Page 231 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.33

FIGURE 5.22 Metal headwall for pipe. (Courtesy of Contech Construction Products.)

Metal pipe is very resistant to such disruptive forces, and can be further strengthened by special
joint fastenings. Short-section rigid pipe may be braced at each end by a heavy headwall, founded
on underlying stable material, or may be cabled together.

Depth. Fills over a culvert, to a depth of 4 to 7 feet, protect it by spreading out the weight of vehicles
on the surface. Deeper fills have diminishing protective effect and impose the load of their own weight,
which at great depths may be sufficient to crush the pipe.
Figures 5.24 and 5.25 contain tables showing the approximate required strength for highway
culvert or sewer pipe under various depths of fill. It should be noted that the required strength listed
must be multiplied by the diameter of the pipe in feet. This is because the increase in crushing
strength with increased size of pipe only partly compensates for its greater surface area, against
which pressure is exerted. In short, although large pipe is stronger in itself than small pipe, it is
weaker in regard to burial loads.
The table also indicates the importance of pipe bedding in determining strength. Corrugated
pipe requires good compaction of fill at the sides, and proper bedding.
It is questionable whether the formed bed shown for the ordinary and first-class installations
is often perfect enough to give the calculated support. However, careful tamping of the fill under
the curve of the pipe will give similar results.

Restricted Height. If there is not sufficient space between the channel bed and the embankment
surface to install a round pipe of adequate size, two or more smaller round pipes, or low-clearance
pipe with a flatter cross section or a pipe arch may be used.
Poured-concrete structures may use a flat rectangle or two or more openings separated by sup-
porting walls.