha was constructed in the mid-1970's. The three-story structure has a lower level that walks out to grade along the south wall line. The original building has dimensions of 240 feet north-south by 210 feet east-west with a bay spacing of 30 feet. A new addition was recently constructed north of the north wall line of the original building.
The lower level slab-on-grade settled over the years, resulting in floor cracks, faulting and uneven surfaces. The floor was mud-jacked twice to re-level the floor, but the settlement continued. An investigation of the floor settlement identified air leaks in the sub-floor heating, ventilation and air conditioning (HVAC) system. Soil borings confirmed that leaks in the HVAC system caused significant drying of the subgrade soils, resulting in soil shrinkage and subsequent settlement of the floor slab.
The selected repair option included removing the existing slab, removing the affected subgrade soils, placing select fill, reconstructing and/or repairing the HVAC system, and then replacing the floor slab. Prior to complete removal of the floor slab, tiebacks had to be installed along the bottoms of the east and west foundation walls to prevent wall movement due to the unbalanced earth pressures.
Soil boring information was available from the construction of the original building, the construction of the north addition, and from floor slab evaluation studies. The tiebacks would generally penetrate native silty clay Peoria loess along the west wall line and silty clay fill soils (with construction debris) along the east wall line.