CS8C - Concurrent Session 08C: A Case Study of Design and Construction Challenges of a Soil-Cement Bentonite Hydraulic Cutoff Wall in a Loose Sand Embankment

Abstract Description: As time goes on, dams are reaching their design life. Owners are challenged with maintaining, inspecting, and investing in these significant assets. Many high-hazard potential dams need time-critical repairs to address deficiencies, changing design standards, and climate change. The average age of the 86,000 non-federal dams in the US is > 60 years. Investments of $157.5B to repair these aging and critical structures is needed (ASDSO, 2023).

The Edenville Dam located on the Tittabawassee and Tobacco Rivers in Michigan’s Lower Peninsula was built in 1925 to provide storage and head water control for the purpose of hydroelectric generation. The Edenville Dam’s earthen embankment did not meet modern design or construction standards. Portions of the Edenville embankments were noted to have seepage and stability issues during the initial filling process. Nearly 100 years later, during a two-day rain event in 2020, the left embankment dam catastrophically failed releasing the reservoir, causing both the Tobacco and Tittabawassee Rivers to flow through the breach channel. Following the failure, the remaining structures were stabilized, and the rivers were returned to flow through the partially demolished and stabilized spillways.

The reconstruction project is ongoing and included construction of a Soil-Cement-Bentonite hydraulic cutoff wall (SCB wall) within the embankment to control seepage. A subsurface investigation program was performed to collect four representative full depth soil profiles within the alignment of the embankments to inform the design. A supplemental subsurface exploration was performed by the contractor to identify the elevation of the top of the hydraulic key layer on 50-foot spacing prior to installation of the SCB wall. The construction dam safety risks associated with the design were evaluated during a supplemental potential failure modes analysis (PFMA) session. Prior to construction, vibrating wire piezometers and inclinometers were installed in the embankment in addition to the existing open standpipes and structural monitoring points to monitor the embankments’ stability during SCB wall construction. The design also incorporates a sheet pile wall that will extend hydraulic cutoff from the SCB wall to the spillway structures. In approximately 46 working days the contractor successfully installed approximately 5,000 lineal feet of SCB wall via the one-pass trenching technique, to a maximum depth of 102 feet, resulting in 217,000 vertical square feet of SCB wall. This paper will discuss the design and construction challenges and lessons learned during construction. Further, the vital role of instrumentation monitoring during construction will be discussed.