The in situ stress field for Problems 34 and 35 changes between 3,000 and 3,500 ft to one described by the formulas v = 1.125h,

The in situ stress field for Problems 34 and 35 changes between 3,000 and 3,500 ft to one described by the formulas σv = 1.125h, σH = 3,500 + 0.33h, σh = 3,500 + 0.33h, where the stresses are in psi and the depth h is in ft. The stresses σv, σH, σh are principal stresses in the vertical and horizontal directions. Find the safety factor at 4,500 ft for an unlined, circular shaft wall. Assume the same rock properties.