Desert tortoises should be hibernating now and they do dig a burough where they can sleep un-interupted and safe from predators. The problem is that when they are in captivity they are subject to the environment that is provided for them and that might not be what they need. They would dig under the soft sand and first layer of hard ground, down to the gravel which is easily excavated. Here they are protected from weather as well as predators. Rain in the desert is minimal and is absorbed before it reaches them and if it does the next layer below them acts as a sump so the water does not collect. If these layers are not present in your area they run the risk of drowning in rain runoff or developing a respiratory infection from the wrong moisture content in the ground. So you have two choices. Build a borough for them to serve their purposes and keep them safe, or dig them out and put them in a cardboard box in a safe cool place where you can check on them from time to time until they wake up and can be placed back outside. We have had and raised desert tortoises for 40 years and they are a joy to have.