  A granular-matter model is exactly solved, where disks of two sizes and
weights in alternating sequence are confined to a narrow channel. The axis of
the channel is horizontal and its plane vertical. Disk sizes and channel width
are such that under jamming no disks remain loose and all disks touch one wall.
Jammed microstates are characterized via statistically interacting particles
constructed out of two-disk tiles. Jammed macrostates depend on measures of
expansion work, gravitational potential energy, and intensity of random
agitations before jamming. The dependence of configurational entropy on excess
volume exhibits a critical point.
