  Compressed frictional granular matter cannot flow without dilation. Upon
forced shearing to generate flow, the amount of dilation may depend on the
initial preparation and a host of material variables. On the basis of both
experiments and numerical simulations we show that as a result of training by
repeated compression-decompression cycles the amount of dilation induced by
shearing the system depends only on the shear rate and on the (pre-shearing)
packing fraction. Relating the rheological response to structural properties
allows us to derive a scaling law for the amount of dilation after $n$ cycles
of compression-decompression. The resulting scaling law has a universal
exponent that for trained systems is independent of the inter-granules force
laws, friction parameters and strain rate. The amplitude of the scaling law is
analytically computable, and it depends only on the shear rate and the
asymptotic packing fraction.
