East Branch Dam: Lessons Learned during Cutoff Wall Verification Drilling

ABSTRACT ONLY - The US Army Corps of Engineers completed a Dam Safety Modification Study (DSMS) for East Branch Dam during 2010. The DSMS identified internal erosion of embankment materials through heavily fractured bedrock as the primary risk driver at the site. An historical incident occurred at East Branch dam in 1957 during which muddy flows were observed at the toe of the dam. Subsequent investigations at that time identified a significant void at the bedrock interface. Grouting of the void was performed following the incident. Performance monitoring since that time indicated that conditions were favorable for initiation of the internal erosion and project risks were below tolerable risk guidelines. As a result, the DSMS recommended construction of a full depth concrete cutoff wall was selected.
Based on this recommendation, further geotechnical investigations were undertaken, installation of new instruments and enhancement to existing instrumentation were performed, and design of the full depth cutoff wall was completed. A single contract was awarded during September 2014 for installation of additional instrumentation, pre-construction grouting, and construction of a full depth concrete cutoff wall. Construction of the cutoff wall began during Spring 2017 and was completed during Fall 2020. Cutoff wall construction was achieved via a performance specification. Based on site conditions the Contractor elected to construct a panel wall. The constructed cutoff wall was 2153 feet long and extended to a maximum depth of 243 feet in the valley section. Specific measures within the contract to ensure quality included verification of the thickness, depth, and overlap of individual panels, fresh concrete testing, and verification drilling at selected locations. Verification drilling specifically included assessing quality of concrete core, performing borehole imagining, and permeability testing.
Following verification drilling in the prescribed Demonstration Section, the Contractor modified their processes and procedures for excavation and cleaning of panels prior to concrete placement. Verification drilling continued into production panel wall construction. When verification findings did not meet specification requirements a risk-based approach was implemented to determine if remediation would be required. The Government and the Contractor partnered to identify and implement innovate approaches for remediation. This paper focus on key findings and lessons learned from the verification drilling process in both the Demonstration and Production Sections. Implementation of these best practices identified at East Branch in future cutoff wall construction projects will result in more efficient risk reduction.