A retaining wall, 4.5m high has a smooth vertical back. The back fill has a horizontal surface in level with top of the wall. There is a uniformly distributed surcharge load of
$$\frac{2t}{m^{2}}$$ intensity over the back fill. The density of the soil is $$\frac{1.9t}{m^{3}}$$,its angle of shearing resistance is $$30^\circ$$ and cohesion is zero. Determine the magnitude of the total active pressure per metre length of wall.