An underground storage cavity is excavated in a massive salt formation by solution mining. Depth to the center of the cavity is 1,340 ft (408 m). Borehole surveying shows the cavity has the shape of an oblate spheroid 100 ft (30.5 m) in diameter and 50 ft (15.25 m) high. Estimate the salt strength necessary for stability. Show the stresses in vertical section at the top and sides of the opening.