Page 103 - Buried Pipe Design

Design of Gravity Flow Pipes 79

Bedding factors and classifications
In Chap. 2 we learned that for rigid pipe, the soil load can be calculated
2
by Marston’s equation W d C d B d . Experience has shown that the
Marston load, to cause failure, is usually greater than the three-edge
bearing strength (Fig. 3.4) and depends on how the pipe was bedded.
The Marston load that causes failure is called the field strength. The
ratio of field strength to three-edge bearing strength is termed the bed-
ding factor since it is dependent upon how the pipe was bedded
(installed). The term bedding factor as used by Marston is sometimes
called the load factor. The two terms refer to the same parameter and
may be used interchangeably.

field strength
Bedding factor (3.1)
three-edge bearing strength
Major pipe manufacturing associations recommend bedding factors
that correspond to those listed in the Water Pollution Control Federation
Manual of Practice, No. FD-5, Gravity Sanitary Sewer Design and
Construction.
These bedding types (classes) are shown in Fig. 3.5, and correspond-
ing bedding factors (load factors) are given in Table 3.2.

Installation design
Equation (3.1) may be solved for the three-edge bearing strength as
follows:

field strength
Three-edge bearing strength (3.2)
bedding factor

The field strength is the Marston load that will cause failure in the
field. Most designers and specifications require a factor of safety in the
design. Thus the required strength is as follows:

Required three-edge bearing strength
design load factor of safety
(3.3)
bedding factor

A design procedure to select the appropriate pipe classification or
strength is outlined as follows:

1. Determine the earth load.
2. Determine the live load.