Bedrock

Bedrock is divided by geologists into three large groups, namely (1) igneous, (2) meta — morphic, and (3) sedimentary. Igneous rocks are those that have resulted from the cooling and crystallization of molten masses of mineral matter and gases either at or below the earth’s surface. Sedimentary rocks consist of the transported and subse­quently indurated products of weathering of previously existing rock types, while metamorphic rocks are frequently defined as those having characteristic textures and mineral compositions that have resulted from high temperatures and pressures and/or hot mineralizing solutions acting on a parent rock. Figures 8.16, 8.17, and 8.18 indi­cate easily recognizable descriptions for field classification of igneous, metamorphic, and sedimentary rock, respectively.

8.3.2 Soils Laboratory Tests

Grain size, shape, and gradation are generally established by sieve analysis. For the finer clays, a hydrometer analysis is necessary. Figure 8.19 depicts a classification of sediment based on grain size.

Atterberg limit tests are performed on fine-grained soils and represent the amount of water present in the voids. The liquid limit (LL), plastic limit (PL), and plasticity index (PI) constitute the Atterberg limits.

FIELD CLASSIFICATION OF IGNEOUS ROCKS
ROCK TEXTURE	ROCK COLOR AND ESSENTIAL MINERALS
Light gray, white, or pink contains orthoclase and quartz	Dark gray or black contains plagioclase, hornblende, and/or biotite	Dark gray or black contains plagioclase and pyroxene	Black or green contains augite and/or olivene and/or hornblende
Granular or (course-grained)	Granite	Diorite	Gabbro	Peridotite
Porphyritic and aphanitic (coarse and fine)	Rhyolite	Andesite	Basalt
Aphanitic (fine-grained)	Felsite (light-colored)	Basalt (dark-colored)
Glassy (amorphous)	Obsidian Pitchstone Pumice (black) (red or brown) (a glass froth)

FIGURE 8.16 Field classification of igneous rock. Note: Consolidated volcanic ash is called tuff if no large fragments are present. If large fragments are present, it is called breccia. (From C. H. Harned, Some Practical Aspects of Foundation Studies for Highway Bridges, U. S. Bureau of Public Roads, January 1959)

FIELD CLASSIFICATION OF METAMORPHIC ROCKS
NONFOLIATED (no parallel alignment of minerals)	FOLIATED (parallel alignment of minerals)
Quartzite conglomerate (from conglomerate) Quartzite (from sandstone) Marble (from limestone) Serpentine (from basic igneous rocks) Anthracite (from bituminous coal)	Gneiss (individual foliation planes are easily distinguishable with the naked eye) Schist (individual foliation planes are distinguishable with a hand lens) Slate (microfoliated)

ESSENTIAL MINERAL COMPOSITION OF THE COMMON METAMORPHIC ROCKS
NONFOLIATED	FOLIATED
ROCK MINERALS Quartzite Quartz Marble Calcite or dolomite Serpentine Serpentine Coal No minerals	ROCK MINERALS Gneiss Quartz, feldspar, muscovite, biotite, pyroxenes, amphiboles Schist Garnet, staurolite, talc, muscovite, biotite, chlorite, epidote Slate Microscopic quartz, muscovite, biotite, chlorite
Note: The names of foliated rocks are frequently modified by designating the conspicuous minerals present, e. g., chlorite schist, talc schist, mica schist, staurolite schist.

FIGURE 8.17 Field classification of metamorphic rock. (From C. H. Earned, Some Practical Aspects of Foundation Studies for Highway Bridges, U. S. Bureau of Public Roads, January 1959)

The triaxial shear test is used to find the shear strength of a soil for the determina­tion of pile lengths and of bearing capacity for spread footings or drilled shafts. Triaxial shear test results are also needed to give soil parameters for the design of retaining walls. High-quality, undisturbed samples are required for triaxial shear tests. Poor samples should be discarded rather than tested, as they will give misleading results.

The direct shear test is sometimes performed in lieu of other shear tests, and the use of its results is the same as that noted above for the triaxial shear test. It is impor­tant to remember that direct shear test results are usually less reliable than those obtained from the triaxial shear test, since the failure line in the direct shear test is imposed by the method of testing, whereas the triaxial method allows the sample to fail in its weakest plane. On occasion, it is desirable to shear soil or rock along a par­ticular plane. In these cases, a direct shear test may be used. High-quality, undisturbed samples are needed for this test.

CLASTIC SEDIMENTARY ROCKS
RESIDUAL AND /OR MECHANICAL SEDIMENT
GRAIN SIZE	UNCONSOLIDATED SEDIMENT	CONSOLIDATED ROCK
Coarse	Boulders, cobbles, gravel, and coarse sand	Conglomerate (rounded particles) Breccia (angular particles) Sandstone (coarse)
Medium	Sand	Sandstone Arkose = +25% feldspar Graywacke = dark colored
Fine	Silt and clay	Siltstone and shale
NONCLASTIC SEDIMENTARY ROCKS
CHEMICAL SEDIMENTS	ORGANIC SEDIMENTS
Gypsum Salt Dolomite Clauconite Some chert (flint) Some iron ores Some phosphate rock	Some limestone Some chert (flint) Some phosphate rock Peat Coal
Note: 1. The cementing agents for sedimentary rocks are calcite, quartz, limonite, hematite, and chalcedony. Clay minerals may also function as binder or semicementing material. 2. Compositional descriptive adjectives such as siliceous, argillaceous, arenaceous, calcareous, carbonaceous, ferrugenous, feldspathic, opaline, and cherty are frequently used. 3. Other descriptive adjectives such as massive, laminated, stratified, varved, cross-bedded, concretionary, and fissile are also used.

FIGURE 8.18 Field classification of sedimentary rock. (From C. H. Earned, Some Practical Aspects of Foundation Studies for Highway Bridges, U. S. Bureau of Public Roads, January 1959)

The unconfined compression test of a soil is a uniaxial compression test in which the test specimen is provided with no lateral support while undergoing vertical com­pression. The test measures the unconfined, compressive strength of a cylinder of cohesive or semicohesive soil, which, indirectly, may be indicative of the shearing strength. The test is usually performed on an undisturbed sample of soil at its natural moisture content. It may also be performed on a remolded sample to evaluate the effects of disturbance and remolding upon the shearing strength.

Unconfined compression tests are relatively quick to perform and relatively inex­pensive. When used in conjunction with the triaxial test, the unconfined compres­sion test is of value. Also, it is sometimes used as an index test because it is easy to conduct.

(a)

GROUPING USED IN TRIANGULAR CLASSIFICATION
CLASS	PERCENT OF SIZES PRESENT
NAMES	SAND	SILT	CLAY
Sand	80-100	0-20	0-20
Silty sand	45-80	0-55	0-20
Sandy silt	0-45	35-80	0-20
Silt	0-20	80-100	0-20
Clayey sand	38-80	0-42	20-30
Clayey silt	0-38	32-80	20-30
Sandy clay	30-70	0-40	30-50
Silty clay	0-30	20-70	30-50
Clay	0-50	0-50	50-100

If gravel is present in appreciable amounts, the term “gravelly” may be added to the class name, vis. “gravelly sand”. The terms “coarse”, “medium”, and “fine”, when used to describe gravel, sand, and silt, refer to standard grade size limits.

(b)

FIGURE 8.19 Classification of soil based on (a) grain size of sediment and (b) standard grain size limits. (From C. H. Harned, Some Practical Aspects of Foundation Studies for Highway Bridges, U. S. Bureau of Public Roads, January 1959)