Page 351 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.13

In general, most oversize stone can be eliminated during the spreading and grading processes
at less expense than pit screening, except in patching work.

Crusher Gravel. Bank gravel which is short of pebbles and long on stones may be run through
a crusher to reduce the oversize to pebbles. The result may be superior to run of bank of similar
size distribution because of the angular shape of the crushed pieces.
Blasted rock which is run through a crusher, without separation of the product, will often pro-
duce a material similar in size, distribution, and performance to the best of bank gravels.
Crusher gravel is usually more expensive than run of bank because of the extra processing.

Similar Materials. Any hard material which is broken into particles of the gravel size range may
be used in its place. The breakage may be from blasting, rooting or digging, burning, or the effect
of heat and cold. Such materials include shale, soft limestone, fine blasted rock of any kind, scoria,
red dog, slag, disintegrated granite, cinders, and shell.
Exposures of shale rock are frequently soft enough to be dug by a small shovel without blasting.
The broken shale has the appearance of excellent road material, but breaks down readily into mud.
Some very expensive road failures have been caused by allowing traffic to use a shale subgrade,
then putting a concrete pavement over it after its usefulness had been destroyed.
Soft limestone is the “coral” of the island military bases. It is often dug from the solid by loosen-
ing with heavy rippers, or hydraulic dozer blades fitted with teeth. It is easy to drill, but blasting
may require as heavy loading as hard rock.
Such limestone is used as it comes from the pit. It should be rolled promptly after placing, as rain
can make a soggy mess of it when loose. After compaction, wetting sets it into a hard surface that
requires less maintenance than gravel.
Rock from tunnels (muck) is well suited for road fills, as the tight, heavily loaded shots cause
fine fragmentation.
Scoria comes from clay beds that have been cooked by the underground burning of adjacent
coal seams. It resembles broken brick. Red dog is a similar material that is produced by the burn-
ing of piles of waste bituminous coal with a high clay content. Both of these substances may break
down into mud under traffic unless protected by some other surfacing.
Disintegrated granite is the standard low-cost road material in many parts of the southwest. It
is a rough, coarse sand with excellent compaction and drainage characteristics.
Slag is a by-product of hot refining of metals, which may be poured molten onto dumps where
it hardens into rock, or may be cooled and broken up by a water spray.
Cinders are of two kinds—refuse from steam power plants burning lump coal, and aerated rock
blown from volcanoes.
Those from power plants are light, easily worked, and free-draining. However, they pound into
mud quickly under traffic, and are useful for light-duty footings and emergency surfaces only.
They are becoming rare because of power plants changing over to powdered coal and other fuels.
The use of fly ash mixed with lime makes an economical slurry to stabilize a wet base. The fly ash
is a by-product of coal-fired power plants. The slurry might have a mix of 4 parts of fly ash with 1
part of lime. It is injected into a depth of up to 4 feet of base. The fly ash drives out the moisture by
filling the voids in the subgrade, and the lime locks the fine-grained soils into a firm, stabilized mass.
Weight of volcanic cinders and ash ranges from 60 to 120 pounds per cubic foot, compacted.
The difference is chiefly in entrained air, the lighter qualities being so full of bubbles that they
have little strength.
Most volcanic cinders from 90 pounds up make good road material, but care may be needed in
selecting them in the pit. They are too resilient for use under rigid pavements, but have good frost
resistance because of their air content.
Shells are dredged in enormous quantities from bars along the Gulf of Mexico, and serve locally
for the principal low-cost road base and surface.

Preparing Subgrade. The subgrade should be finished as accurately as possible. Ridges or
hummocks of subsoil which extend up into the gravel weaken it. If the subgrade is clay or silt, it