Depoe Bay Marina Seawall Distress
Depoe Bay, Oregon
On the east side of the Depoe Bay boat basin, a slow-moving landslide was tilting and deforming a structural sea wall that was originally built by the Portland District Corps of Engineers in 1939 and improved in 1952. The 750-foot long wall was designed as a gravity concrete retaining wall with a 21-foot height and a 13-foot base width. By 1993 the landslide had displaced the southern 200 feet of the wall 3 feet horizontally and ½ foot downward into the bay. Cracking and settlement were also impacting the City-owned pavement and utilities, including a main sewer line in the parking lot behind the wall. Long-term continuation of the slide movement was also a potential threat to the boat basin. In 1983 and again in 1995 the Portland District Corps of Engineers performed investigations and attempted to remediate the damage to the wall.
Under contract with the Portland District Corps of Engineers, Cornforth Consultants performed geotechnical investigations and analyses and confirmed that the wall distress was related to a landslide that was larger and deeper-seated than previously anticipated. It was moving on a slip surface that was deep within the mudstone bedrock foundation. Recurrent movement over the years was triggered by increased pore-water pressures within the landslide mass during periods of intense rainfall.
Through traditional means of investigation and analysis, a non-traditional method was selected to stop the slide movement – a subsurface shear pile wall. Cornforth Consultants designed the wall, prepared plans and specifications, and provided engineering support during construction. The construction of the shear pile wall was successfully completed, within budget and schedule. Instrument measurements indicate that the landslide has stopped moving the seawall toward the bay.