Page 381 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.43

FIGURE 8.31 Screening oversized rock from fill.

One or two workers are needed to free oversize stones stuck between the bars and to coordi-
nate the trucks. If the stones are a substantial part of the bulk of the soil, smaller trucks may be
used under the grizzly than on top of it.
If sticking of stones or sliding off of chunks of earth is much of a problem, a vibrating grizzly,
or a standard grizzly with a vibrator bolted to it, may be desirable. The flat slope illustrated is suit-
able only for loose soil and large openings.

Rock Fill. Various results are obtained from all-rock fills. If the largest pieces are smaller than
the depth of the fill, and sizes are mixed, including a good proportion of fines, a solid fill with a
good surface may be obtained by pushing piles off an edge with a bulldozer. Large pieces tend to
move ahead and over the bank, while smaller ones drift under the blade to form a topping.
If there are not enough small pieces to provide a working surface, finer material should be
brought to fill surface holes and even off the top.
Rocks too large to fit in the fill can be rolled ahead of it until a hole is found or is made to bury
them, or they are reduced by splitting or blasting.
Rock fills are generally almost incompressible, exceptions being when rock is soft or fissured, and
very heavy weights are used. However, they are apt to be subject to only minor and local settlement,
where fines are shaken or washed into spaces between rocks below them.
Rock is desirable fill material for the bottom layer in crossing water or mud, as it is not softened
by contact with water and spreads surface loads over large areas of the base. In such locations it
may settle due to displacement or compression of the ground under it.
The volume of fill is greater than the unbroken rock in the bank. The difference will vary with
the quality of rock, type of fragmentation, and amount and kind of compaction. And 50 percent is
a rule-of-thumb average that can be used except where there are indications to the contrary.
If the rock must be used in the fill, it is best placed at the bottom. Unfortunately, rock is ordi-
narily the last material to be taken from a cut as soil is stripped prior to blasting.

SCRAPER FILLS

The standard method of building a fill with scrapers is to start with an area sufficiently leveled to
allow the scrapers to travel on it, and to build it up in thin layers, starting at the outside edges or
at low spots.
Spreading depth may vary from 2 inches to the maximum lift of the bowl—8 inches to 1 foot.
Thin layers favor compaction (see Fig. 8.32), particularly if the scrapers are depended on for the
rolling and facilitate smooth building up of the grade. Chunky, sticky, or rocky fill will not spread
thinly, or can be made to do so only by very slow travel during the dump.