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 Peorian loess along the west wall line and silty clay fill soils (with construction debris) along the east wall line.
The tieback system, designed by Grip-Tite, included two tiebacks per bay along the west wall and three tiebacks per bay along the east wall, each with a design load of 13,000 pounds (13 kips). Minimum length criteria from the wall face to the closest helix blade were also specified at 10 feet and 15 feet for the west and east walls, respectively. Tiebacks were not required within the sunken mechanical room in the southeast building corner and within the areas of the east bathrooms where the foundation walls were braced by interior walls.
Twenty-six (26) helical tiebacks were installed. The tieback configuration consisted of 1.5-inch round corner square bar with a triple-helix lead section (10-inch, 12-inch, and 14-inch diameter helix blades) followed by one (1) double-helix extension (two 14-inch diameter helix blades). Standard extensions advanced the tiebacks to lengths of 25 to 30 feet. The tiebacks were installed at angles of 15 to 20 degrees for ease of installation and to avoid utilities that parallel the foundation walls.
Tieback capacities, estimated by correlation to installation torque, were at least twice the design load. The tiebacks were connected to the bottoms of the walls with a custom-designed bracket and pre-tensioned to at least 1.5 times the design load. The 26 tiebacks were installed and pre-tensioned in five days.