Page 97 - Buried Pipe Design

Design of Gravity Flow Pipes 73

a method is used, the soil must be free-draining. Also no load (soil on
top of pipe) should be placed until, after vibration, the soil has drained
and has developed its strength. If the soil is not free-draining, parti-
cles flow into place, but settle only under buoyant weight. The result
is the same as ponding. The soil gradation must be controlled just as
concrete aggregate is controlled. Flotation must be avoided.

Flooding (ponding). A lift of freedraining soil is placed up to the
spring line of the pipe, then the soil is irrigated. A second lift to the
top of the structure is often specified. Enough water must be applied
that the lift of soil is saturated. The soil should be free-draining and
must be dewatered to settle the soil. The compaction mechanism is
downward seepage stress which compacts the soil. Soil is washed
into voids and under the haunches of the pipe. The pipe must not
float out of alignment. This is the least effective method (yet often
adequate) for compaction.

Jetting. Soil density greater than critical can be achieved by jetting.
This technique is particularly attractive for soil compaction around
large buried structures. Soil is placed in high lifts, such as 3 to 5 ft, or
to the spring line (midheight) of large diameter pipes. A stinger pipe
(1-in diameter and 5 or 6 ft long, attached to a water hose) is injected
vertically down to near bottom of the soil lift. A high-pressure water
jet moves the soil into place at a density greater than critical if the
soil is free-draining and immediately dewatered. Jet injections are
made on a grid every few feet. Five-foot grids have been used suc-
cessfully for 5- or 6-ft lifts of cohesionless soil. Gangjets can be
mounted on a tractor. They can be injected into a lift of sidefill up to
the spring line. To fill holes left when jets are withdrawn, the stingers
are vibrated. A second lift up to the top is jetted in a similar manner.
The technique works well in sand. As with vibration, no load (soil on
top of pipe) should be placed until, after jetting of the soil around the
pipe, the soil has drained and has developed its strength. Pipe overde-
flection and pipe failures have been reported when soil, several feet in
depth, was placed on the pipe before jetting took place. In such a case,
jetting causes the soil to collapse and liquefy around the pipe, giving
no support to the pipe and no arching to help support the soil on the
pipe. In any compaction technique, it is absolutely essential that the
required pipe zone density be achieved before overburden is placed on
the pipe.

Flushing. Soil densities greater than critical can be achieved if a high-
pressure water jet is used to flush soil into place against the pipe. A
high-pressure water jet plays the stream onto the inside slope of the