(c) A long excavation with vertical sides and depth-to-width ratio of 0.5 is to be made in a deep deposit of soft clay, having an undrained shear strength of 20 kPa and a saturated bulk unit weight of 17 kN/m. Assuming that construction traffic will impose a surcharge of 10 kPa outside the excavation, determine the maximum depth of excavation that can be achieved. with side support, if a factor of safety of 1.5 against base heave is required