Shear Behavior of Mangrove Soils in Louisiana and Texas: A Dual Approach Using Large-Scale Direct Shear Testing and In Situ Cone Penetration Testing

Mangrove expansion in the northern Gulf of America (Mexico), driven by climate change, has introduced complex root-soil interactions into coastal systems previously dominated by saltmarshes. However, the geotechnical contributions of mangrove root structures to soil stability remain underexplored. This study assesses the role of belowground biomass (roots) in reinforcing wetland soils at two environmentally distinct sites: River-dominated deltaic system (Port Fourchon, Louisiana), and Tide-dominated lagoonal system (East End Lagoon, Galveston, Texas). We applied a novel, custom-built large-scale Direct Shear Testing (DST) apparatus alongside Cone Penetration Testing (CPT) to assess the shear behavior of root-reinforced soils. The DST results revealed strain-hardening behavior in upper-layer samples (0–15 cm) from both sites, where peak shear strength 55 kPa in the river-dominated site and 51 kPa in the tide-dominated site, reflecting the reinforcing effect of dense root networks. These values were more than double those of the mid-layer (15–30 cm; ~20–22 kPa) and about five to six times greater than the bottom layer (30–45 cm; ~8–16 kPa), where samples exhibited strain-softening responses, consistent with reduced root influence and a transition to soil-governed failure mechanisms. The CPT sleeve friction values aligned well with DST results in root-rich upper layers and showed elevated resistance at depth in the tide-dominated lagoonal system, suggesting sensitivity to sediment heterogeneity. The findings demonstrate the effectiveness of DST in capturing the structural contribution of belowground biomass to soil stability and reinforce its value as a mechanistic tool for evaluating wetland geomechanics. Future work will integrate root biomass measurements and explore a broader range of geomorphic and hydrological conditions to inform restoration strategies and enhance coastal resilience under changing climatic conditions.