Des Moines, Iowa
Restoring 50 Year Old Wall in Des Moines, IA
Long-time Mercy employees can remember the wall leaning since before the 1960's. Our dealer was awarded the project in lieu of a complete removal and wall replacement. The Grip-Tite® Helical Tieback System restored the wall and the dealer saved the client over $200,000.
The purpose of the project was to design and install a helical tieback system to straighten the existing 4' to 6' tall, 320' long concrete retaining wall. A sidewalk was located on the low side of the wall in addition to the adjacent very busy University Avenue. An asphalt drive/parking lot was located on the high side of the wall, which was to remain open for hospital use during construction.
The project included coring 3.5" diameter holes through the wall and removal of the backfill soils. The clay backfill soils contained varying amounts of cinders, concrete, brick and highly expansive clay shale content. Removal of the backfill soils exposed the old "deadman" system, which consisted of steel straps encased in concrete. Some of the straps appeared to have corroded over time and broke. The broken deadman straps along with heavy clay backfill soils likely added to the wall failure.
The Helical Tie-Back system included installing sixty-four (64) helical tie-backs at about a 5' O/C spacing. The tiebacks generally consisted of either a single 10", 12" or 14" diameter helix blade lead section or a 8" diameter helix blade lead section followed by a single 14" diameter helix blade extension. The tiebacks were installed to lengths on the order of 20' to 35' beyond the wall face. Torque readings in the range of 3000 to 4400 ft-lbs were achieved with ultimate capacities on the order of 30 to almost 44 kips.
To reduce lateral earth pressures, free-draining coarse granular material was utilized to replace the existing cohesive (clay) backfill soils. Despite limited working space, obstructions and areas of rubble fill soils, the project was completed in less than 1 week