Page 380 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

8.42 THE WORK

A compacted fill will not absorb rainwater readily, so that the fill should remain hard enough
to work even after heavy rains. Whether the surface will become greasy depends on the clay con-
tent and the possible presence of a layer of dry uncompacted dust before the rain.

Rollers. Rollers may be smooth steel wheel, tamping sheeps-foot, or grid, or rubber-tire. Some
have rubber on two drive wheels, and a steel roll, rough or smooth. Rollers may depend entirely
on weight and shape for effectiveness, or may have vibrators also. See Chap. 19.
The smooth steel-wheel models are just known as rollers. They are usually self-powered, and
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may have either two or three rolls. Weights range from 1 ⁄ 2 to over 20 tons. They are primarily finish-
ing machines, used more often on surfaces than on subgrades. Tops of thick layers may be better
compacted than bottoms.
These machines have little traction, particularly with tandem construction, and are not suited
to rough ground. Grid or segmented drive rolls overcome this difficulty.
Towed tamping or sheepsfoot rollers are steel drums 4 or 5 feet long and 40 to 60 inches in
diameter, fitted with projecting lugs (feet or legs) 7 or more inches long. There may be three lugs
to every 2 square feet of drum surface.
Feet may have expanded soles (which may kick up soft dirt), or taper from a wide base to a flat
end. Either way, they penetrate soft fill until weight is carried on the sole, where compaction
begins. As the ground is compressed on successive passes, the feet do not sink as far and start to
“walk out” of the ground.
Drums may be filled with sand or water. Sole pressures range from 250 to 750 pounds per
square inch. The drums are mounted in box frames fitted with drawbars. Up to three may be
mounted side by side, and two pairs may follow each other. Working speed is 2 to 3 miles per
hour. Power requirements are high, particularly in soft fill.
These units are being replaced by self-powered units with one or two tamping drums, and a
pair of rubber-tire drive wheels. This is in line with a general move away from towed equipment.
Pneumatic tired rollers are ballast boxes supported by wheels with smooth-tread tires. The wheels
may roll straight, vibrate, or move up and down or wobble as they revolve. They compact by a
combination of weight and kneading action of the soft tire walls. Weights vary up to more than
80 tons. They can compact single fill layers as deep as 24 inches.
Fill Bases. It is desirable that a fill be firmly bonded to the surface on which it rests to prevent
formation of saturated zones, water channels, and possible sliding downslopes. This is usually
accomplished by removing vegetation and topsoil, and plowing ridges across any slopes.
Methods of removing humus and other muds from the location to be filled, and of stabilizing
such muds when removal is impractical, are discussed in Chap. 3.
When the area to be filled is wet, rough, or otherwise impassable to machinery, the first layer
is built by dump trucks and dozers to a height at least sufficient to carry the hauling units over the
soft spots or obstacles.
If the surface is uneven but passable, low spots may be built up first with compacted layers, or
high spots removed, before the main fill is placed.
Rock Separation. Handling and compaction of fill material are rendered difficult by the pres-
ence of loose stone. Rocks of even small sizes interfere with grading. If their diameter is greater
than that of the fill layer, they will project from the top. If two or more rocks are in contact, they
are liable to prevent even distribution and compaction of fill under their adjoining edges.
For this reason, the size and number of rocks present in thin or layered fills are often limited.
This may be done by using selected borrow, or by putting bouldery material through a grizzly.
The arrangement shown in Fig. 8.31 represents a minimum of equipment for screening. A
truck on a high level dumps on a sloping grizzly, dirt falls between the bars into a truck parked
below, and boulders roll to the side.
Oversize material may be allowed to roll directly into trucks, be loaded from beside the grizzly,
dozed away from it to a stockpile, or, if the grizzly can be located on the edge of an abandoned
pit, allowed to accumulate.