Sandy River

The main water supply for the Portland metropolitan area is carried through three major pipelines from the Bull Run Watershed to the City of Portland. These three conduits cross the Sandy River on two bridges, which expose the pipes to natural hazards including earthquakes, volcanic debris flows, and landslides. Cornforth Consultants was retained by the City of Portland Bureau of Water Works to determine the feasibility of relocating the existing and any future conduits into underground crossings of the Sandy River.

The natural hazards were geologically mapped and geotechnical explorations were performed. Borings were drilled deep into rock and angled beneath the river, and geophysical surveys were conducted on the ground surface. Rock samples were collected and tested to determine the rock mass characteristics, and permeability tests were performed to estimate the flow of groundwater through the rock. Environmentally clean exploration methods were used to protect the wild and scenic Sandy River and included a helicopter, pontoon-barges, and low-impact mobile drilling rigs.

Initially, Cornforth Consultants and the design team helped the City of Portland decide on the most environmentally sensitive and cost effective tunneling technique. Once the tunnel was conceptualized, preliminary engineering was undertaken to design the tunnels.