|
PB has provided final design and design services during construction. (Photo by S. Bednarz) |
|
The 3.5-mile soft-ground tunnel will help to reduce CSOs from the Willamette River. |
|
|
Delving Deeper than Ever
|
One of PB's main challenges was conducting the deepest-ever exploration of the Portland region's underlying geologic formations. A tunnel route averaging 120 feet below the ground presented significant engineering and construction challenges for wall stability during the construction of tunnel openings in and out of the various shafts.
The firm undertook a detailed geotechnical investigation program to determine the tunneling properties of the city's subsurface ground conditions. Using innovative techniques such as rotosonic drilling, large-diameter augured boreholes, single-well tracer tests and tidal lag time analysis, PB became familiar with the geology of the city and learned how to distinguish its different formations and conditions that range from densely packed gravels and boulder fields to loose sand-filled lake beds and stream channels.
|
|
