Page 237 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.39

Tie lines may consist of three rods, cables,
or chains, hooked around the end pipes. Turn-
buckles or load binders are used to tighten them.
They may be internal or external, as shown in
Fig. 5.28.
The inside ties will reduce culvert capacity
slightly, and may cause jamming of debris and
complete stopping. However, they are accessible
for inspection and tightening. Outside instal-
lations are difficult to service.
A loose cable is sometimes left inside a
small-diameter culvert for use if it becomes
plugged with silt. Pulling the cable back and
forth will make a small hole that can be enlarged
FIGURE 5.28 Cable ties on concrete pipe.
by forcing water through.

OTHER FORMS OF CONSTRUCTION

Wood Culverts. Culverts may be constructed of wood when they are for temporary use, or when
time or expense prohibits obtaining more permanent materials.
Construction may be to almost any desired strength. Life expectancy will vary with the type
and size of wood, preservative applied, and moisture conditions. In general, the parts that are per-
manently wet will have a much longer life than those that are exposed to air.
Several designs are shown in Fig. 5.29.

Casual Placement. There are many situations in which it may be unnecessary or impossible to
place culverts with the care required for permanent installations. These would include light-traffic
driveways and farm lanes; pioneer or access roads to be used for only a short period; and urgent
construction in which it is necessary to get traffic through without delay, even at the cost of pos-
sible repair or reconstruction later.
Good standards should be approached as closely as possible, however.
If heavy traffic will ride directly on the pipe, or very closely over it, a strong construction
should be used. If silting and trash are not a problem, several small pipes will be better than one
large pipe, as they are more resistant to concentrated loads and they can be provided with an ade-
quate depth of cover more readily.
If the foundation is unstable so that a part of the culvert will sink, oversize pipe may be used
to provide adequate capacity after settlement and silting. If silting can be prevented, a badly sag-
ging pipe may act as an inverted siphon.
The pipe should be long enough not to require large headwalls, unless they can be easily built
with big stones or logs available on the site. Wingwalls, where required, can be made of rocks, of
saplings hammered in as piling, or of brush mats.
On wet bottoms, pea gravel or crushed stone should be used under the haunches. Ordinary
earth can be used as soon as the gravel has been built up above water level, but it will not con-
solidate under water.

BACKFILL

Proper placing and compacting of backfill affects both the strength of the pipe and the load it has
to carry.