  In order to understand the nature of friction in closely-packed granular
materials, a discrete element simulation on granular layers subjected to
isobaric plain shear is performed. It is found that the friction coefficient
increases as the power of the shear rate, the exponent of which does not depend
on the material constants. Using a nondimensional parameter that is known as
the inertial number, the power-law can be cast in a generalized form so that
the friction coefficients at different confining pressures collapse on the same
curve. We show that the volume fraction also obeys a power-law.
