Can we Put a 6 feet hole in a reinforced concrete slab of 8 inches?

That would depend on where the slab was and what it was doing. How close to a wall is it? What's under the slab? Rock, plumbing? Can you do it? Yes. Should you? Good question. What may happen varies with each slab and circumstance. You may bust it out and find a large rock under it and have to replace it.