Impact of Shoreline Restoration Fill Overburden on Localized Sediment Consolidation at the Caminada-Moreau Headland

Project conducted in coordination with Gahagan & Bryant to evaluate the geological framework of deltaic deposits at the Caminada-Moreau headland in south Louisiana and monitor the impact of fill placement on the rate of consolidation settlement for the top 90 feet of the geologic column.  Geodetic survey measurements were collected to monitor subsidence associated with various depth horizons at three locations within the beach restoration design template.

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During Phases 1 and 2 of the Caminada Headland Subsidence/Settlement project, four deep rotary borings (~120 ft deep) were acquired to document the geologic character of subsurface sediment along a 3-mile section of headland beach. A large-scale beach restoration project was completed for the area and a primary question regarding restoration efforts related to subsurface consolidation associated with overburden from the sand restoration project. Although greatest consolidation was expected within near-surface mud layers, quantifying differences in compaction at various locations within the shallow geologic section provided a method for predicting consolidation settlement associated with proposed beach restoration projects. This information is critical for designing placement elevations to maintain post-construction elevations that meet environmental requirements for the restoration effort.

Once benchmarks were established using geodetic GPS procedures (+/- cm-level accuracy) and secured to avoid vandalism and destruction during beach restoration sand placement (Phase 3), position and elevation measurements were collected at regularly scheduled intervals to monitor fill-induced consolidation at various depth horizons (Phase 4). Surface and subsurface benchmarks were established at three boring locations as follows: 1) settlement plates were placed at all locations by the restoration contractor; 2) subsurface Boros anchor locations for boring site A were installed at -19 ft, -41 ft, and -62 ft NAVD; 3) subsurface Boros anchor locations for boring site B were installed at -19 ft, -48 ft, -64 ft, and -88 ft NAVD; and 4) subsurface Boros anchor locations for boring site C were installed at -24 ft, -41 ft, and -85 ft NAVD.  All subsurface benchmarks were sleeved and anchored so consolidation settlement for each anchor was isolated from downdrag frictional forces above the anchor, thereby allowing settlement associated with specific geologic horizons to be quantified. Background subsidence, independent of consolidation settlement associated with the fill, was recorded at a nearby benchmark away from the fill-impact area. Background subsidence was subtracted from elevation change measurements for each Boros anchor location to isolate consolidation settlement associated with the fill only.  A total of 13 survey measurements were collected for each survey epoch. Initial position and elevation measurements were recorded during Phase 3, prior to placement of the fill. After initial survey measurements, 10 survey epochs were completed to evaluate the impact of beach restoration sand placement on consolidation at the project site.

Caminada Beach, Louisiana, Subsidence, Replenishment