Direct and Interface Shear Behavior of an Authigenic Glauconite Sand from the Coastal Plain of New Jersey

Glauconite sand, a green-colored, iron–potassium micaceous peloid known for its high crushability, has been found at locations in Europe and the United States where offshore wind development is currently underway. Crushing the material increases the fines content and transforms its behavior from sandlike to claylike, which can affect shaft resistance and end bearing during pile driving and long-term axial loading. The strain rate during pile driving can exceed 106% per hour, in which undrained viscous effects dominate the shaft resistance. Following installation, shaft resistance can exhibit a drained or an undrained response, depending on the nature of static and cyclic operational loads and the degree of particle crushing of the soil and its associated drainage properties. Direct shear and interface shear tests were conducted to study the effects of shear rate, interface roughness, and extent of particle crushing on the shear behavior of glauconite sand. Tests were conducted on natural and artificially degraded glauconite sand as well as Ottawa 20-30 sand and Boston Blue Clay to benchmark the results against typical sand and clay behavior. Results show that the peak and residual shear stress decrease as the shear rate increases, transitioning from drained to partially drained conditions. With continued increases in shear rate, viscous effects were observed to increase the shear resistance of degraded glauconite sand. The peak and residual shear stress also decreased with increasing degradation, reaching a minimum after mixing the soil in a dispersion cup for 60 min. The drained peak and residual shear stress between soil and steel followed expected trends related to surface roughness, while roughness effects under undrained conditions were less conclusive. Results are discussed in the context of implications for offshore pile foundation design.