A retaining wall 9 m high retains granular fill weighing 18 kN/m³ with level surface. The active thrust on the wall is 180 kN per metre length of the wall. The height of the wall is to be enhanced and to keep the force on the wall within allowable limits; the backfill in the top-half of the depth is removed and changed by cinders. If cinders are used as backfill even in the additional height, what additional height may be allowed if the thrust on the wall is to be limited to its initial value? The unit weight of the cinders is 9 kN/m³. Assume the friction angle for cinders the similar as that for the soil.