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15th St NW Bridge

By: City of Bondurant
Date Published: Wednesday, September 26, 2018
 

The bridge on 15th Street NW was damaged during the flood that occurred June 30/July 1st. The area had received an extreme amount of rainfall in the storms that had passed through, which caused this site to become inundated with flood water. The City engaged an engineer to perform a special flood damage assessment and the bridge was barricaded off to eliminate any traffic passing over the bridge.

The existing bridge was built in 1927 and has experienced typical structural deterioration over its life. Prior to the flood, it had a 5-Ton load limit imposed. In the 91 years since this bridge was designed and built, the area has seen immense development and therefore increased flow in the stream. The increase in flow has led to some stream degradation, scour, and erosion under and around the substructures of the bridge. It also means that his bridge is hydraulically undersized.

During the inspection, the engineer noted that there were locations of undermining at both abutments. It appears that the recent floods advanced the levels of scour under both abutments. The west abutment had an average depth of scour of 1' which extended back approximately 1' over the majority of the abutment but expanded at the northwest corner to nearly 3' back from the front edge of the footing. The east abutment scour width varied from approximately 2' at the southeast corner to 3' at the centerline and then back to about 1' wide at the northeast corner. The depth of scour along the east footing varied from 1' to 1.5' along the length of the abutment.

The rest of the bridge (mostly the visible parts of the bridge) did not appear to have suffered any structural damage from the high water. The wings and backwalls, while showing signs of age with some cracking did not show any indications of new damage or movement. The surrounding banks and fill behind the wings had not eroded and showed no visible signs of erosion/movement.

While reviewing the history of this bridge, it is clear that the bridge is hydraulically undersized. Approximately 500' downstream is a triple 12'x12' reinforced concrete box culvert which provides more than twice the hydraulic capacity as this bridge.

There are no plans available for the existing 15th St. structure to provide insight into the existence of piles supporting the abutments. The continued degradation of the stream, scouring out, under and around the abutment footings and the ongoing deterioration of the structure is creating concerns about the structural stability of the bridge.

At this time, the engineer recommendations to return the bridge to its pre-flood condition would be to remove the loose silt under the footings and install flowable mortar or grout under each abutment to fill the existing scour holes. Normally the placement of riprap to provide protection to the mortar would be appropriate, however, this is not possible in this case because placement of revetment would further reduce the waterway opening and increase the water's velocity through the structure. In this instance, since the bridge is so vastly undersized hydraulically, the high velocities through the bridge would most likely wash away any riprap placed.

The City Council researched and discussed the cost of undertaking bridge repairs. Due to the limited remaining life of this bridge and the difficulty in being able to properly fix it, and the cost to repair it, the decision was made to forego repairing the bridge and move forward with demolition of the structure and construction of a new, appropriately-sized structure, in a slightly different location. (The current bridge’s location outlets vehicles onto a curve, which does not meet current safety standards.) The process to design, bid, and construct the necessary bridge and street improvements will take between 18 and 24 months.