Mitigation of CPT thrust force through glauconite sand

Glauconite sands, which easily crush under shear and compressive stresses, can lead to high piezocone penetration testing (CPTu) penetration forces due to a combination of high tip resistance and high rod friction. Various methods to reduce thrust such as larger cones and friction reducers are routinely used but can still lead to early refusal in some soil conditions. This study presents the results of a CPTu field investigation at a New Jersey test site in two different geological formations of glauconite sand. In addition to larger cones and standard friction reducers, longer reducer length, larger reducer diameter, multiple standard reducers spaced evenly along the rod, and water jetting with a standard reducer were used to investigate their impact on reduction in thrust. Use of a larger 15 cm² cone, larger diameter friction reducer on a 10 cm² cone, and water jetting from the upper bevel on a 10 cm2 cone significantly reduced thrust, allowing greater penetration, though effectiveness was layer-specific. Neither the longer standard reducer nor multiple standard reducers showed any improvement. The results provide guidance for reducing thrust in glauconite sand, which can lead to deeper and more cost-effective geotechnical site characterization where thick glauconite sand layers are found.