No excavation shall exceed a depth greater than nine feet below the grade of the street or streets adjacent thereto, as may be established by the City Engineer, unless the side walls of such excavation be maintained at an incline of not less than forty-five degrees from horizontal, or the side walls be supported by piling or other retaining equipment equal to that specified for building excavations. (Ord. 57-1952. Passed 6-23-52.)