Ditto, you have to till tons of organic matter directly into the soil.
Topsoil is good, but it will wash away if added alone. Even if it is tilled in, rain separates topsoil from clay. You need to add in "chunky stuff" and a layer of leaves, pine needles, and safe mulch on top to prevent this from happening. Your plant roots should help to anchor the soil.
Give the turtles a HUGE pile of leaves and/or pine needles. This will provide a place for them to escape from the heat. Water it a few times a week, and the clay under the pile should stay soft enough for them to dig. You'll have to keep adding leaves/needles as the pile settles.
You can also make little turtle houses. You must use untreated lumber. I've built them before using untreated 2X10's. The sides can be 10 inches tall and I make them around 2 feet long. Paint the top with a light colored, non-toxic paint to reflect light. I've placed them directly on top of the ground and also have sunken them in until the tops are level with the ground. They rot quickly, which is why I've switched to using logs as shelter(plus rotting logs attract slugs). However, they're cheap and easy to make so replacing is simple. When I have used them, my turtles absolutely LOVED them. You can stuff leaves or pine needles inside for added insulation. Quite frequently, the turtles will use these almost exclusively. You can use this basic concept and make very elaborate and pretty houses. The clay may wash down into the sunken houses during rains. You may need to periodically scoop excess clay out.(Welcome to the Carolinas - NEVER stick your hand into one of these hiding spots without a line of sight. Copperheads abound!)
One more thing...don't till unless you are adding a large quantity of organic matter(nearly as much as the soil you are displacing). Tilling clay will usually end up making it MORE compact if nothing is added. When tilling, you're basically destroying the little tunnels and pockets of air that are created by organisms living in the soil. After the next rain, the clay will pack down again and often become like a rock. It takes decades for those tunnels to be re-constructed. This is by far the most common mistake of gardeners working in heavy clay soil. My neighbors will say, "I tilled this area, and now that it rained it is lower than the surrounding soil". Much of these air pockets and tunnels are smaller than the human eye can see.
P.s. Don't listen to the garden center people who tell you to add sand or "clay soft".
Sorry for the long post.
