Excavation shall be of such width as to allow for the necessary space on each side for setting of strips, which must be of steel or wood. If wood is used for forms, the boards must be true and even and not less than one and one-half (1½) inches in thickness, and twelve (12) inches in width for the top boards and six (6) inches in width for the bottom boards if used. A sufficient number of pegs must be used to hold the strips to a straight and true line and to the proper grade. The excavation must be made eighteen (18) inches in depth. In the event the foundation is soft or spongy, it must be tamped until thoroughly compact.