In all, inmate overcrowding and increasing staff shortages forced California's corrections department to spend more than a half-billion dollars on overtime pay in 2006, the San Francisco Chronicle reported Sunday.

The 35 percent surge in overtime from the previous year came as California prepares a major expansion of its prison system to improve conditions. The improvements must satisfy federal judges poised to impose a population cap and potentially release thousands of inmates early.

The Chronicle's analysis found nearly 15 percent of the department's 56,000-member workforce earned at least $25,000 in overtime in 2006. That is more than eight times the amount paid to the average state worker during the same period.

The analysis also showed 36 workers, including prison guards, nurses, doctors and other medical staff, each received more than $100,000 in overtime in 2006.