§ 6-123. Subgrade preparation for building construction.

(a)

Subgrades for concrete construction shall be evenly graded to provide a uniformly firm foundation. All loose or extraneous material shall be removed from excavations and soft spots in the subgrade shall be corrected before the concrete is placed.

(b)

Whenever concrete is placed on fill, the entire fill beneath the concrete shall be compacted as specified by the building official. It should be recognized that the degree of compacting specified must be that required to obtain a stable fill (one that will not be subject to detrimental shrinkage or expansion under varying moisture conditions) and adequate to support the loads it must bear. The following comments are of a general nature and when unusual soils (especially expansive soils) are encountered, the degree of compaction to be specified should be determined after proper soil analyses have been made:

(1)

Nonexpansive soils. Fills of such soils generally should be compacted to at least ninety-five (95) percent of maximum density as determined by AASHO T-99 or ASTM D-698. Adequate compaction can usually be obtained when the moisture content of the soils is within a range of three (3) percent below to three (3) percent above optimum moisture for maximum density at the time of compaction.

(2)

Expansive soils. Fills of such soils normally encountered in the city area generally should be compacted to a minimum of ninety (90) percent but not in excess of ninety-five (95) percent of maximum density as determined by AASHO T-99 or ASTM D-698. Relatively stable fills usually can be obtained when the minimum moisture content at time of compaction is in excess of three (3) percent above optimum for maximum density. This moisture content should be maintained in the top twelve (12) inches of fill until covered by concrete.

(3)

Granular soils. Fills consisting of sand, gravel or crushed stone should be compacted to one hundred (100) percent of maximum density as determined by AASHO T-99 or ASTM D-698. Generally the moisture content is not as critical as when compacting clay or silt soils and may vary with the compactive equipment available. Vibratory equipment has proven the most suitable for compaction of granular materials.

(c)

Over-excavation for slabs shall be corrected by replacing with well-graded crushed stone, with suitable soil, or with cement-stabilized soils mechanically compacted in not more than six-inch lifts to ninety-five (95) percent of maximum density. For hand compaction, lifts shall not exceed four (4) inches measured before compaction.

(d)

Footing excavations shall be extended to undisturbed soil below the frost line and shall be level or stepped level increments.

(e)

Footings shall not be placed on fill except as directed by the building inspector.

(f)

Expansive soils may be treated with a cementitious or lime-type material for stabilization as directed by the building official.

(Code 1974, § 5-12)