Our house sits on clay. The builder put down a layer of sand to pour the slab on. They just dug down through the garage floor and removed it to make it deep enough. They laid down a grid of rebar and tied it together. They used 3,000 PSI concrete.
One of the smart thing I did was hire processionals to do it. I have zero experience in that type of work. They cut the concrete, broke it up, and hauled the old concrete and dirt away. My only part was putting in the PVC conduit to run the hydraulic hose and some Romex in to have 110 volts in the pit. I have that circuit off unless I need to use it.
The top of the pit has a lip recessed down 1/2 inch so the plywood sits level with the concrete floor. 3/4 inch plywood is just too heavy to move around, so I went with 1/2 inch marine grade plywood from a lumber yard, not big box store.
