Page 304 - Buried Pipe Design

Rigid Pipe Products 275

assurance. A distinction should be made between the ultimate
strength design limit to be used in developing the basic pipe design
and the requirements for W 3-edge testing of pipe for quality assurance.
The pipe reinforcement designs given in ASTM C 76 are based on
strength factors of 1.5 to 1.25 over the required service load (W D load ) 0.01 ,
but this standard does not require that quality control W 3-edge tests be
taken to ultimate. The same applies to ASTM C 506 and C 507.
The required W 3-edge strengths in these indirect designs are obtained
from the loads and bedding factors calculated using the Marston-
Spangler soil-structure interaction analyses for earth loads. In sum-
mary, under the indirect design procedure, the required pipe will have
a W 3-edge service and strength as determined from actual three-edge
bearing tests, from empirical evaluations of former tests (as given in
ASTM standards), or from design procedures derived from reinforced
concrete theory and evaluations of appropriate tests.

The Direct Method
The American Concrete Pipe Association (ACPA) recommends a design
practice for pipe-soil installations based on a direct design of the pipe
for its installed conditions. New standardized installation types are
given that differ significantly from those originally developed by
Marston and Spangler. The four new standard installations and a
direct design procedure are found in a 1993 American Society of Civil
Engineers standard entitled ASCE Standard Practice for Direct
Design of Buried Concrete Pipe in Standard Installations (SIDD).
The four standard installation types are as follows:
Type 1 requires select granular soils in bottom haunch and outside
bedding zones with high levels of compaction.
Type 2 permits coarse or fine granular soils with some silts, includ-
ing some native soils in the haunch and outside bedding zones.
Compaction requirements remain high for native soils and are
reduced for select granular soils.
Type 3 permits coarse or fine granular soils with some silts or silty
clay in haunch zones. Compaction requirements vary with soil type
and are reduced for select granular soils to levels where testing is
optional. Compaction requirements are high for nonplastic soils
with clay particles.
Type 4 has no requirements for embedment soils in haunch and bed-
ding zones, unless clays are used in the haunch zone. Silty clay
requires limited compaction with testing being optional. Plastic
clays are not recommended.