A tabular ore body is 32 ft (9.75 m) thick, strikes due north, and dips 22◦ east. The premining stress field is due to gravity alone. Stopes are excavated at a depth of 2,590 ft (179 m) in a regular room and pillar configuration. A lane and pillar method is used, so the system appears two-dimensional in section. Lanes (rooms) are 52 ft (16.9 m) wide. Rock properties are: Co = 18, 500 psi (128 MPa), To = 1, 650 psi (11.5 MPa), E = 11 × 106 psi (75.9GPa), v = 0.19 A safety factor of 1.5 with respect to the maximum shear stress is required for pillar design. Find the extraction ratio and pillar dimensions assuming no size effect on strength.