Page 51 - Buried Pipe Design

External Loads 29

Solve for

H
C d
B d

Thus the prism load is a special case of the Marston-Spangler trench
load. In Fig. 2.2, C d H/B d is plotted as a straight 45° line. One of the
advantages of the prism load is that it is independent of trench width.

Embankment condition. The load on a flexible pipe in an embankment
may be calculated by the Marston-Spangler theory via Eq. (2.5).

2
W c C c B c
This equation does not include a trench width term since a trench is not
involved. Again it is interesting to set this load equal to the prism load.

Prism load B c PB c HB c
2
Marston embankment load W c C c B c
Equating the two loads,

2
HB c C c B c
or

H
C c
B c
and C c can be determined from Fig. 2.5. The above equation plots as a
straight 45° line on Fig. 2.5. This is the line shown for r sd p 0. Thus
for an embankment, the prism load is the same as the Marston load
for r sd p 0.

Tunnel loadings. There are few documented data dealing with loads on
flexible pipes placed in unsupported tunnels. However, since a flexible
pipe develops a large percentage of its load-carrying capacity from pas-
sive side support, this support must be provided, or the pipe will tend
to deflect until the sides of the pipe are being supported by the sides of
the tunnel.
When a flexible pipe is jacked into undisturbed soil, the load may be
calculated by either the prism load, Eq. (2.11), or Eq. (2.9).

B t B c