Design and Construction Challenges with a Multi-Level Seepage Cutoff Barrier

Seepage cutoff walls have been used successfully for many years. In Florida, the authors have experience with soil-bentonite (SB) and soil-cement-bentonite cutoff walls in multiple reservoirs in Pasco, Hillsborough, and Hendry counties. Single-phase cutoff walls are standard and most often used for construction. However, in some cases geology and site conditions dictate modifications to the standard installation processes.
The C43 Reservoir in Labelle, Florida is a 170,000 acre-foot reservoir and a critical piece of the Comprehensive Everglades Restoration Plan (CERP) – a Federal/State partnership critical to preserving the South Florida Everglades ecosystem. The SB Cutoff Wall at C43 extends more than 16 miles around the perimeter of the reservoir and is designed for installation in 2 distinct construction phases. Preconstruction explorations identified a continuous subsurface clay layer as an appropriate key-in material. However, the occasional presence of well-cemented limestone above the clay layer and the anticipated difficulty in excavation from an elevated constructed embankment dictated the need for the multi-phase wall construction approach. The first phase (Phase I) extends from near foundation grade, and the second phase extends from near the embankment crest and overlaps Phase I.
This paper describes the concepts used to design a multi-phase SB Wall at the C43 Reservoir and evaluates some of the challenges discovered during construction. Topics reviewed include: anticipating (design) challenges with connecting the wall in separate phases, adjusting to subsurface conditions including occasional clay discontinuity, field identification of suitable key material, consolidation of soft backfill materials during placement of overlying embankment fill, laboratory testing, and other challenges encountered during construction.