Bonneville Dam New Lock Upstream Cofferdam
Cascade Locks, Oregon
One of the most challenging design features of the new Bonneville Navigation Lock on the Columbia River was the upstream cofferdam. This feature consisted of an earth embankment with sloping surfaces on the upstream side and a retained vertical wall on the downstream side. The retaining wall was “L-shaped” in plan. A seepage cutoff wall, built using slurry trench construction techniques, prevented water from entering the excavation via the Bonneville pool.
A preliminary design of the retaining wall included soil anchors and soldier piles along the entire length of the wall, with excavated wall heights up to 70 feet. However, this design required some of the anchors to penetrate through the seepage cutoff wall. As a result, Cornforth Consultants conducted a study to review other alternatives for the retaining wall structure. A review of several options resulted in two alternatives: (i) soldier piles and tieback anchors (similar to the preliminary design), with anchors placed only where they would not impact the seepage cutoff wall, and internal bracing elements (corner bracing members) used where anchors would not fit; and (ii) an internal bracing system that would be supported by a rock slope exposed by the excavation. The advantages and disadvantages of both alternatives were evaluated. The first option was subsequently selected for construction.