How can I avoid water and drainage pipes in or under the slab? Are these
pipes usually imbedded in the concrete slab or are they underneath the
concrete?

Thanks

Please do not laugh at this answer. I learned it on a fire site from a
gas utility worker who made these dowsing rods out of coat hangers to
locate a gas shutoff in the front yard of a house that was ablaze.

You will need two steel coat hangers.

Take some utility pliars or wire cutters to cut through the hangers
where the twisted wire meets the rest of tha hanger and at one of the
bent "corners. Straighten them out a little until they look like two
capital L s. You can see a picture at
http://www.seanachaidh.com/dowsing.html

Holding the short ends loosely in your fists as if they were toy guns,
pointing straight ahead of you with your fists 8 to 16 inches apart.
Long ends should be parallel to each other. As you walk across your
basement floor, the wires will cross over hollow spots in the floor,
usually above pipes. Mark the floor directly below where the wires
cross. Continue this on paths about three feet apart until you map the
floor.

This will not be 100% accurate. You may find more hot spots than there
are pipes, but you certainly will not find less.

If you do not believe this, cut the hangers and test it for yourself.
Try it in your kitchen and walk across the floor until you find your
water supply pipe. Even if you hold the wires firmly in your hands,
they will take on a life of their own above the pipe.

It works in houses and in front yards, too.