Page 519 - Moving the Earth_ The Workbook of Excavation

PIT OPERATION

PIT OPERATION 10.25

In any calculation about humus it should be remembered that 1 yard in the wet deposit or pile
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means ⁄ 2 yard or less in the cured pile.

GRAVEL, SAND, AND CLAY

Bank Gravel. Bank gravel is a useful and highly varied material. It consists chiefly of sand, pebbles,
and cobbles, but may also contain clay, silt, and boulders, mixed in or in accompanying layers or
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pockets. The gravel proper is the pebbles and cobbles in sizes from ⁄ 4 to 2 inches.
The specifications which gravel must meet to do certain jobs, and the proportions found in
deposits, vary widely.
Bank gravels consist mainly of deposits laid down by fast-running streams, often of glacial origin,
but they are also formed by waves on the seashore. The quality depends on the original stone, the
proportion of sizes, and the angularity of the particles. Wave-formed gravels are predominantly
rounded, glacial ones subangular, and product of other streams variable.
Talus gravels, formed at the foot of cliffs by falling and sliding, may be coarse and angular,
but are often weak stone.
If gravels are not sufficiently angular for their job, and contain oversize stones, they may be
run through a crusher which will produce angular fragments.
Fines in bank gravel act as a cement or binder, holding it together when dry. Gravel without
binder becomes too loose for road use in hot, dry weather.
Fines in excess of 8 or 10 percent may cause a gravel to become sloppy after repeated freezing
and thawing when wet. Fines over 15 percent may cause it to soften under prolonged soaking.
Softening is made more likely by a high proportion of fine sand in the mixture, and less likely if
thorough compaction precedes the freezing or soaking.
Any gravel will become sloppy if soaked when freshly dug, but if of good quality, should drain
and firm quite quickly.
Gravels derived from continental glaciers are largely of hard rock. River and mountain glacier
gravels are derived from upstream formations, and occasionally include too much shale or other
soft rock for some purposes.
There are a number of tests for gravel, for field and laboratory use. A sample, with stones over
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⁄ 4 inch removed, can be shaken up with water in a glass jar, then allowed to stand. The pebbles will
form a layer in the bottom, with coarse and then fine sand on top. Silt and clay will settle out more
slowly, and may take an additional day to compact. The relative amounts of the different-size par-
ticles can then be determined by inspection.
In the laboratory, gravel is dried, weighed, and put through a vibrating screen with many dif-
ferent meshes. The particles caught on each tray are weighed. Any lumps have to be broken up.
This operation gives a classification of the specimen for size gradation.
Gravel can be tested for abrasion resistance by rolling in a cylinder with steel balls or other
hard weights. Resistance to breaking up by freezing can be tested with cold, or with chemicals
which duplicate its effect.
Clean bank gravel of the proper sand-gravel proportions is frequently mixed directly with
cement for concrete.

Sand. Most bank gravel deposits are more than half sand. In addition, sand deposits occur in
many areas where no gravel is found.
Ocean beaches are typically sand, and river deposits usually contain high proportions of it. If
the river flows slowly, the sand may be mixed with silt and clay, which usually must be separated
before use.
Most sand is largely particles of silicon dioxide, best known in the form of quartz. It is very hard
and withstands the abrasion of water working, which reduces other minerals occurring with it to
fines. Calcium carbonate, mica, feldspar, gypsum, and many other minerals may also occur as sand.
Many sandbanks are clean enough for use without processing, but in most cases it is safer to
screen and wash before using in concrete.