I went to a presentation they did this morning for employees. The clamshell is used to dig down to bedrock, about 100 feet down, and then they bring in the big grinder thing (actually called a hydromill) to go down 2 feet into bedrock. They circulate the slurry throughout this process and separate the ground-up rock etc out of it with the big gray machine in the back corner of the site. The rebar cages go down about 20' below the bottom of the parking garage and it's just concrete below that The interior pilings are done similarly except there's a huge I-beam inside them.