Figure 6a: The basic lay-out
of a room and pillar mine plan showing nonuniform working characteristics
of many small abandoned mines. (P.B. DuMontelle, 1981, Mine Subsidence
in Illinois: Facts for the Homeowner, I.G.S., Environ. Note 99.)Figure 6a: The basic lay-out
of a room and pillar mine plan showing nonuniform working characteristics
of many small abandoned mines. (P.B. DuMontelle, 1981, Mine Subsidence
in Illinois: Facts for the Homeowner, I.G.S., Environ. Note 99.)