Page 96 - Buried Pipe Design

72 Chapter Three

provides ideal support but often requires vibration or compaction to
move the stone under the haunches and in contact with the pipe.
Below the water table, soil density is extremely important. At void
ratios greater than the critical void ratio, the addition of water will
cause relative shifting of soil particles and tends to “shake down” the
soil grains into a smaller volume. When a loose soil is saturated, the
volume decrease and the voids left are occupied by only the noncom-
pressible water that cannot support stresses. The soil mass becomes
liquefied and the pipe may collapse. If the soil has initially been placed
at a density greater than the critical density (void ratio less than crit-
ical), under disturbance (vibration) the soil volume tries to increase
but is confined, and cannot increase. For many soils, the critical den-
sity is fairly high and is in the range of 88 to 92 percent standard
Proctor density.

Compacting techniques
Select embedment. Carefully graded select soils, such as pea gravel
and crushed stone, fall into place at densities greater than critical den-
sity. The only requirement is to actually move the soil in against the
pipe especially under the haunches, to provide intimate contact
between embedment and pipe.

Mechanical compaction. Mechanical compaction of the soil in lifts (lay-
ers) is an effective method for densifying soils. Mechanical compactors
densify the soil by rolling, kneading, pressing, impacting, vibrating, or
any combination. Instructions are available on mechanical compactors
and on procedures such as optimum heights of soil lifts, and moisture
content. Efficiencies of various compactors in various soils have been
studied. In most cases, density tests are required to ascertain that the
specified density is being achieved. Heavy equipment (compactors, load-
ers, scrapers, etc.) must not operate close to the structure—especially
flexible structures, since misalignment, deflection, and high induced
stress may occur.

Vibration. Loose soil can be compacted by vibrating it with vibroplates
and vibrating rollers on each soil lift. Some compaction of the embed-
ment can be achieved by vibrating the pipe itself. Concrete vibrators
are effective in the placement of embedment around pipes if enough
water is mixed with the soil to form a viscous, concretelike mix. The
contractor may saturate a lift of sidefill and then settle it with con-
crete vibrators. This technique places, but does not compact, the soil.
Saturated soil is noncompressible, therefore, noncompactable. If such