It happened at the Hegewisch neighborhood Water Reclamation Plant at 130th Street.
Firefighters say one man was already working in the hole when Jeff Andrews slipped off a ladder and landed on top of him.
Andrews was taken out on a stretcher and airlifted to an area hospital. His relatives says he is in very critical condition and will undergo brain surgery to relieve pressure in his head.
"He's in bad shape, for sure," said Tammy Booe, victim's sister. "That's why we need a lot of prayers.
The second injured worker was able to walk out of the trench on his own and is in fair condition.
Officials say both men were contract workers for a cement company.