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Northwest Outfall Siphon Rehabilitation

The Northwest Outfall Siphon Rehabilitation Project in Tucson, Arizona, led by Pima County RWRD, addressed odor and design issues in a 30-inch and 39-inch dual-barrel sewer siphon under the Santa Cruz River. Dibble devised corrosion-resistant structures to smooth flow and prevent corrosive gas release. Hunter Contracting Co. facilitated digital collaboration, surpassing expectations in aligning with the project vision. The scope included a flow management plan with a 270-foot temporary bridge, demolition of Silverbell Siphon structures, installation of new fiberglass-lined precast structures, a 48-inch storm drain, headwall, and access route. Project elements encompassed traffic control, surveying, quality testing, public relations, and carbon fiber wraps. Restoration efforts considered impacts on the Silverbell Golf Course and The Loop bike path, achieving success in odor control while enhancing regional infrastructure resilience and environmental considerations.

Pima County RWRD
September 2022
Delivery Method:
Design Engineer:
  • 270-foot single-span temporary bridge to redirect flows over the Santa Cruz River
  • Construction of a 48-inch storm drain, headwall, and access route to the outlet structure
  • Demolition and removal of existing Silverbell Siphon’s inlet and outlet structure
  • Development of two corrosion-resistant structures
  • Extensive bypass flow management to facilitate the work
  • Installation of fiberglass lined inlet and outlet structures on the Santa Cruz River
  • Repair and reconstruction of the Loop bike path
  • Preservation of native plants
  • Public outreach
  • Quality control testing
  • Survey
  • Traffic control
  • Tying over existing house connection sewers to the new 54-inch main


American Public Works Association (APWA) Arizona Chapter

2023 Local Public Works Projects of the Year – 

Environment Less than $5 million

The B1M

Construction Story of the Year – 2023 Top Three Finalists

American General Contractors (AGC) Arizona Chapter

Build Arizona: Reconstruction – Under $5 million – Utilities