I gave up on the spa electrical trench when I ran into the huge piece of concrete that was right in the middle of the trench area. I called the electrician thinking it would be better to pay for a trench than waste my time on the impossible or at least super slow. I thought the trench had to be 18″ deep, but the electrician said it was almost deep enough and only had to be 12″ deep for residential on just a lawn area. So I was encouraged to try again as it will save money when the hook up is done.
Dave and the electrician both suggested I use water to help dig the heavy clayey soil. That really did help. I hooked up the hose from the outside well hydrant and dampened the soil to soften it. It was still difficult to get the shovel in more than an inch at a time though. But finally I got the whole trench to a 12 inch depth.
I used a 36″ pry bar and heavy hammer to whale away at the dirt under the concrete. Also spraying the dirt with the hose and a spray attachment helped weaken the dirt enough to chip away at it. I dug out the dirt with a trowel. It felt like I was scratching away at a prison escape tunnel, but eventually the pry bar went though and I was able to scrape open a hole for the electric conduit at the 12″ depth.
The small victory was sweet; from an impenetrable barrier to a solution and a savings on the electrician. Not too shabby.