Page 305 - Buried Pipe Design

276 Chapter Five

SIDD design assumes the same design may be used for embank-
ment, trench, and subtrench installations of the same type. The fol-
lowing reasons are given in the Concrete Pipe and Technology
Handbook:
1. “This assumption precludes the need to specify a maximum
allowable trench width for any installation. It is often difficult to con-
trol the actual trench width in the field and impossible to restore the
specified in-situ trench width after an over-width trench has been con-
structed.”
2. “With a narrow trench, access for compacting the embedment soil
in the haunch region usually is too limited for assurance that the spec-
ified minimum compaction level will be achieved for many types of
placed soils (except in type 4 installations which do not require haunch
zone compaction). Thus, higher quality trench installations require suf-
ficient trench width for access to properly compact soils in the haunch
zone below the pipe, and narrow trench installations should be limited
to type 2 or type 3 installations with ‘self-compacting’ types of granu-
lar embedment soils (i.e., crushed stone or pea gravel), or to type 4
installations.”
3. “Although the vertical earth load on a pipe in a narrow trench
typically is lower than the earth load on a similar embankment pipe,
the lateral load on the trench pipe typically is also lower than the lat-
eral load on the pipe in a comparable embankment. Since the differ-
ence between vertical and lateral loads in a given installation is the
primary influence on design requirements, the comparative design
requirements for pipe in the two installations are not as favorable to
the trench condition, as the comparative magnitudes of vertical earth
load.”
4. “It is conservative to use the embankment installation criteria for
trench installations.”

The four standard installations are defined by the types and densi-
ties of the bedding and embedment soils required for each installation
type. The soil zones that define the locations of soil types and densities
specified in the ASCE SIDD Practice are shown in Fig. 5.10 for the
embankments and in Fig. 5.11 for trenches. The soil types and densi-
ties that are required, or permitted, in the various zones are given in
Table 5.12 for embankments and in Table 5.13 for trenches. Note that
several alternative combinations of soil types and compaction densi-
ties are sometimes permitted for the various zones in each installation
type. See Table 3.1 for a description of the soils that are included in
each of the standard soil classifications.
The following general descriptions of each installation type provide a
summary of the major characteristics of the ASCE standard installations.