Page 135 - Buried Pipe Design

Design of Gravity Flow Pipes 109

engineering solutions may be available, and economic inputs may
suggest a proper solution.
For example, suppose PVC sewer pipe (ASTM D 3034 DR 35) with a
2
minimum pipe stiffness of 46 lb/in is to be installed where the native
soil is a class IV clay. Ninety percent of the line will be at depths as
great as 20 ft. The engineer has selected 7.5 percent deflection as his
design limit. According to Table 3.9, the native class IV material could
be used for that portion of the pipeline with less than 14 ft of cover if
compacted to 75 percent of standard Proctor thereby ensuring deflec-
tions less than 7.5 percent. However, groundwater conditions may
make compaction difficult, even impossible, or may result in subse-
quent reduction in soil strength. If this is the case, class I, II, or III
material may be imported and used with appropriate embedment pro-
cedures to limit deflection to 7.5 percent. The choice will be based on
availability, convenience, and consequently, cost. For the deep portion
of the line, class III material compacted to 85 percent, class II material
compacted to 80 percent, or class I material without compaction could
be used successfully.

Pipe Design Criteria

Design methods for installation design have been discussed. However,
no design can be affected without performance criteria. Performance
criteria are usually established by the design engineer based upon
required performance and capabilities of specified products. When a
capability of a product is reached or exceeded, it is said that a perfor-
mance limit has been reached. Each product will exhibit one or more
performance limits for each application. Performance limits are estab-
lished for each product to prevent conditions that may interfere with
the design function, including the life of the product.

Performance limits
For buried pipes, as for most structures, performance limits are
directly related to stress, strain, deflection, or buckling. It is not
implied that stress, strain, deflection, and buckling are independent,
but only convenient parameters on which to focus attention. For a par-
ticular product, certain performance limits are not considered because
others will always occur first. The following is a list of performance
limits that are often considered in design and could be thought of as pos-
sible responses to soil pressure:

Wall crushing (stress)
Wall buckling
Reversal of curvature (deflection)