Prior to the election of Sheriff Joe Arpaio in 1993, the prisoner population in Maricopa County Jail, Arizona, the 4th largest jail system in the world, exceeded the maximum number of inmates allowed in its facilities. Prisoners were routinely released from custody prior to completing their sentence due to the overcrowding. In a study conducted in 1993 it was estimated that construction of a new facility would cost approximately $70,000,000. Sheriff Arpaio, concerned about the cost of a new facility and reasoning that military tents were good enough for the men and women of the U.S. armed forces who fought in Operation Desert Storm, ordered that a Tent Jail be constructed utilizing inmate labor. It consisted of Korean War era tents donated by the United States armed forces, and a 50 ft (15.4 m) observation tower with a vacancy sign mounted on the front. The final cost of the project was approximately $100,000 and it is capable of housing over 2400 Inmates.
All inmates housed outside in the tents (N yard for the males and O yard for the females) are "volunteers" in the "Working Inmate Program" and must agree to work an assigned job and comply with the Sheriff's grooming standards. Inmates who decline to work or refuse to groom themselves are relocated inside a hardened facility along with the rest of the jail population.
Prisons 
All inmates housed outside in the tents (N yard for the males and O yard for the females) are "volunteers" in the "Working Inmate Program" and must agree to work an assigned job and comply with the Sheriff's grooming standards. Inmates who decline to work or refuse to groom themselves are relocated inside a hardened facility along with the rest of the jail population.
http://vice.typepad.com/vice_magazine/2009/08/tent-city-phoenix-hades-is-full-of-tents.html