"The type of ground in the area is red clay, which is extremely hard to dig through. This makes it very very costly to dig basements, especially for large buildings like apartments and schools.
Sad as it is, the cost is so high as to be prohibitive when it comes to tornado emergency shelters. Tornadoes are rare, short-lived, and almost never as destructive as the one from yesterday. Most large buildings are built to withstand up to an EF3, which protects against most tornadoes." on another site.
I was looking for the same answer and found this:
"The type of ground in the area is red clay, which is extremely hard to dig through. This makes it very very costly to dig basements, especially for large buildings like apartments and schools.
Sad as it is, the cost is so high as to be prohibitive when it comes to tornado emergency shelters. Tornadoes are rare, short-lived, and almost never as destructive as the one from yesterday. Most large buildings are built to withstand up to an EF3, which protects against most tornadoes." on another site.