A large vertical finished (inside) shaft diameter of 32 ft is required for hoisting capacity in a planned high volume underground oil shale mine. An aquifer is encountered at a depth of 1,270 ft where the water pressure is 240 psi.

(a) Determine the thickness of a concrete liner needed for a liner safety factor of 2.5 when the concrete compressive strength (Co) is 3,500 psi, Young’s modulus is 5.6(10⁶) psi, Poisson’s ratio is 0.27. Explain, defend your answer.

(b) If changes in the inside diameter of the shaft are monitored for safety, what “reading” in inches would indicate impending failure of the liner?