Title page for ETD etd-07122012-232344

Type of Document Master's Thesis
Author Boudreaux, Jacques Pierre
Author's Email Address jboud43@tigers.lsu.edu
URN etd-07122012-232344
Title Shear Strength Evaluation of an Erosional Soil System at Fourchon Beach
Degree Master of Science in Civil Engineering (M.S.C.E.)
Department Civil & Environmental Engineering
Advisory Committee
Advisor Name Title
John Pardue, Ph.D., PE Committee Chair
Clinton Willson, Ph.D., PE Committee Member
Guoping Zhang, Ph.D., PE Committee Member
  • coastal erosion
  • land loss
  • coastal Louisiana
  • erosion
  • wetlands
  • Fourchon
  • Fourchon Beach
  • Lafourche Parish
  • shear strength
  • soil shear strength
  • Louisiana
Date of Defense 2012-06-29
Availability unrestricted
South Louisiana is vanishing. Subsidence due to relative sea level rise with erosion of weak wetland soils together produce devastating rates of land loss for this area. It is believed that high rates of erosion are due to weak strength properties of fine-grained sediments in the beaches, marshes, and other wetlands in coastal Louisiana. Wave action is known to initiate the movement of weak coastal soils in a manner that is related to the difference between the shear stresses applied by waves and the critical shear strength of erosional sediments.

Direct shear tests were performed on samples obtained from the field at Fourchon Beach, Louisiana, as well as on samples that were cultivated in a known soil media within a controlled environment. The role of plant roots on soil shear strength was studied by examining changes in the shear strength of vegetated soil-root composites (SRCs). Two species were used to create SRC direct shear test specimens: Scirpus americanus and Scirpus acutus. Samples were grown in fully-saturated conditions in a greenhouse, and tests were conducted on samples after 4, 8, and 12 weeks of growth after planting. A matured sample of Scirpus acutus, which contained a highly developed root system, was also sampled and tested. Both species were observed to add benefits in shear strength with increased effectiveness after longer growth periods. Findings indicated that Scirpus americanus, being more resilient during cultivation and having higher growth rates, provided the most benefits by increasing shear strength upwards of 30 percent in as little as eight weeks after planting.

Field samples were obtained from five areas at Fourchon Beach across an elevational gradient from the intertidal shoreline area to the heavily vegetated marshes containing a variety of plants, specifically Avicennia germinans and Spartina alterniflora. A sample of relict marsh clay was also obtained from the shoreline area and tested in addition to beach sand that had been treated by workers in the aftermath of the BP oil spill. Erosion rates were calculated using a method developed for fine-grained estuarine sediments.

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