Contact micromechanics in granular media with clay

Many granular materials, including sedimentary rocks and soils, contain clay particles in the pores, grain contacts, or matrix. The amount and location of the clays and fluids can influence the mechanical and hydraulic properties of the granular material. This research investigated the mechanical effects of clay at grain-to-grain contacts in the presence of different fluids. Laboratory seismic wave propagation tests were conducted at ultrasonic frequencies using spherical glass beads coated with Montmorillonite clay (SWy-1) onto which different fluids were adsorbed.; For all bead samples, seismic velocity increased and attenuation decreased as the contact stiffnesses increased with increasing stress demonstrating that grain contacts control seismic transmission in poorly consolidated and unconsolidated granular material. Coating the beads with clay added stiffness and introduced viscosity to the mechanical contact properties that increased the velocity and attenuation of the propagating seismic wave. Clay-fluid interactions were studied by allowing the clay coating to absorb water, ethyl alcohol, and hexadecane. Increasing water amounts initially increased seismic attenuation due to clay swelling at the contacts. Attenuation decreased for higher water amounts where the clay exceeded the plastic limit and was forced from the contact areas into the surrounding open pore space during sample consolidation. The decreased clay thickness at the contacts increased the contact stiffness and decreased attenuation. Attenuation was larger for clay with absorbed ethyl alcohol and hexadecane than for similar water amounts.; Both compressional and shear waves cause the clay-coated contacts to move towards and away from one another resulting in pumping loss due to viscous deformation of the clay. Pumping loss attenuation will depend on the contact microgeometry and material viscosity.; The grain contacts were shown to behave as low-pass filters by use of a Butterworth mathematical filter. Analyses of the mechanics of contact stiffness were made using effective modulus and discontinuity theories. Numerical modeling was performed on idealized columns of grains coupled by discontinuous displacement and discontinuous velocity boundary conditions. The numerical trials accurately simulated the laboratory velocity and attenuation variations, confirming the experimental interpretations of the dominance of contact stiffness on wave propagation.