Gerry Frank Amphitheater
Geotechnical services for a premiere attraction at Salem’s Riverfront Park
In 2017, the Rotary Club of Salem, Oregon announced a plan to build a new open-air amphitheater at the city’s Riverfront Park. The $4 million project would deliver a premiere venue for outdoor community events, complete with a vendor plaza, elevated stage and pavilion shell. GeoEngineers provided geotechnical services for the project as sub-consultant to project lead CB Two Architects.
A GeoEngineers team had worked on earlier park improvements including the Salem Carrousel, Willamette Queen dock, west edge of the old amphitheater and a landmark pedestrian bridge. This previous subsurface experience at Riverfront Park proved valuable. GeoEngineers knew that a building had once stood on part of the proposed amphitheater site, and that demolition rubble and other material was likely still below the surface. The team’s geotechnical investigation confirmed that demolished or partially demolished debris and even complete utility tunnels were scattered throughout the site beneath several feet of silty fill.
Although in most cases concrete debris would be removed prior to construction, the upper soil at the amphitheater site was partially subject to residual environmental contamination from site use decades before. To avoid this potential complication and keep costs manageable, GeoEngineers recommended foundation designs that would bear on underlying dense native gravel—and require much less excavation.
The potential of contaminated soil meant careful planning and regulatory oversight from the Oregon Department of Environmental Quality (DEQ). GeoEngineers worked with the City of Salem to develop a Contaminated Media Management Plan (CMMP) and submit it to the DEQ for approval. The team also provided geotechnical recommendations for the amphitheater’s foundation, on-grade slabs, earthworks, construction and more.
- Initial Data Review: The team started by reviewing existing subsurface information from public sources and previous GeoEngineers’ work.
- Field Investigation and Soil Characterization: GeoEngineers completed seven boreholes on the site, six of which hit solid material or debris, including a large object thought to be a concrete slab from a partially demolished building. The investigation validated the team’s concerns about construction debris and the potential for contaminated soil. The team concluded that a layer of extremely variable construction debris lay 3-5 feet beneath much of the site’s surface. To avoid disturbing contaminated soil, no additional investigations were performed.
- Contaminated Media Management Plan and DEQ Support: GeoEngineers worked with the City of Salem to develop a Contaminated Media Management Plan (CMMP) and submit it to the DEQ for approval. Excavated soil was evaluated for potential contaminants prior to disposal off site, and care was taken to avoid using any potentially contaminated materials as structural fill.
- Earthwork and Construction Recommendations: Recommendations for temporary excavation support, earthwork construction, including use of on-site and imported structural fill and fill placement and compaction requirements. In areas where fill would reach the finished grade, the team recommend first compacting the upper soils.
- Foundations and On-Grade Slabs: Significant uplift loads from the timber amphitheater shell meant large spread foundations were required. These large base foundations required relatively low allowable bearing pressures, readily managed by recompacting the upper fills over native gravels. Although the team initially recommended deep foundations consisting of driven steel pipe piling to support the amphitheater, the large mass footing was a simpler solution that limited drilling through unknown remaining construction debris.
GeoEngineers overcame the challenges of this complex site with widespread subsurface construction debris, finding efficient and effective foundation design and ground improvement options for the project by working closely with the City of Salem, the Rotary Club, and the site contractor. The successfully completed Gerry Frank Amphitheater will be a community hub and premiere Riverfront Park attraction for many years to come.