(A) Method A. If protection for underground facilities does not require clay for backfill, then the entire depth of the excavation shall be filled with granular material in 12-inch layers with each layer being thoroughly compacted.
(B) Method B. Selected clay material free from rocks may be used in the bottom two feet of the excavation under circumstances where clay material is desired for the protection of underground facilities, but such clay material shall not be used within 18 inches of the finished grade. Clay backfill must be placed in layers not to exceed four-inch thickness and each layer must be thoroughly compacted by mechanical means. The remaining depth shall be filled with granular material and compacted in no more than six-inch layers.
(`95 Code, § 66.12)