The clay in the tunnels has taken on a rock hard consistency because it is dry. This is the type of clay that is supporting our home.
The hard clay was discovered when the trench was dug for the French Drain which was created to drain the water from the back yard and under the house. The digging was very very hard. Actually, when the kitchen was remodeled on moving in 1996, the crawl area was a little moist for putting the piping and heat duct changes in, but I was still a young 61 year old guy and was able to manage.

In 2004 we found it necessary replace the cold water line of galvanized pipe to the kitchen. At the same time we added the pantry to the new back porch, build out the Laundry area. Both which required more electrical wiring.

The crawl area had dried out, providing a very hard surface, so I decided to make it easier to work in the crawl area.
I dug down several inches, making sure to only expose one side of the house support base. The TUNNELS were created and the dirt was handy for fill in making The WALL and a level front lawn.

Our grandchildren enjoyed the tunnels. Here is Leo who liked to dig, in 2012.
The tunnels were created in 2004, after the back porch was added on in 2003.
The soil being of hard, rock like clay, made it possible to dig out and
 not reduce the support of the home.