Soil Bentonite Wall Mixing: Tuning Materials, Sampling, and Testing for a Construction Symphony

Containing 55 billion gallons of water amongst geology rife with granular material is no simple feat. This was the challenge in designing the C43 Reservoir (C43) in Hendry County, Florida. The reservoir is a key part of the Comprehensive Everglades Restoration Program (CERP), which is a Federal/State partnership critical to preserving the South Florida Everglades ecosystem.
The Soil-Bentonite seepage cutoff wall (SB Wall) is a proven seepage mitigation measure and was selected as the primary seepage barrier at C43. Typical SB Wall construction methods mix native materials with bentonite slurry and/or dry bentonite, and some construction conditions may increase the potential for flaws in the containment barrier.
Thorough mixing is critical to the performance of an SB Wall, and installation through media with low fines content typically requires adding borrow clay, bentonite, or both to the mixture. Note the terms “fines” and “fines content” denote the dry weight percent passing the US Standard No. 200 Sieve. The condition of the borrow material prior to mixing, and especially the mixing process itself, is key to the ultimate success of the reservoir. Further, traditional testing procedures for fines contents and hydraulic conductivity (permeability) have produced misleading results that artificially indicate higher wall quality if thorough mixing is not achieved. If contractor, quality teams, and testing agencies can find a way to overcome the challenges, the resulting process can flow together like a fine orchestra playing a symphony. (Continued)