Page 65 - Buried Pipe Design

External Loads 41

Restraint Wedges
Load Wedge Load Wedge

H
Top

Shoulders

Unit Weight
of Soil

Figure 2.21 Flexible ring in the process of collapse under minimum dead load soil cover
showing the load wedges advancing against the ring, and lighter restraint wedges being
lifted.

Dead load. Cohesionless soil cover is minimum if the pipe is unable to
resist the variation in soil pressure. This concept is shown in Fig. 2.21,
where top pressure is H but shoulder pressure is greater than H. If
the pipe cannot resist the difference in pressures, shoulder wedges
slide in against the pipe, deforming the ring which lifts the top wedges.
Collapse of the pipe is catastrophic. If the pipe is rigid (brittle), col-
lapse is fragmentation. If the pipe is flexible, equations of equilibrium
of soil wedges provide values of minimum soil cover. For typical gran-
ular backfill, based on analysis confirmed by tests, minimum cover is
about H D/10. An often specified minimum allowable is H D/6, but
this applies to a perfectly flexible ring. In fact, pipes have ring stiffness
and so provide resistance to dead load collapse.

Pyramid/cone soil stress. The Boussinesq and Newmark procedures
for calculating live load pressure on a buried pipe are based on
the assumption that soil is elastic. The assumption does not apply to
minimum-cover analysis. Pipe damage due to surface loads on less-than-
minimum cover occurs after a truncated soil pyramid or cone is
punched through. Figure 2.22 shows a truncated pyramid and cone. If
the loaded surface area is circular, a truncated cone is punched
through. If the loaded surface area is a rectangle, a truncated pyra-
mid is punched through. Pyramids are imperfect because sharp cor-
ners do not form. Nevertheless, using a conservative pyramid slope ,
the analysis is applicable. The tire print of dual wheels is nearly rec-
tangular.