Measuring the Shear Strength of Spartina alterniflora Vegetated Soil in Delaware Salt Marshes Using Field Vane Shear Tests

Coastal wetlands are crucial ecosystems for their capacity to protect inland areas from the effects of storms by reducing tidal amplitude, dissipating wave energy, and providing mechanical support between the mainland and open water. The importance of the shear strength of vegetated soil in such environments stems from its critical role in stabilizing the shoreline. Measuring the shear strength of natural soils in wetlands and coastal areas is essential for assessing erosion risks. One of the methods in geotechnical engineering for measuring soil shear strength is by in situ vane shear testing. This study evaluates the peak shear strength of soil reinforced with roots from Spartina alterniflora using the field vane shear test (VST). Tests were conducted using vane shear blades of different sizes, i.e., 20 × 40 mm (standard vane), 25.4 × 50.8 mm, and 50.8 × 101.6 mm. The effectiveness of each blade size was analyzed for different soil depth intervals. The study site is a temperate tidal salt marsh at the St. Jones Reserve in Delaware, USA. Furthermore, the measured shear strengths at different study site locations are compared for the same vegetation type. Although the results of the field tests showed considerable spatial variation, they revealed a clear trend. The field shear strength of the saturated soils reinforced by roots from S. alterniflora increased at shallow depths where the belowground biomass was higher. However, at depths greater than approximately 25 cm, the shear strength decreased significantly.