A vertical circular shaft will pass through a water-bearing stratum at a depth of 2,780 ft where the water pressure is estimated to be 210 psi. Finished shaft diameter must be 18 ft. The pre-shaft stress state is attributable to gravity alone. Rock properties are: Co = 6,750 psi, To = 675 psi, E = 2.4 million psi, ν = 0.25, γ = 156pcf. What concrete liner thickness is indicated, assuming the maximum allowable concrete liner stress is 3,500 psi?