Page 240 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

5.42 THE WORK

Tamping. Fill must be tamped under the pipe haunches. It should be free of lumps, stones, and
trash, and should contain enough moisture to pack, but not enough to make it rubbery. It is placed
with a hand shovel in thin layers.
The tamper should have a narrow edge to enable it to get well under the pipe and, if the trench
is narrow, may require a curved handle.
Filling and tamping are done evenly on both sides of the trench, to avoid shifting the pipe to
the side. It may be necessary to wedge it in place temporarily with rocks or other blocking. Such
material may be left and buried if the pipe is rigid, but removed if it is flexible.
Tamping blows should not be so vigorous as to wedge the pipe out of position.
When sufficient fill has been placed that its surface is out from under the pipe, mechanical
tampers can be used. Fill should be compacted to the full width of the trench, or, if the pipe is on
the surface, for one pipe diameter on each side. Layers should be 4 to 6 inches deep.
When the pipe is nearly or wholly covered, layers of 6 to 12 inches may be placed, and tamping
continued, until the trench is filled to grade. A compacting bucket, like the one manufactured by
Felco Industries Ltd., mounted on a backhoe, can be useful.
Side Fill. When an embankment is to be built up on each side of the pipe and above it, much of
the compaction can be done by machinery first moving parallel with the pipe, then across it.
A roller working parallel must be kept far enough away so as not to exert a horizontal thrust
that will move the pipe. Fill and compaction should be kept even on both sides. Since there is
often only one roller available, and it may not be able to get across, the other side may be com-
pressed with a truck or by tamping.
Soil between the rolled strips and the pipe, and between the rolled strips and over the pipe,
must be thoroughly tamped.
Successive layers can be rolled closer to the pipe centerline. It is good practice to postpone
crossing it until the fill is as deep as the outside diameter of the pipe, to avoid pushing it out of line.
Material is pushed to the pipe by a dozer or grader.

Loosened Backfill. A compacted embankment may be built up one diameter above the pipe,
then ditched over it. This trench is filled in loosely, and layers of compacted fill are built to the
top of the embankment in the regular manner.
The soft fill over the pipe causes the load to be transferred to the solid sidewalls.
If any trench containing pipe is backfilled by a dozer, care should be taken not to drop rocks
on the pipe.

FORDS AND DIPS

Farm or pioneer roads sometimes cross a shallow stream on its bottom so that vehicles must drive
in the water. This type of crossing is called a ford. It may be satisfactory for light or occasional
traffic, but it is subject to interruption by high water and ice, and may develop bad bottom condi-
tions that it would be difficult to remedy.
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Crusher rock, in mixed sizes from ⁄ 2 to 2 ⁄ 2 inches, makes a good patching and paving material.
If bank gravel is used, thorough raking will allow the water to take away excessive fines.
In arid regions, many watercourses are dry most of the time, but will occasionally carry such
large volumes of water that adequate bridges would be very expensive. In such cases the road may
run across the channel at its natural grade, with no provision being made for passing water under
it. The section of road in the channel is usually heavily built, with reinforced-concrete slabs up to
2 feet in thickness sometimes being used. This slab should be sloped on its upstream side, and may
have a cutoff or curtain wall extending below its main mass.
Occasionally a culvert may be placed under or beside the dip, to pass small water flows, or a
culvert structure may include a spillway for floodwater.
Second-class roads may cross such a streambed on graded local material that must usually be
worked over after each flow of water. Roads may also run considerable distances in streambeds,
as this may be the only route that is practicable without heavy blasting and grading.