Hand written solution required

A 2.5m thick horizontal ore body is located at a depth of 80m with the overlying rock having a unit weight of 25kN/m3. An initial mining layout is based on 6.0m rooms and 5.0m square pillars with the full thickness of 2.5m being mined. The pillar strength at the site is empirically defined by the formula 5 = 7.18W0.46h-0.66 Where S is in MPa, and W and h are in m. (i) Determine the pre-mining stress at the pillar location. (ii) Comment on the stability of the pillars for the initial design. (iii) What options would you make in terms of redesigning the layout if the safety factor were to be set at 1.5