About This Project

This project included the construction of a new three-story library building in the southeast portion of Bend, Oregon. The site was located in an area known for subsurface voids which include known cave systems developed from the formation of underground lava tubes. Several prominent caves were located in the immediate vicinity of the site, some of which had previously collapsed. In order to develop an understanding of subsurface conditions at the site, CGT initially oversaw the completion of a series of electrical resistivity (ER) geophysical surveys within the footprint of the building. The resulting 3D model produced from the electrical resistivity surveys identified potential anomalies which represented potential subsurface voids that could impact the safety and constructability of the new library building.   

This project is funded by a $195 million dollar bond that Deschutes County voters approved in November of 2022. Geotechnical services for the project were solicited by the Deschutes Public Library (DPL) in an RFP which was issued in July of 2021. CGT was awarded the project in August of 2022.   

Discussion of Findings

The results of the ER surveys indicated the presence of two, large, subsurface anomalies within the footprint of the new library building, at depths of approximately 20 feet below grade. Based on the results of the geophysical surveys, and our previous experience in the vicinity of the site, we concluded that there was a high likelihood that these anomalies were large air-filled voids (i.e. lava tubes).

In order to further refine our understanding of these anomalies and their potential impact on the project, CGT advanced a series of percussion air-rotary (“air-track”) borings into the anomalies. Then, using a downhole camera equipped with an LED light and DVR, CGT was able to visually inspect the anomalies to confirm their composition.

Geotechnical Conclusions and Findings

Video was recorded within each of the boreholes advanced through the anomalies, and was compared to the results of the geophysical surveys and the mapped geology the vicinity of the site to determine the composition of the anomalies. Based on our analysis, CGT concluded that the anomalies did not represent air-filled voids, but instead were natural structural variation in the composition of the basalt of Newberry, namely flow contacts between successive flows of basalt deposited over time. No special considerations were recommended to mitigate the presence of these flow contacts.

By utilizing a range of geophysical exploration methods and a backlog of local expertise, CGT was able to identity, analyze, and eliminate what could have been a very costly subsurface hazard, allowing the project to move forward with final design and construction.

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