Santamarina, J. C. and Shin, H. (2009a). "Frictional Phenomena in Granular Media." Batsheva de Rothschild Seminar on Shear Physics at the Meso-scale in Earthquake and Landslide Mechanics
Fine-grained and coarse grained granular materials exhibit normal-stress dependent frictional shear strength. The mineral-to-mineral friction mobilized at interparticle contacts emerges at the macroscale through a complex sequence of competing particle level processes. The observed frictional response of the soil mass varies with strain level; we can distinguish the constant volume friction angle, dilation angle, peak friction angle, residual friction angle after grain alignment, and post-granular-segregation friction angle. Compiled experimental data and particle-level simulations help identify the most relevant soil parameters that affect the frictional response in each case (including interfacial friction at boundaries). Other sediment conditions that affect frictional strength include: grain crushing, inherent anisotropy, intermediate stress, temperature, strain rate, vibration, pore fluid and contact-level adhesive forces. The measurement of soil friction is a boundary-value problem; information-intensive measurement methods may help overcome measurement limitations related to incomplete knowledge of boundary conditions.