exposed to the atmosphere, it undergoes a physical andchemical process called WEATHERING, which, over asufficient length of time, disintegrates and decomposesthe rock into a loose, incoherent mixture of gravel, sand,and finer material.Soil QualityThe intended use of the soil is the determining factorin the quality required. In general, soil used for fills andsubgrades do not have to meet the same specificationsas those used for compacted rock surfaces, base courses,or pavements.Seven properties of rock are used to help select rockand aggregates for construction. Briefly, these rockproperties are as follows: toughness, hardness,durability, chemical stability, crushed shape, surfacecharacter, and density. Toughness, hardness, anddurability are commonly checked in the field with asimple field test.Hardness is the resistance of a rock to scratching orabrasion. This property is important in determining thesuitability of aggregate for construction. Hardness canbe measured using the Mob’s scale of hardness (table5-2). The harder the material, the higher its number onthe Moh’s scale. Any material will scratch another ofTable 5-2.-MOH’S Scale of HardnessMineralHardnessDiamond . . . . . . . . . . . . . . . . 10Corundum . . . . . . . . . . . . . . . 9Topaz or beryl . . . . . . . . . . . . . 8Quartz . . . . . . . . . . . . . . . . . . . . . 7Feldspar . . . . . . . . . . . . . . . . . 6Apatite . . . . . . . . . . . . . . . . . . 5Fluorite . . . . . . . . . . . . . . . . . 4Calcite . . . . . . . . . . . . . . . . . . 3Gypsum . . . . . . . . . . . . . . . . . . 2Talc . . . . . . . . . . . . . . . . . . . . 1ExpedientsPorcelain . . . . . . . . . . . . . . . . 7.0Steel file . . . . . . . . . . . . . . . . . . 6.5Windowglass . . . . . . . . . . . . . . 5.5Knife blade . . . . . . . . . . . . . . 5.0Copper coin . . . . . . . . . . . . . . . . 3.0Fingernail . . . . . . . . . . . . . . . . . 2.0equal or lesser hardness. In the field, hardness may bemeasured using the common expedients shown in table5-2; for example, when you are able to scratch a rockwith a knife blade, the rock has a hardness of 5.0 or less.A rock which can be scratched by a copper coin has ahardness of 3.0 or less.Aggregates for general construction should have ahardness of 5 to 7 and should be difficult or impossibleto scratch with a knife. Material with a hardness greaterthan 7 should be avoided since they cause excessivewear to crushers, screens, and drilling equipment.Material with a hardness of less than 5 may be used ifother sources of aggregate prove uneconomical.The requirements as to toughness, durability,crushed shape, and other properties vary according tothe type of construction. Chemical stability has specificimportance when considering aggregates for concrete.Several rock types contain impure forms of silica thatreacts with alkalies in cement. This reaction forms a gelthat absorbs water and expands to crack or disintegratehardened concrete. These reactive materials may beincluded in some gravel deposits as pebbles or ascoatings on gravel. Potential alkali-aggregate reactionsmay be anticipated in the field by identifying the rockand comparing it to known reactive types or byinvestigating structures in which the aggregate has beenused. Generally, light-colored or glassy volcanic rocks,chert, flints, and clayey rocks should be consideredreactive unless proven otherwise.An additional property of rock is gradation (fig.5-1). This property is also important for evaluating rockas possible construction material. Gradation is thedistribution and range of particle sizes that are presentin, or can be obtained from, a deposit. The gradation ofpit materials can be readily determined from a simpletest. Quarry materials may be more difficult to evaluate.Figure 5-1.-Types of soil gradation.5-3
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