Constant-Volume Direct Simple Shear Behavior of Reconstituted Natural and Degraded Glauconite Sand

Abstract This paper presents results from a laboratory investigation of constant-volume direct simple shear (DSS) behavior of reconstituted authigenic and allogenic glauconite sands. This complex soil is formed in shallow marine depositional environments and primarily exists in situ as peloidal sand-sized grains but upon progressive degradation releases clay minerals and transitions to a plastic fine-grained soil. It has been studied extensively in the geosciences, but there is limited knowledge of its geotechnical engineering behavior. Natural samples of two glauconite sands with different geologic histories were collected from outcrops in New Jersey. Air-dried samples were reconstituted into DSS specimens using air pluviation. The samples were also degraded by mixing in a hydrometer dispersion cup, and the resulting slurry was reconstituted into DSS specimens. The DSS test program explored variations in reconstitution method, initial density, effective stress state, and overconsolidation ratio. Results show that the authigenic glauconite sand degraded much more readily than the allogenic glauconite sand. For the same applied degradation duration of 60 min, the authigenic glauconite transitioned from a silty sand to a high plasticity clay whereas the more resilient allogenic glauconite remained primarily a sand with a transition from a poorly graded sand to a clayey sand. These differences in the nature and composition of the glauconite grains in each soil controlled the normally consolidated DSS consolidation and shear behavior with a clay-like contractive behavior for the authigenic glauconite versus a sand-like dilative behavior for the allogenic glauconite. The results on the intrinsic behavior of two glauconite sands with different geologic histories inform interpretation of the shear behavior of intact and degraded glauconite, supporting development of a framework for constitutive model development.