I'm looking to use 18" grass stakes (link) for my safety pool cover. Being that I'm putting them around the pool (grass is approx. 5-10' from the pool) and pump and heater is on the southern side about 15-20' from the pool. How can I tell that I'm not going to puncture a pipe? Is there a standard as to how deep they are (such that 18" is safe?) or to know what spots are safe (no pipes below) or other way to avoid risk?