Coal is mined 15 ft high underground in flat strata by the long wall method at a depth of 1,500 ft. Panels are 750 ft wide and 7,500 ft long. Assume US conditions, then considering trough subsidence, estimate (in ft):
a. Maximum subsidence over a single panel (ft),
b. Critical width (ft),
c. Maximum subsidence over an area undermined by six adjacent panels (ft),
d. Maximum tensile strain over a single panel,
e. Maximum compressive strain over an area undermined by six adjacent panels.