Before there can be a hole in the roof at the new Cowboys stadium, there had to be a big hole in the ground. It was a year ago Tuesday that dirt excavation began at the site of the $1 billion, 80,000-seat stadium that is scheduled to completed for the 2009 season. On Monday, workers installed the first of more than 3,000 precast concrete risers to which the stadium seats will be mounted. It will take 16 months to install all the seating risers, which will have a total weight of nearly 30,000 tons. "We're right on schedule," said Jack Hill, stadium construction manager for the Cowboys. "We couldn't ask for a better project."