PORTLAND'S WESTSIDE LIGHT RAIL TUNNEL: AN EVALUATION FIVE YEARS AFTER COMPLETION

The Westside Light Rail Tunnel is 5000 m long and has a 7 m wide twin bore which was constructed between 1994 and 1997 through the Tualatin Mountains west of downtown Portland, Oregon. Geologic investigations for the tunnel were conducted from 1988 to 1993 and included 87 borings spaced along the alignment at approximately 250 m intervals. Total test drilling was 5000 m. The eastern two thirds of the tunnel lie in the tectonically deformed Columbia River Basalt Group. The western third of the alignment contained more Columbia River Basalt but also Pleistocene Boring lavas and thick deposits of loess with paleosols. Conventional mining techniques were chosed for the western section. RQD values well below 40% in many parts of the eastern section should have dictated similar mining techniques for that portion of the tunnel, but a tunnel boring machine (TBM) was selected because it producced less noise. It was felt that TBM techniques would be quieter and would elicit fewer complaints from the large population near the east portal. The TBM ran into problems of being plugged with rock fragments and was averaging 30 cm/day. After months of delays, the head was redesigned and headway was regained. Politics had dictated choice of tunneling technique, not rational use of early investigations. Interesting findings from construction: Oatfield fault trace on the west side of the Tualatin Mountains; an unknown Boring lava cinder cone at the west end of the tunnel; interfingering of loesses and lavas at the west end; and the Washington Park Station shaft (deepest train station in North America) penetrating a landslide that was moving a few centimeters/month at a depth of 15 m.