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Engineering | Architecture | Planning

SR 99 Bored Tunnel

TKDA* supported the Seattle Tunnel Partners, A Joint Venture (STP-JV), with design and surveying services for a 2.2-mile tunnel project. SCE held four contracts, which included designing the utility relocation and street restoration work, utility monitoring QA, manual survey for the settlement monitoring program, and QA Structural and Material Controls inspection. We served as the utility civil/structural engineer for the relocation of the following:

  • Seattle City Light (SCL) 115kv, 26kv, and the 13.8kv electrical circuits
  • SPU’s water main
  • SPU’s AWS 24-inch temporary CSO re-route and lift station during the TBM rescue operation
  • PSE and other utilities to clear the way for the excavation of the open-cut and bored tunnel at both north and south portals


TKDA designed the supports for the critical relocation of the 115 KVa main service loop from the Massachusetts Substation to the Union Substation along the waterfront. The design was to support this critical infrastructure for the four-year duration of the tunnel boring and utility project in support of HNTB and the design-build team. This design required the team to design for crossing seismic expansion joints, anchoring into the 1960s elevated SR 99 structure, and included an elevated level of seismic and safety importance factors. Our design had the supports and safety shrouds to protect cars and the public from this high-voltage power line.

*Professional services for this project were provided by Saez Consulting Engineers, Inc., which is now a wholly owned subsidiary of TKDA. Learn more about the acquisition here.

CLIENT: HNTB/Seattle Tunnel Partners for Washington State Department of Transportation
LOCATION: Seattle, Washington