Page 94 - Buried Pipe Design

70 Chapter Three

may not be the lowest-cost alternative when consideration is given to
total lifetime cost. Soil-structure interaction influences pipe perfor-
mance and is a function of both the pipe properties and embedment
soil properties, and therefore impacts total system costs. The design
engineer should consider soil-structure interaction in the installation
design and lifetime cost estimates.
The soil-pipe system is highly statically indeterminate. This means
that the interface pressure between the soil and pipe cannot be calcu-
lated by statics alone—the stiffness properties of both soil and pipe
must also be considered. The ratio of pipe stiffness to soil stiffness
(PS/E ) determines to a large degree the load imposed on the conduit.
For example, a “rigid pipe” will have a much greater load than a “flex-
ible pipe” installed under the same or similar conditions.
Soil to be placed in the pipe zone should be capable of maintaining
the specified soil density. Also, to eliminate pressure concentrations,
the soil should be uniformly placed and compacted around the pipe.
Various placement methods can be used depending upon system
parameters such as soil type, required density, burial depth, pipe stiff-
ness, and pipe strength. The following are suggested as methods which
will achieve desirable densities with the least effort.
Certain manufactured materials may be placed by loose dumping
with a minimum of compactive effort. These materials must be angular
and granular such as broken coral, crushed stone or rock, crushed
shells, crushed slag, or cinders and have a maximum size of 1 in
1
2
(40 mm). Care should be taken to ensure proper placement of these
materials under pipe haunches.
With coarse-grained soils containing less than 5 percent fines such
as GW, GP, SW, SP, GW-GP, and SW-SP, the maximum density will be
obtained by compacting by saturation or vibration. If internal vibra-
tors are used, the height of successive lifts or backfill should be limited
to the penetrating depth of the vibrator. If surface vibrators are used,
the backfill should be placed in lifts of 6 to 12 in (150 to 300 mm). This
material may also be compacted by hand tamping or other means, pro-
vided that the desired relative density is obtained.
Coarse-grained soils which are borderline between clean and those
with fines containing between 5 and 12 percent fines, such as GW-GM,
SW-SM, GW-GC, SW-SC, GP-GM, SP-SM, GP-GC, and SP-SC, should
be compacted either by hand or by mechanical tamping, saturation or
vibration, or whichever method meets the required density.
Coarse-grained soils containing more than 12 percent fines, such as
GM, GC, SM, SC, and any borderline cases in the group (that is, GM-
SM), should be compacted by hand or by mechanical tamping. The back-
fill should be placed in lifts of 4 to 6 in (100 to 150 mm). Fine-grained
soils such as MH, CH, ML, CL, SC-CL, SM-ML, and ML-CL, should be