Shear band formation and poromechanical properties; application to unlithified sand, Humboldt County, CA

In response to shear strain, porous granular media may fail in tabular zones of grain deformation, commonly referred to as shear bands. Previous researchers have argued that shear bands form via cataclasis and strain hardening, and that once formed they do not accommodate additional shear-strain. If correct, this hypothesis requires that shear bands are stronger than their parent material, and that parent material strengthens in response to shear-driven cataclasis, each of which may alter the effective permeability. We report on laboratory experiments designed to resolve the frictional strength and permeability of shear bands formed in well-sorted nearshore marine sand and their unlithified parent material, and to elucidate the strength and deformation properties of parent material under conditions of shear-band formation. Experiments were conducted on in situ shear bands and parent material from late Quaternary nearshore marine sand in the footwall of the active McKinleyville thrust fault, Humboldt County, CA. Shear bands are exposed in positive relief, a consequence of reduced grain size and cementation, which result in decreased permeability. Permeability was measured under hydrostatic stress conditions at effective confining pressures from 0.2 MPa - 5.0 MPa . Shear bands have permeabilities of 8.2*10 -15 - 1.3*10 -17 m 2 , which represents roughly a 1 to 3 order of magnitude decrease relative to that of the parent material (7.0*10 -14 - 2.0*10 -14 m 2 ). We sheared parent material and in situ shear bands in the single-direct shear geometry over a range of probable in situ normal stresses (0.5-1.8 MPa). Shear bands have greater strength than parent material, with the coefficient of internal friction being μ i = 0.623 and μ i = 0.525, respectively. We sheared parent material in the double-direct shear geometry under conditions approximating shear band formation (sliding velocity = 10 μm/s-10 mm/s, σ n = 0.75-1.75 MPa, saturated/dry, shear strain = 0.5-20). We find that parent material strengthens as a function of shear strain throughout individual experiments; frictional yield strength increases by 1 to 9% over a range of shear strain from 0 to 10. We attribute the increase in strength to increased grain angularity and abundance of small particles, both of which are a consequence of pervasive cataclasis. Our results support the hypothesis that shear bands are stronger than their parent material and that parent material exhibits strengthening as a function of shear-driven cataclasis. Further, cataclasis may be responsible for the observed permeability contrast of in situ shear bands and parent material. Ongoing work will analyze the role of cataclasis on parent material permeability as a function of shear strain.