Page 49 - Buried Pipe Design

External Loads 27

Example Problem 2.3 For Example Problem 2.1, what would be the load if
the pipe and side soil had approximately the same stiffness?

42
18
W c C d B c B d 1.6 (120) 1008 lb/ft (2.10)
12
12
Prism load. Again, Eq. (2.4) represents a maximum-type loading con-
dition, and Eq. (2.10) represents a minimum. For a flexible pipe, the
maximum load is always much too large since this is the load acting
on a rigid pipe. The minimum is just that, a minimum. The actual load
will lie somewhere between these limits.
A more realistic design load for a flexible pipe would be the prism
load, which is the weight of a vertical prism of soil over the pipe. Also,
a true trench condition may or may not result in significant load reduc-
tions on the flexible conduit since a reduction depends upon the direc-
tion of the frictional forces in the soil. Research data indicate that the
effective load on a flexible conduit lies somewhere between the mini-
mum predicted by Marston and the prism load. On a long-term basis,
the load may approach the prism load. Thus, if one desires to calculate
the effective load on a flexible conduit, the prism load is suggested as a
basis for design. The prism or embankment load is given by the follow-
ing equation (see Fig. 2.13):

P H (2.11)

where P pressure due to weight of soil at depth H
unit weight of soil
H depth at which soil pressure is required
Example Problem 2.4 Assume an 8-in-OD flexible pipe is to be installed in a
24-in-wide trench with 10 ft of clay soil cover. The unit weight of the soil is
120 lb/ft . What is the load on the pipe?
3
For the Marston load, use Eq. (2.10) for minimum W:

W d C d B c B d
where C d 2.8, from Fig. 2.3
120 lb/ft 3
B d trench width 2 ft
2
B c OD 8 in ft
3
2
Marston load W d (2.8) (120) (2) ( ) 448 lb/ft
3
For the prism load, use Eq. (2.11):

P H 120 (10) 1200 lb/ft 2