Permeability of Cutoff Wall Seepage Barriers in Earthen Dams: A Case Study of East Branch Dam Safety Modification

摘要

Permeability has historically been a quantitative parameter used to determine the acceptability of concrete panels installed as part of cutoff wall seepage barriers in earthen dams. During cutoff wall construction, permeability has been viewed as a significant factor in determining panel acceptability because many other parameters, such as voids, anomalies, joint cleanliness, and bottom contact, are qualitatively measured and difficult to standardize. However, low calculated panel permeabilities do not necessarily deem an adequate cutoff wall panel. This was observed during the construction of East Branch Dam's cutoff wall, when a panel that passed the permeability test was considered unacceptable, while a panel that failed permeability testing was deemed acceptable. Evaluation methods used to calculate permeability of East Branch Dam's cutoff wall include Kansas Geologic Survey (KGS), Daniel and Choi, and Hvorslev. Based on the results of falling head slug tests, permeability results, using multiple calculation methods, lack a trend and can vary largely from each differing method. This lack of trends and permeability results differing as much as an order of magnitude, make permeability a potentially unreliable and inconsistent parameter for cutoff walls. In addition, no permeability calculation or test methods account for the high hydraulic loading factor, due to reservoir pool elevation, on the cutoff wall. The lack of trends and inconsistencies in results calculated with these differing methods provide evidence that permeability, while still vital and important, may not need to be treated as such a significant factor in future cutoff wall construction projects. Instead, there should potentially be more focus on better defining the types and sizes of defects, which could cause continuous flow paths through the cutoff wall leading to poor performance as a seepage barrier.