In order to prevent freezing the plumbing codes require all lines to or from a structure to be buried a minimum of 6" below the average frost depth for your region.

There is no risk of freezing under a slab if the living space above the slab is heated.

If you really want to insure an access to the lines under a slab for future maintenance you can install PVC pipe with long radius bends up to the slab, then pull continuous roll copper or plastic pipe through the PVC. In this configuration should it be necessary to replace a line in future you can pull it through the conduit.

If you install copper pipe under a slab you are required to use wrought copper fittings and all joints must be brazed. They may not be soldered. That is why we normally use continuous roll copper pipe under a slab to eliminate joints, but be careful here, you must use type K or type L copper pipe, either rigid or roll but you may not use type M copper or copper tubing.