Page 238 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

5.40 THE WORK

FIGURE 5.29 Wood culverts. (Courtesy of U.S. Army Engineers.)

Load Distribution. If a round pipe lies on a hard, flat surface and is subjected to load, the entire
pressure falls on the line of bottom contact. If the surface is curved, the area of contact is greatly
increased and the load per square inch reduced correspondingly. As the haunches curve upward,
the amount of vertical support to each square inch of surface decreases until it is zero at the widest
point.
Corrugated pipe is flexible and requires horizontal support as well. A normal load tends to flatten
the top and spread the sides. If the sides are held in firmly, the arch form of the load-carrying section
is retained and strength kept at a maximum,
No part of the foundation or backfill touching a flexible pipe should be rigid, as the whole pipe
should be able to change cross-section shape as it deflects under load, and any rigid support will
cause excessive strains, particularly at the edges of the contact.
Rigid pipe receives only nominal support from the side fill.
Surface loads on soil masses are ordinarily distributed over increasing areas and reducing pressures
on lower levels, as in Fig. 5.30(A). If there is a difference in bearing power of the soils within the
affected cone, the more rigid soil may carry most or all of the load.