Figure 20. Block Diagram of a typical sag subsidence event: A. Wooden frame house in tension zone. Foundation has pulled apart and dropped away from the superstructure in one corner. B. Road in compression zone. Asphalt has buckled. C. Brick house in tension zone. Walls, ceilings, and floors have cracked. (P.P. DuMontelle, 1981, Mine subsidence in Illinois: Facts For the Homeowner. I.G.S., Environ. Notes 99)