Case Studies Case Studies: Chequamegon-Nicolet Forest Pedestrian Bridges

Located in the Wisconsin Northwoods, the Chequamegon-Nicolet National Forest covers more than 1.5 million acres of forest preserve across five counties. Severe storms and runoff damaged walking trails within two of the counties. This damage also included the total destruction of three pedestrian bridges located at stream crossings. It was anticipated that future storm events of similar or greater magnitude would occur more frequently, with higher levels of flooding. Therefore, the designs for the replacement bridges would need to consider the increased flood potential, with anticipated scour depths of 3 to 6 feet below the new abutments. The bridges were located in extremely remote areas, being several hundred yards to over a mile from any access roads, which restricted the use of concrete abutments. The construction for the new bridges would also require smaller equipment to limit disturbance to the protected forest and streams. Geotechnical investigations at the new bridge locations generally showed very loose sand to depths of about 2.5 to 5 feet with shallow refusal at some of the abutment locations and cobble formations at others. Given the potential for future flooding, the abutments would need to resist uplift and lateral loads in addition to supporting vertical compression loads.

Helical piles and anchors were chosen as the most economical solution for supporting the bridge abutments given the unique challenges and environmental constraints. The final foundation design included two vertical helical piles and four helical anchors (installed at a 45-degree batter) for each abutment. The abutments and bridge superstructure would be constructed with timber and a custom timber bracket was designed to connect the helical piles and anchors to the timber abutments. The custom bracket incorporated the vertical piles and battered anchors into the same bracket.

The vertical piles and anchors had design working loads of 10 kips and 3.2 kips, respectively. A factor of safety of 2.0 is generally considered for helical pile and anchor design; however, due to uncertainties in the soil profile and the anticipated installation challenges, higher factors of safety were specified. The minimum required ultimate capacities were 27 kips for the vertical piles and 10 kips for the anchors. Pile buckling analyses were performed for the vertical piles given the varying potential scour depths. The deep foundation design included the Model HP288 (2.875-inch O.D. by 0.276-inch wall thickness) shaft with a 10”-12” helix plate configuration for the vertical piles and the Model HA150 shaft (1.5-inch solid square shaft) with an 8”-10” plate configuration for the battered anchors. Hot-dip galvanized material was specified for additional corrosion protection. 

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