15 AUGUST 2005
can be achieved by internal vibration (for example, by using concrete vibrators).
Downward drainage is required to maintain seepage forces in a downward
direction if the placed material is saturated to aid in compaction.
Inorganic clays, inorganic silts, and very fine sands of low to medium plasticity
are fairly easily compacted in open zones with sheepsfoot or rubber-tired rollers
in the 67 kN (15,000 pound) and above wheel-load class. Some inorganic clays
can be adequately compacted in confined zones using rammer or impact
compactors with a static weight of at least 0.445 kN (100 pounds) provided close
control of lift thickness and water content is maintained.
Fine-grained, highly plastic materials, though not good backfill materials, can
best be compacted in open zones with sheepsfoot rollers. Sheepsfoot rollers
leave the surface of the backfill in a rough condition, which provides an excellent
bond between lifts. In confined areas the best results, which are not considered
good, are obtained with rammer or impact compactors.
Lift Thickness. The loose-lift thickness will depend on the type of backfill
material and the compaction equipment to be used.
As a general rule, a loose-lift thickness that will result in a 0.15 m (6 in) lift when
compacted can be allowed for most sheepsfoot and pneumatic-tired rollers.
Cohesive soils placed in approximately 0.25 m (10 in) loose lifts will compact to
approximately 0.15 m (6 in), and cohesionless soils placed in approximately 0.20
m (8 in) base lifts will compact to 0.15 m (6 in). Adequate compaction can be
achieved in cohesionless materials of about 0.300 to 0.381 m (12 to 15 in) loose-
lift thickness if heavy vibratory equipment is used. The addition of vibration to
rolling equipment used for compacting cohesive soils generally has little effect on
the lift thickness that can be compacted, although compaction to the desired
density can sometimes be obtained by fewer coverages of the equipment.
In confined zones where clean cohesionless backfill material is used, a loose-lift
thickness of 0.100 to 0.150 m (4 to 6 in) and a vibratory plate or walk-behind,
dual-drum vibratory roller for compaction is recommended. Where cohesive soils
are used as backfill in confined zones, use of rammer compactors and a loose-lift
thickness of not more than 100 mm (4 in) should be specified. Experience has
shown that "two-by-four" wood rammers, or single air tampers (commonly
referred to as powder puffs" or "pogo sticks") do not produce sufficient
Density Requirements. In open areas of backfill where structures will not be
constructed, compaction can be less than that required in more critical zones. Compaction to
90 percent of ASTM D 1557 maximum dry density should be adequate in these areas. If
structures are to be constructed on or within the backfill, compaction of cohesionless soils to
within 95 to 100 percent of ASTM D 1557 maximum dry density and of cohesive soils to at
least 95 percent of ASTM D 1557 should be required for the full depth of backfill beneath these