exposed to the atmosphere, it undergoes a physical and
chemical process called WEATHERING, which, over a
sufficient length of time, disintegrates and decomposes
the rock into a loose, incoherent mixture of gravel, sand,
and finer material.
Soil Quality
The intended use of the soil is the determining factor
in the quality required. In general, soil used for fills and
subgrades do not have to meet the same specifications
as those used for compacted rock surfaces, base courses,
or pavements.
Seven properties of rock are used to help select rock
and aggregates for construction. Briefly, these rock
properties are as follows: toughness, hardness,
durability, chemical stability, crushed shape, surface
character, and density. Toughness, hardness, and
durability are commonly checked in the field with a
simple field test.
Hardness is the resistance of a rock to scratching or
abrasion. This property is important in determining the
suitability of aggregate for construction. Hardness can
be measured using the Mob’s scale of hardness (table
5-2). The harder the material, the higher its number on
the Moh’s scale. Any material will scratch another of
Table 5-2.-MOH’S Scale of Hardness
Mineral
Hardness
Diamond . . . . . . . . . . . . . . . . 10
Corundum . . . . . . . . . . . . . . . 9
Topaz or beryl . . . . . . . . . . . . . 8
Quartz . . . . . . . . . . . . . . . . . . . . . 7
Feldspar . . . . . . . . . . . . . . . . . 6
Apatite . . . . . . . . . . . . . . . . . . 5
Fluorite . . . . . . . . . . . . . . . . . 4
Calcite . . . . . . . . . . . . . . . . . . 3
Gypsum . . . . . . . . . . . . . . . . . . 2
Talc . . . . . . . . . . . . . . . . . . . . 1
Expedients
Porcelain . . . . . . . . . . . . . . . . 7.0
Steel file . . . . . . . . . . . . . . . . . . 6.5
Windowglass . . . . . . . . . . . . . . 5.5
Knife blade . . . . . . . . . . . . . . 5.0
Copper coin . . . . . . . . . . . . . . . . 3.0
Fingernail . . . . . . . . . . . . . . . . . 2.0
equal or lesser hardness. In the field, hardness may be
measured using the common expedients shown in table
5-2; for example, when you are able to scratch a rock
with a knife blade, the rock has a hardness of 5.0 or less.
A rock which can be scratched by a copper coin has a
hardness of 3.0 or less.
Aggregates for general construction should have a
hardness of 5 to 7 and should be difficult or impossible
to scratch with a knife. Material with a hardness greater
than 7 should be avoided since they cause excessive
wear to crushers, screens, and drilling equipment.
Material with a hardness of less than 5 may be used if
other sources of aggregate prove uneconomical.
The requirements as to toughness, durability,
crushed shape, and other properties vary according to
the type of construction. Chemical stability has specific
importance when considering aggregates for concrete.
Several rock types contain impure forms of silica that
reacts with alkalies in cement. This reaction forms a gel
that absorbs water and expands to crack or disintegrate
hardened concrete. These reactive materials may be
included in some gravel deposits as pebbles or as
coatings on gravel. Potential alkali-aggregate reactions
may be anticipated in the field by identifying the rock
and comparing it to known reactive types or by
investigating structures in which the aggregate has been
used. Generally, light-colored or glassy volcanic rocks,
chert, flints, and clayey rocks should be considered
reactive unless proven otherwise.
An additional property of rock is gradation (fig.
5-1). This property is also important for evaluating rock
as possible construction material. Gradation is the
distribution and range of particle sizes that are present
in, or can be obtained from, a deposit. The gradation of
pit materials can be readily determined from a simple
test. Quarry materials may be more difficult to evaluate.
Figure 5-1.-Types of soil gradation.
5-3
