The subgrade shall be excavated to a depth of about four inches below the finished grade, and thoroughly settled and compressed by wetting and tamping. All fills under sidewalk shall be made of suitable material spread in layers not to exceed one foot in thickness, each layer to be thoroughly flushed with water and tamped or rolled until a hard unyielding surface is obtained. In fills, the subgrade shall extend at least nine inches past the edge of the sidewalk, and shall have a natural slope of one and one-half feet to one foot, unless the fill is against some permanent structure. No board retaining walls shall be used. (1957 code § 8.12.060(e).)