If those are harvester ants, which I think they are, they do that because they get attacked (isolation of the enemy), and the chambers are grow areas for their gardens. Keeping them separated is about always having resources regardless of what happens anywhere in the colony as well... it is never just for one reason. They go deep for temperature control as well, the queen goes deep with the eggs.
Their mounds are often mistaken for fire ants for some odd reason.
http://creatures.ifas.ufl.edu/urban/...vester_ant.htm
http://www.discover.com/issues/nov-0...-life-of-ants/
"Myrmecologist Walter Tschinkel of the University of Florida holds up a partial zinc cast of a seven-foot-deep Florida harvester-ant nest. The flat chambers are living quarters as well as storerooms for seeds, which the foraging harvesters collect, shuck, and deliver to the workers underground for stocking and sorting."