Case Studies Case Studies: Pedestrian Bridge - Slope Stabilization

A bridge was proposed to span over the Little Papillion Creek and allow pedestrian access to a new community sports arena. Temporary sheet pile walls were installed to construct the bridge bents, which would be supported by deep foundations. However, when the sheet piles were removed, one of the bridge bents began to translate and lean toward the creek, indicating a global slope failure and halting construction. The project designers proposed installing a permanent, 67-feet-long sheet pile wall upslope from the leaning bent. A tieback system would need to be installed through the sheet pile wall to bear beyond the failure plane of the slide mass.

The soil boring closest to the proposed tiebacks identified clay fill to a depth of five feet, underlain by very soft to medium stiff clay to a depth of 43 feet. Below the clay, medium dense sand with gravel and cobbles was identified to a depth of 55 feet, followed by upper-end stiff to hard clay to a depth of 93 feet. Groundwater was encountered at a depth of 23 feet at the time of the investigation.

Helical tiebacks were chosen over grouted tiebacks due to the limited access and working area. The helical tieback configuration consisted of a square-bar “stinger” lead section, Model 200 (2.00-inch round corner square bar) with an 8”-10”-12” triple-helix arrangement, transitioning (via a special welded coupler) to a Model 350 (3.50-inch OD by 0.313-inch wall) hollow round shaft extension with two additional 14” helix plates. The remaining lengths of the tiebacks consisted of blank Model 350 extensions. The square-to-round “stinger” lead section was used to better penetrate the sand with gravel and cobbles to bear the helix plates within the underlying hard clay. The 8” helix plate was 0.5-inch-thick and utilized a V-style cut on the leading edge to also help the tiebacks advance through the gravel and cobbles. The tiebacks were positioned 5.3 feet down from the top of the wall, spaced at four feet two inches, and installed at downward angles from 35 to 40 degrees. The tiebacks were advanced to lengths from 84 to 105 feet behind the wall and to installation torque values correlating to ultimate capacities of at least two times the design working load of 67 kips (FOS≥2).

Twenty (20) helical tiebacks were installed to anchor the sheet pile wall. The continuous waler consisted of two C7x14.75 steel channels with welded cleats. Performance tests were performed on two of the production tiebacks, while standard proof tests were performed on all of the remaining tiebacks.