The in situ stress field for Problems 38 and 39 changes between 914 and 1,067 m to one described by the formulas σv = 25.45h, σH = 24,138 + 7.47h, σh = 24,138 + 7.47h where the stresses are in kPa and the depth h is in m. The stresses σv, σH, σh are principal stresses in the vertical and horizontal directions. Find the safety factor at 1,372 m for an unlined, circular shaft wall. Assume the same rock properties.