A retaining wall with sloping back, of vertical height 9m retains soil of density 1.6 Mg/m3, and angle of friction 30o. The soil slopes upwards at 20o to the horizontal from the back edge of the wall, the angle between the slope and the back of the wall is 100o. Cohesion 10kN/m2, angle of friction between soil and wall is 25o but there is no cohesion between them. Find the thrust on the back wall per metre run using trial values of 25, 30, 40, and 45o for the inclination for the plane of rupture to the vertical.