An existing DC three-wire system is to be converted into a 3-phase, 4-wire system by adding a fourth wire equal in cross-section to each outer wire of the DC system. For the same supply and load voltages to neutral and balanced conditions, the extra power at unity power factor that can be supplied by the AC system will be
a
What is the maximum depth to which a trench of vertical sides can be excavated in a clay stratum with c = 50 kN/m², y = 16 kN/m³, β = 90°, ¢ = 0°, Fc = 1 and N = 0-261?