About This Project

This project included the construction of an approximately 875 square-foot basement addition beneath the historic Liberty Theater in downtown Bend, Oregon. The theater building was originally constructed in 1917, and was supported predominantly on stacked lava rock foundations and individual pier pad footings, which were added to the building in the early 2000’s to support improvements to the three stories situated above the ground floor. In fact, the building was sandwiched between two other historic buildings, which were similarly supported on stacked lava rock foundations.   

This project was privately funded, and was completed in collaboration with a local architectural firm, Neal Houston & Associates and a local structural engineering firm, Froelich Engineers. CGT was awarded the project in March of 2021.  

Recognizing the distinct possibility of undermining the existing stacked lava rock foundations and shallow pier pad foundations, and the limited overhead space available to exploration equipment, CGT completed a geotechnical investigation using a small, limited-access drill rig to advance several drilled borings within the interior of the theater building. The borings were advanced though the existing slab and into the underlying subsurface materials.

Discussion of Findings

The results of our geotechnical investigation within the Liberty Theater building indicated that the existing column pad footings were supported on a historic slab-on-grade associated with the original theater construction, which was underlain by bedrock at depth. Our analysis also indicated that the existing, stacked lava rock foundations were at risk of collapse if excavation of the basement proceeded as planned. In order to ensure the stability and support of the existing structural features, CGT worked in coordination with a local shoring and underpinning contractor to develop a permanent shoring plan for the lava rock foundations and a permanent underpinning plan for the existing column pad footings.

Geotechnical Conclusions and Findings

Based on the results of our geotechnical investigation, a permanent shoring plan was developed that included the installation of rock anchors through the lava rock foundations, followed the installation of a shotcrete wall to further stabilize the existing foundations, and finally the construction of concrete retaining walls to form the basement level. This plan also included supporting the column pad footings on micropiles advanced deep into the underlying bedrock layer, so that excavation of the basement level could proceed adjacent to the footings.

Construction of the basement began in February of 2022 and ended in June of 2022. CGT provide geotechnical (soils) special inspections during the construction phase of the project, and oversaw the installation of the shoring and underpinning measures, as well as the construction of the basement level. CGT’s expertise in underpinning and shoring helped the project reach a successful completion in the summer of 2022.

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