No concrete curbing shall be less than eight inches on the top thereof, nor the thickness from the bottom of excavation to the grade line of the street less than nine inches, where the top does not exceed eight inches. It is the intent and meaning of this section that the thickness of the concrete shall in all cases be determined by the width of the top of the curb when finished, and that the bottom of excavation to the grade line of the street, it shall not be less than 1 1/4 times that of the top of curb as aforesaid.