  Understanding intermittency, an ubiquitous behavior in flows of packed
grains, is pivotal for establishing the rheology of granular matter. A
straightforward explanation has been missing despite the long development of
theories at various levels of abstraction. Here, we propose the use of a
Stribeck-Hertz model that starts with the classic Coulomb friction but takes
into account the inter-particle tribology, i.e. the reduction of friction
coefficient with sliding speed as is commonly observed. Our numerical
experiments reveal a state diagram that covers a wide range of packing
fractions and show that incorporating the tribology enables the occurrence of
quaking intermittency in the mid-range of a newly established dimensionless
shear rate, in consistence with prior experimental observations. Further study
of the discontinuities in the evolution of mean contact number leads to our
discovery of two types of quaking, that are distinguished by the abrupt
increase or decrease of neighboring contacts and reveal different pathways of
microstructural change underlying these discrete events. In contrast to the
prevailing paradigm in which shear is believed to promote jamming at
intermediate densities, our study demonstrates that shear can also unjam a
granular system, and this occurrence depends on the shear rate.
