Page 102 - Buried Pipe Design

78 Chapter Three

Rigid Pipe Analysis
Three-edge bearing strength
Rigid nonpressure pipes are of four general types and are covered by
four ASTM specifications. Asbestos-cement pipe is governed by ASTM
C 428. Vitrified clay pipe is specified in two strength classes by ASTM
C 700. Nonreinforced concrete pipe is covered by ASTM C 14. Reinforced
concrete nonpressure pipe is specified by its so-called D load strength
as given in ASTM C 76. The D load is the three-edge bearing strength
divided by diameter. Designs of the various types of rigid pipe for non-
pressure applications to resist external loads require knowledge of
available pipe strengths as well as the construction or installation
conditions to be encountered.
Rigid pipe is tested for strength in the laboratory by the three-edge
bearing test (see Fig. 3.4 for diagram of test). Methods for testing are
described in detail in the respective ASTM specifications for the spe-
cific pipe product. The three-edge bearing strength is the load per
length (usually pounds per foot) required to cause crushing or critical
cracking of the pipe test specimen. This strength is the load at failure
in a testing machine. It is not necessarily the load that will cause failure
in the soil.

W

D

R C R

Figure 3.4 Three-edge bearing (wood block)
schematic.