Page 95 - Buried Pipe Design

Design of Gravity Flow Pipes 71

compacted by hand or by mechanical tamping in lifts of 4 to 6 in (100
to 150 mm).

Embedment
Soil is a major component of the soil-pipe interaction system and is
actually part of the structure that supports the load. The following are
some basic rules of thumb that may be of use in the evaluation of
buried pipe structures.

1. A common installation is a narrow trench with only enough side
clearance to align the pipe and to permit placement of embedment.
A trench should never be so narrow that it is difficult to place and
compact soil in the haunch zone of the pipe.
2. The arching action of the soil helps to support the load. The soil acts
like a masonry arch. No cement is needed because the soil is con-
fined in compression. Soil protects the pipe. The sidefill is the soil
arch. It must be compacted up and over the pipe, in order to create
a soil arch. Bedding provides abutments for the soil arch so the bed-
ding must be compacted.
3. If mechanical compactors are used, the soil arch should be compacted in
lifts of less than 1 ft on alternate sides of the pipe, so that the compaction
surfaces are at the same elevation. Soil should not be compacted directly
on top of the pipe. Compaction right over the pipe creates a load con-
centration and can produce a worst-case Marston load.
4. Very good buried pipe installations are those which disturb the native
soil the least. A bored tunnel of exact pipe OD, into which the pipe is
inserted, would cause the least disturbance. Microtunneling, with a
bore slightly greater than the inserted pipe, is used successfully.
5. In saturated soil, most pipes tend to float rather than sink.
6. All voids in the backfill should be eliminated. Voids can cause pres-
sure concentrations against the pipe and may become channels for
groundwater flow along the pipe (under the haunches). Full contact
of embedment against the pipe should be achieved.
7. Soil density is the most important soil property to ensure that the
soil will provide the structural support for the pipe. Its importance
cannot be overemphasized—in actual tests, the ring deflection of
flexible pipes 3 ft in diameter, in an embedment of loose silty sand,
was reduced to approximately one-half by merely stomping soil
under the haunches. For many soils the required density can only
be achieved by mechanical compaction. For select embedment such
as pea gravel, compaction can be achieved by merely moving the
gravel into place in contact with the pipe. Crushed angular stone