Council approves $119,675 design amendment to rework Carpenter Creek culvert replacement

Mount Vernon — The city council on Wednesday approved an amendment to the David Evans and Associates design contract for the Carpenter Creek/Hickox Road fish-barrier improvement project after engineers recommended a different structural approach to control escalating construction costs.

Public works capital projects manager Frank Briner told council the 60% design had revealed steep ravine conditions on the northwest side of the site that were driving ever-escalating temporary shoring requirements. Rather than continue with a plan that required large temporary excavation and shoring, Briner said the consultant performed a value-engineering review and proposed a new approach that makes some shoring permanent and integral to the culvert structure, reducing excavation and anticipated construction cost escalation.

Nut graf: The council authorized addendum No. 4 for additional structural and geotechnical design services costing $119,675, bringing the contract total to a not-to-exceed $574,740. Briner said the added design work is expected to avoid construction costs “in excess of 10 times” the amendment by reducing the need for extensive temporary shoring.

Council members asked about culvert width and whether the revised approach would change inspection or maintenance responsibilities. Briner said the culvert top measures about 30 feet; state design standards for larger culverts apply, but long-term maintenance and inspection requirements will not materially change. He described the new wing walls as concrete-faced soldier pile retaining walls instead of previous precast options.

Ending: Council approved the amendment; staff said the redesign should produce a more constructible and cost-effective project and will return with subsequent design steps.