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  1. Roy E. Hunt (Author of Geologic Hazards)
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  3. Characteristics of Geologic Materials and Formations : A Field Guide for Geotechnical Engineers
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Pocket knife blade has a hardness of about 5. Brass pinpoint has a hardness a little over 3 can scratch calcite. Fingernail is a little over 2 can scratch gypsum. Summary The identification characteristics of some common minerals including streak, luster, color, hardness, specific gravity, cleavage, and fracture are given in Table 1. Calcite, a constituent of limestone, is differentiated from most other minerals by its vigorous effervescence when treated with cold hydrochloric acid.

Dolomite will react to hydrochloric acid only if the specimen is powdered. Talc 2.

Roy E. Hunt (Author of Geologic Hazards)

Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Feldspar 7. Quartz 8. Topaz 9. Corundum No cleavage Earthy appearance. Transparent to opaque Effervesces in HCl Cleavage as in calcite. Effervesces in HCl only when powdered Good cleavage in four directions. Cubic crystals. Transparent to translucent Perfect cleavage one direction, producing thin elastic sheets Soapy feed, foliated or compact masses No cleavage visible Plastic when wet Conchoidal fracture also nepheline Perfect cubic cleavage.

Soluble in water. Salty Perfect cleavage in one direction. Transparent Cleavage in two directions. Striations on some cleavage planes Conchoidal fracture, six-sided crystals, transparent to translucent Translucent to opaque. To prepare a thin section, a sample about 25 mm in diameter is ground down to a uniform thickness of about 0. At this thickness it is usually translucent. The specimen is enveloped in balsam and examined in polarized light. The minerals are identified by their optical properties. Other Methods Minerals are also identified by a table microscope, the electron microscope, blow-pipe analysis, or X-ray diffraction.

Near the surface a volcanic vent is formed, the pressures decrease, the gases are liberated, and the magma cools and solidifies. Igneous rocks occur in two general forms see also Section 2. Intrusive The magma is cooled and solidified beneath the surface, forming large bodies plutons that generally consist of coarser-grained rocks; or small bodies such as dikes and sills, and volcanic necks, which generally consist of finer-grained rocks because of more rapid cooling.

Classification Igneous rocks are classified primarily according to mineral content and texture as presented in Table 1. Mineral Composition and the Major Groups The important minerals are quartz, feldspar, and the ferromagnesians, as given in Figure 1. Modern classification is based primarily on silica content SiO2 Turner and Verhoogan, Reprinted with permission of Wiley. Excludes pyroclastic rocks. From Pirsson, L. Porphyries are phanerites with large conspicuous crystals phenocrysts.

Aphantic aphanites contain grains too small to be perceived with the unaided eye. Glassy rocks have no grain form that can be distinguished. Porphyritic, in which phenocrysts are embedded in the ground mass or finer material the term refers to grain size, not shape. Grain Shape Grains are described as rounded, subrounded, or angular. Characteristics Photos of some of the more common igneous rocks are given as Plates 1. The characteristics of igneous rocks are summarized in Table 1. PLATE 1. Cloud, Minnesota. Rock Decay or Weathering Processes see also Section 2.

In chemical weathering, the rock mass is acted upon chemically by substances dissolved in water, such as oxygen, carbon dioxide, and weak acids, causing the conversion of silicates, oxides, and sulfides into new compounds such as carbonates, hydroxides, and sulfates, some of which are soluble. Materials Resulting The residue can include rock fragments of various sizes, consisting essentially of unaltered rock; particles of various sizes, consisting of materials resistant to chemical decomposition, such as quartz; and clays or colloidal particles, which are insoluble products of chemical decomposition of less-resistant rocks such as feldspar and mica.

Soluble products of decomposition go into solution. Transport and Deposition Clastic Sediments Detritus The particle products of weathering are transported primarily by flowing water to be deposited eventually in large water bodies or basins. The products are generally segregated by size as defined in Table 1. Wind currents provide transport for finer sand grains and silt sizes. Chiefly quartz and feldspar appearing separately as large grains ranging from a centimeter to as large as a meter in diameter Granite The most common and widely occurring igneous rock.

Fabric roughly equigranular normally. Light colors contain chiefly quartz and feldspar; gray shades contain biotite mica or hornblende Syenite Light-colored rock differing from granite in that it contains no quartz, consisting almost entirely of feldspar but often containing some hornblende, biotite, and pyroxine Diorite Gray to dark gray or greenish, composed of plagioclase feldspar and one or more of the ferromagnesian minerals.

Equigranular fabric Gobbro Dark-colored rock composed chiefly of ferromagnesian minerals and plagioclase feldspar Peridotite Dark-colored rocks composed almost solely of ferromagnesian minerals. Olivine predominant: negligible feldspar. Hornblende or pyroxenes associated. Readily altered Pyroxenite As above but pyroxene alone or predominant Hornblendite As above but hornblende alone or predominant Dunite Major constituent is olivine, which alters readily to serpentine Dolerite or Dark-colored rock intermediate in grain size between gabbro and basalt.

Abundant diabase as thick lava flows that have cooled slowly. Pure andesite is relatively rare, and it is usually found with phenocrysts. Colors range from grayish to greenish black to black. Fine-grained with a dense compact structure. Often contains numerous voids vesicular basalt The microcrystalline equivalent of granite formed at or near the surface. Characteristically white, gray, or pink, and nearly always contains a few phenocrysts of quartz or feldspar Occurs as dikes, sills, and lava flows.

The term felsite is used to define the finely crystalline varieties of quartz porphyries or other light-colored porphyries that have few or no phenocrysts and give but slight indications to the unaided eye of their actual mineral composition. Chemical Precipitates nondetrital Materials are carried in solution in flowing water to the sea or other large water bodies where they precipitate from solution. Chemical precipitates include the immense thickness of marine carbonates limestones and dolomites and the less abundant evaporites gypsum, anhydrite, and halite.

In addition to being formed from physical—chemical processes, many nondetrital rocks are formed from the dissolved matter precipitated into the sea by the physiological activities of living organisms. Seldom found in rock masses —64 Same as source rock Along stream bottoms. Deposited as alluvial fans and in river channels 64—4 As for cobbles or sand As for cobbles: also deposited in beaches 4—2 As for cobbles or sand As for pebbles and sand 2—0.

Some channels, fans, floodplains, locales: hornblende, pyroxene, beaches deltas. Occasionally shell fragments aeolian 0. Salt water: clay result of decomposition of particles curdle into lumps and unstable minerals yielding settle quickly to the bottom. Show complex hydrous silicates no graded beds. Freshwater: Settle see Section 1. Organics Beds of decayed vegetation remain in place to form eventually coal when buried beneath thick sediments.

Depositional Characteristics Horizontal Bedding Under relatively uniform conditions, the initial deposition is often in horizontal beds. Cross-Bedding Wave and current action produce cross-bedded stratification as shown in Figure 1. Ripple Marks Wave and current action can also leave ripple marks on the top of some beds. Unconformity An unconformity exists when a stratum is partially removed by erosion and a new stratum is subsequently deposited, providing an abrupt change in material. Disconformity A lack of parallelism between beds, or the deposition of a new stratum without the erosion of the underlying stratum after a time gap, results in a disconformity.

Lithification Rock forms by lithification, which occurs as the thickness of the overlying material increases. The detritus or precipitate becomes converted into rock by: compacting; the deposition of cementing agents into pore spaces; and physical and chemical changes in the constituents. Classification Sedimentary rocks have been divided into two broad groups: detrital and nondetrital. A general classification is given in Table 1. A special classification system for carbonate rocks formed in the middle latitudes is given in Table 3.

Detrital Group Clastic Sediments Classified by particle size as conglomerate, sandstone, siltstone, and shale. Arenaceous rocks are predominantly sandy. Argillaceous rocks are predominantly clayey. Chemical precipitates are classified by texture, fabric, and composition. Organics include only the various forms of coal.

Characteristics Photos of some of the more common sedimentary rocks are given as Plates 1. The characteristics of the detrital rocks are summarized in Table 1. Tom, Massachusetts. Cementing agent chiefly silica, but iron oxide, clay, and calcareous material also common Gap-graded mixture of large particles in a fine matrix First member of a series; deposited unconformably Angular fragments of any rock type.

Resulting from glaciation, rock falls, cave collapse, fault movements 1. Color depends on cementing agent: yellow, brown, or red — iron oxides predominate: lighter sandstones — silica or calcareous material predominates. Seldom forms thick beds, but is often hard 1. Red shales are colored by iron oxides and gray to black shales are often colored by carbonaceous material. Commonly interbedded with sandstones and relatively soft. Many varieties exist Hard, indurated shales devoid of fissilily; similar to slates but without slaty cleavage Contain carbonates, especially calcite.

With increase in calcareous content becomes shaly limestone Black shales containing much organic matter, primarily carbon, often grading to coal formations Contain carbonaceous matter that yields oil upon destructive distillation Commonly contain montmorillonite clays that are subject to very large volume changes upon wetting or drying see Section 2.

Sandstones and siltstones are frequently interbedded and grade into one another unless an unconformity exists. Flysch: A term used in Europe referring to a very thick series of sandstone, shales, and marls impure limestones well developed in the western Alps. The calcite can be precipitated chemically or organically, or it may be detrital in origin.

There are many varieties; all effervesce in HCl Relatively pure, coarse to medium texture, hard Calcareous Precipitates Crystalline limestonea Micrite Oolitic limestone Fossiliferous limestone Coquina Chalk Dolomite Gypsuma Anhydritea Halitea Microcrystalline form, conchoidal fracture, pure, hard Composed of pea-size spheres oolites , usually containing a sand grain as a nucleus around which coats of carbonate are deposited Parts of invertebrate organisms such as mollusks, crinoids, and corals cemented with calcium carbonate On Barbados, dead coral reefs reach 30 m thickness, and although very porous, are often so hard as to require drilling and blasting for excavation Weak porous rock consisting of lightly cemented shells and shell fragments.

Currently forming along the U. Best known are of Cretaceous Age Harder and heavier than limestone bulk density about pcf compared with pcf for limestone. Ranges from microcrystalline to phanerocrystalline. Normally a splintery fracture, pearly luster, and white color An evaporite: a crystalline aggregate of salt grains, commonly called rock salt.

Soft, tends to flow under relatively low pressures and temperatures, and forms soil domes. Since the salt is of substantially lower specific gravity than the surrounding rocks it rises toward the surface as the overlying rocks are eroded away, causing a dome-shaped crustal warping of the land surface. The surrounding beds are warped and fractured by the upward thrust of the salt plug, forming traps in which oil pools are found 1. Coalification results from the burial of peat and is classified according to the degree of change that occurs under heat and pressure.

Lignite brown coal changes to bituminous coal soft coal which changes to anthracite hard coal Biogenic and Chemical Origin Siliceous Rocks Chert Diatomite Formed of silica deposited from solution in water both by evaporation and the activity of living organism, and possibly by chemical reactions.

Can occur as small nodules or as relatively thick beds of wide extent and is common to many limestone and chalk formations. Hardness is 7 and as the limestone is removed by weathering, the chert beds remain prominent and unchanged, often covering the surface with numerous rock fragments.

Flint is a variety of chert; jasper is a red or reddish-brown chert Soft, while, chalklike, very light rock composed of microscopic shells of diatoms one-celled aquatic organisms which secrete a siliceous shell : porous Other Materials Often Included by Geologists Duricrusts Caliche Laterite Ferrocrete Silcrete Loess Marl a Discussed in Chapter 3 Rocks readily soluble in groundwater.

Metamorphism Effects Tremendous heat and pressure, in combination with the activity of water and gases, promote the recrystallization of rock masses, including the formation of minerals into larger grains, the deformation and rotation of the constituent grains, and the chemical recombination and growth of new minerals, at times with the addition of new elements from the circulating waters and gases. Metamorphic Forms Contact or thermal metamorphism Figure 1.

Away from the intrusive body, the effect diminishes rapidly. Cataclastic metamorphism Figure 1. These forces cause plastic flow, intense warping and crushing of the rock mass, and, in combination with heat and water, bring about chemical changes and produce new minerals. Regional metamorphism combines high temperatures with high stresses, during which the rocks are substantially distorted and changed. Classification Classification is based primarily on fabric and texture as given in Table 1.

Massive fabric is homogeneous, often with equigranular texture. Formed by differential shearing movement between beds Banded or lenticular as shown in Figure 1. Schistose as shown in Figure 1. Slaty cleavage as shown in Figure 2. It is seen that some metamorphic rocks can be derived from a large number of other rock types, whereas a few are characteristic of a single other rock type.

Characteristics Photos of some of the more common metamorphic rocks are given as Plates 1. Characteristics of metamorphic rocks with foliate fabric are summarized in Table 1. Depends on impurities. Competent Rock Intact rock that is fresh, unweathered, and free of discontinuities and reacts to applied stress as a solid mass is termed competent or sound rock in engineering nomenclature.

Permeability, strength, and deformability are directly related to hardness and density, as well as to fabric and cementing. The general engineering properties of common rocks are summarized in Table 1. Decomposed Rock Decomposition from weathering causes rock to become more permeable, more compressible, and weaker. As the degree of decomposition advances, affecting the intact blocks and the discontinuities, the properties approach those of soils. The final product and its thickness are closely related to the mineral composition of the parent rock, the climate, and other environmental factors see Section 2.

Nonintact Rock Discontinuities or defects, representing weakness planes in the mass, control the engineering properties by dividing the mass into blocks separated by fractures such as faults, joints, foliations, cleavage, bedding, and slickensides, as described in Table 1.

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Joints are the most common defects in rock masses. They have the physical properties of spacing, width of opening, configuration, and surface roughness. They can be tight, open, or filled with some material, and can display the strength parameters of cohesion and friction along their surfaces see Section 2. Blocks will have the characteristics of intact rock. As the degree of decomposition increases, the significance of the discontinuities decreases, but even in highly decomposed rock and residual soils, relict fractures can represent potential failure surfaces.

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In general, experience shows that the response of a rock mass during tunneling operations will be governed by intact properties, and the rock may be considered as competent if its joints are tight, their spacing is about 1 m or more, and the rock is fresh Hartmann, In slopes and excavations, however, any size block of fresh rock within an exposed wall can fail if the bounding planes of the block incline downward and out of the slope.

The foliation causes lenticular planes of weakness resulting in slabbing in excavations see Figure 2. Chief minerals are quartz, and feldspar, but various percentages of other minerals mica, amphibole, and other ferromagnesians are common. The identification of gneiss includes its dominant accessory mineral such as hornblende gneiss see Plate 1. The important platy minerals are muscovite, chlorite, and talc. Schist is identified by the primary mineral as mica schist see Plate 1.

Garnet is a common accessory mineral to mica schist and represents intense metamorphism. Schists and gneisses commonly grade into each other and a clear distinction between them is often not possible Consist largely of amphibole and show more or less schistose form of foliation. Composed of darker minerals and in addition to hornblende, can contain quartz, plagioclase feldspar, and mica. They are hard and have densities ranging from 3. Association with gneisses and schists is common in which they form layers and masses that are often more resistant to erosion than the surrounding rocks Soft, with a satin-like luster and extremely fine schistosity.

Composed chiefly of chlorite. Very unstable in cut slopes. Grades to schists as the coarseness increases see Figure 1. Generally hard plates split from formations: once used for roofing materials 1. Descriptions are made from the examination of outcrops, exploration pits and adits, and boring cores. Intact Rock Characteristics Descriptions should include the hardness, weathering grade, rock type, coloring, texture, and fabric. Discontinuities Joint spacing and joint characteristics are described, and details of joint orientations and spacing should be illustrated with photographs and sketches to allow for the preparation of two- or three-dimensional joint diagrams see Section 2.

Metamorphosed limestone does not normally develop cavities. Very hard Derived from serpentine. Generally compact, dull to waxy luster, smooth to splintery fracture, generally preen in color and often soft unless it contains significant amounts of quartz. Can have foliate fabric Derived from talc; generally gray to green color, very soft and easily trimmed into shapes with a knife, without cleavage or grain, and resists well the action of heat or acids Rocks baked by contact metamorphism into hard aphanitic material, with conchoidal fracture, dark gray to black color, often resembling a basalt Quartzite Marble Serpentinite Soapstone Hornfels Plate 1.

Common along the base of overthrust sheets and can range from very thin, to a meter or so, to several hundreds of meters thick. Shale mylonites form very unstable conditions when encountered in cut slopes or tunneling. They are formed by differential movement between beds 1. Groundwater Conditions Observations of groundwater conditions made in cuts and other exposures must be related to recent weather conditions, season, and regional climate to permit judgments as to whether seepage is normal, high, or low, since such conditions are transient.

Rock-Quality Indices Indices of rock quality are determined from a number of relationships as follows: 1. Significant as aquifers for water supply, or seepage beneath dams Increases with degree of decomposition Water moves with relative freedom along fractures, and flow quantity increases as joint openings, continuity, and pattern intensities increase Significant in cut slopes and other excavations, seepage pressures beneath dam foundations, or seepage loss, and for water supply Seldom exceeded in the confined state, but in an unconfined condition, such as slopes or tunnels, strength can be very low along weakness planes.

Normally controls mass strength Rupture strength Most rocks essentially nonrupturable in the confined state, although under conditions of high tensile stresses and high pore pressures under a foundation not confined totally, rupture can occur, especially in foliated rocks Deformability from Compression under stress increase foundation loads essentially elastic, although some rocks such as halite deform plasticly and undergo creep. Plastic deformation can also occur along foliations in a partially confined situation such as an excavation, or from high loads applied normal to the foliations Expansion stress Occurs in shales with decrease montmorillonite clays or pyrite, causing excavation and foundation heave, slope collapse, and tunnel closing, when in contact with free water or moisture in the air Decreases with degree of decomposition Increases with degree of decomposition Occurs from the closure of fractures and displacement along the weakness plane.

When confined, displacements are usually negligible. In open faces, such as tunnels and slopes, movements can be substantial and normally control mass deformation Increase with degree of Residual stresses are locked decomposition into the rock mass during formation or tectonic activity and can far exceed overburden stresses, causing deflections of walls and floors in excavations and underground openings, and even violent rock bursts in deep mines intact or nonintact Summary A suggested guide to the field description of rock masses is given in Table 1.

Low foliations. Low parallel to foliations parallel to foliations As for gneiss As for gneiss Weaker than gneiss Weaker than gneiss Very low Very high Very low Very high Fresh intact condition. Anisotropic fabric. A strength classification system must consider the rock type and degree of decomposition because the softer sedimentary rocks such as halite can have strengths in the range of decomposed igneous rocks.

Jaeger relates strength to the degree of decomposition, also given in Figure 1. Deere proposed an engineering classification system based on strength, which he related to rock type Table 1. It is to be noted that very low strength rock is given as less than tsf, which covers the category of rocks composed of soft minerals as well as those in the decomposed state. Waterlogged zones of crushed rock are a cause of running ground in tunnels Shiny, polished surfaces with striations. Often the weakest elements in a mass, since strength is often near residual Can be present as open joints or merely orientations without openings.

Strength and deformation relate to the orientation of applied stress to the foliations Thin zones of gouge and crushed rock occur along the weaker layers in metamorphic rocks Found primarily in shales and slates: usually very closely spaced Often are zones containing weak materials such as lignite or montmorillonite clays Strong laminations: original mineral constituents and fabric crushed and pulverized In limestone range from caverns to tubes. In rhyolite and other igneous rocks range from voids of various sizes to tubes Joint Faults A fracture along which displacement has occurred due to tectonic activity Slickensides Preexisting failure surface: from faulting, landslides, expansion Foliation planes Continuous foliation surface results from orientation of mineral grains during metamorphism Shear zone resulting from folding or stress relief Stress fractures from foldings Foliation shear Cleavage Bedding planes Mylonite Cavities Contacts between sedimentary rocks Intensely sheared zone Openings in soluble rocks resulting from ground water movement, or in igneous rocks from gas pockets Suggested Classification Systems Hardness classifications based on simple field tests and related to uniaxial compressive strength ranges are given in Table 1.

Weathering grade, class, and diagnostic features are given in Table 1. Rock-Mass Classification General Historically, rock-mass classification has been based on percent core recovery, which is severely limited in value. Core recovery depends on many factors including equipment used, operational techniques, and rock quality, and provides no direct information on hardness, weathering, and defects.

Even good core recovery cannot provide information equivalent to that obtained by field examination of large exposures, although ideal situations combine core recovery with exposure examinations. Coarse, medium, fine, very fine Table 1. Give orientations of major joint sets.

Solid, massive, blocky, fractured, crushed mass Table 1. Building Codes Many codes classify rock in terms of hardness using nomenclature such as sound, hard, medium hard, and soft, but without defining the significance of the terms. Point-load index values from Hock, E. City Building Code Table 1. Building Code, City of New York, Simple Classification Systems Early workers in rock mechanics developed systems to classify joints according to spacing, as given in Table 1. Complex Classification Systems Systems have been developed to provide detailed information on rock quality that includes joint factors such as orientation, opening width, irregularity, water conditions, and filling materials, as well as other factors.

They are most applicable to tunnel engineering. The rock mass rating system RMR for jointed rock masses, proposed by Bieniawski , , , is given in Table 1. Rock Mech. Uniaxial compressive strength Core Logging Comm. Rings under hammer impact Slight discoloration inward from open fractures, otherwise similar to F Discoloration throughout. Weaker minerals such as feldspar decomposed. Strength somewhat less than fresh rock, but cores cannot be broken by hand or scraped by knife. Texture preserved Most minerals to some extent decomposed.

Specimens can be broken by hand with effort or shaved with knife. Core stones present in rock mass. Texture becoming indistinct but fabric preserved Minerals decomposed to soil but fabric and structure preserved saprolite. Specimens easily crumbled or penetrated Advanced state of decomposition resulting in plastic soils. Rock fabric and structure completely destroyed. Allowable bearing value applies only to massive crystalline rocks or to sedimentary or foliated rocks where strata are level or nearly level, and if area has ample lateral support.

Tilted strata and their relation to adjacent slopes require special consideration. Allowable bearing for hard to intermediate rock applies to foundations bearing on sound rock. Rock cored with double-tube, diamond core barrel, in 5 ft 1. With permission and Bieniawski, Z.

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Characteristics of Geologic Materials and Formations : A Field Guide for Geotechnical Engineers

Reprinted with permission. Engineering classification of rock masses for tunnel support design Q-system , proposed by Barton , , is given in Table 1. It is based on a very detailed grading of six basic parameters including RQD, description of joint sets, joint roughness, joint alteration, joint water conditions, and a stress reduction factor which provides for rating major zones of weakness in the mass, residual stresses, squeezing rock, and swelling rock.

The rock mass quality Q is calculated from Equation 1. Barton et al. Sample lengths of 5 or 10 m are usually used. Several of the more common and current classification systems are given in Table 1. Major Groupings On the basis of grain size, physical characteristics, and composition, soils may be placed in a number of major groups: 1. Boulders and cobbles, which are individual units. Granular soils, including gravel, sand, and silt, are cohesionless materials except for apparent cohesion evidenced by partially saturated silt.

Clay soils are cohesive materials. Organic soils are composed of, or include, organic matter. For example, if infilling is present, the roughness of the surface will be overshadowed by the influence of the gouge. In such cases use A. Source: From Bieniawski, Z. Also chlorite, talc, gypsum, graphite, etc. Value of Ja depends on percent of swelling clay-size particles, and access to water, etc. For portals use 2. Descriptions refer to small-scale features and intermediate-scale features, in that order.

Increase Jw if drainage measures are installed. Special problems caused by ice formation are not considered. Few case records available where depth of crown below surface less than span width. SRF increase from 2. Source: From Barton, N. Additional Notes: When rock-mass quality Q is estimated, the following guidelines should be followed, in addition to the notes listed in parts a to f : 1. When borecore is unavailable, RQD can be estiamated from the number of joints per unit volume, in which the number of joints per meter for each joint set are added.

The parameter Jn representing the number of joint sets will often be affected by foliation, schistocity, slatey cleavage, or bedding, etc. The parameters Jr and Ja representing shear strength should be relevant to the weakest significant joint set or clay filled discontinuity in the given zone. When a rock mass contains clay, the factor SRF appropriate to loosening loads should be evaluate part f.

In such cases the strength of the intact rock is of little intrest. A strongly anisotropic stress field is unfavorable for stability and is roughly accounted for as in Note g, part f. The compressive and tensil strengths c and t of the intact rock should be evaluated in the saturated condition if this is appropriate to present or future in situ conditions. A very conservative estimate of strength should be made for those rocks that deteriorate when exposed to moist or saturated conditions.

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Coarse-grained soils including gravel and sand 2. Fine-grained soils including silt and clay 3. Cohesive soils, which are clays mixed with granular soils or pure clays 1. Gravel, sand, and silt respond to stress as a mass and are the most significant granular soils. T and British Standards Institute 0. The shape results from abrasion and in some cases solution, and is related to the mode and distance of transport.

Subangular sand grains are illustrated in Figure 1. Behavior is mass-derived because of pore spaces between individual grains which are in contact. Properties Cohesionless, nonplastic. Grain Minerals Types The predominant granular soil mineral is quartz, which is essentially stable, inert, and nondeformable. On occasion, sands and silts will include garnet, magnetite, and hornblende. In climates where mechanical disintegration is rapid and chemical decomposition is minor, mica, feldspar, or gypsum may be present, depending on the source rock.

Shell fragments are common in many beach deposits, especially in areas lacking quartzrich rocks, and offshore, in the middle latitudes, calcareous or carbonate sands are common see Section 3. The weaker minerals such as shells, mica, and gypsum have low crushing strengths; calcareous sands can have deleterious effects on concrete. Identification Simple tests to identify grain minerals include the application of hydrochloric acid to test for calcareous materials, and the determination of specific gravity Table 1.

Silts are classified as inorganic, ranging from nonplastic to plastic, or organic, containing appreciable quantities of organic matter. The smooth texture of wet silt gives it the appearance of clay. Properties Dilantancy: Silts undergo changes in volume with changes in shape, whereas clays retain their volume with changes in shape plasticity. Grains are fine, but compared with clays, pore spaces are relatively large, resulting in a high sensitivity to pore-pressure changes, particularly from increases due to vibrations.

For example, a moist, near-vertical cut slope will temporarily stand stable to heights of 10 ft. Behavior is controlled by surface- rather than mass-derived forces. A spoon sample of lacustrine clay is illustrated in Figure 1. Mass Structures Clay particles form two general types of structures: flocculated or dispersed, as shown in Figure 1. A flocculated structure consists of an edge-to-face orientation of particles, which results from electrical charges on their surfaces during sedimentation.

In salt water, flocculation is much more pronounced than in fresh water since clay particles curdle into lumps and settle quickly to the bottom without stratification. In fresh water, the particles settle out slowly, forming laminated and well-stratified layers with graded bedding. A dispersed structure consists of face-to-face orientation or parallel arrangement, which occurs during consolidation compacting. Properties Cohesion results from a bond developing at the contact surfaces of clay particles, caused by electrochemical attraction forces.

The more closely packed the particles, the greater is the bond and the stronger is the cohesion. Cohesion is caused by two factors: the high specific surface of the particles surface area per unit weight , and the electrical charge on the basic silicate structure resulting from ionic substitutions in the crystal structure Table 1. Adhesion refers to the tendency of a clay to adhere to a foreign material, i.

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Plasticity refers to the tendency of a material to undergo a change in shape without undergoing a change in volume, with its moisture content held constant. Consistency: With decreasing moisture content, clays pass from the fluid state very soft through a plastic state firm , to a semisolid state, and finally to a hard brick-like state.

The moisture contents at the transitions between these various states are defined by the Atterberg limits, which vary with the clay type and its purity. Clay soils are commonly identified by the relationship between the plasticity index and the liquid limit. Activity refers to an affinity for moisture, resulting in large volume changes with an increase in moisture content swelling or decrease in moisture content shrinking Figure 5.

A clay classification based on activity is given in Table 1. Clay Mineralogy and Chemistry Clay Minerals Clays are hydrous aluminum silicates that are classified into a number of groups based on their crystal structure and chemistry. Common groups include kaolinite, halloysite, illite, and montmorillonite. Less common groups include vermiculite and chlorite, which although common in decomposing rock masses, transform readily to the other types.

Characteristics of the common clay minerals are summarized in Table 1. Classification of the clay minerals based on chemistry and crystal structure is given in Table 1. Shrinkage is caused by capillary tension developing in porewater during drying Fronteira, Piaui, Brazil. I, , pp. Sodium clays may be the product of the deposition of clay in seawater, or of their saturation by saltwater flooding or capillary action.

Calcium clays are formed essentially in freshwater deposition. Hydrogen clays are the result of prolonged leaching by pure or acid water, with the resulting removal of all other exchangeable bases. Base exchange refers to the capacity of colloidal particles to change the cations adsorbed on the surfaces. Thus, a hydrogen clay can be changed to a sodium clay by the constant percolation of water containing dissolved sodium salts. The permeability of a clay can be decreased by such changes and the sensitivity increased.

Soils with a high percentage of sodium cation relative to calcium and magnesium cations appear to have a high susceptibility to dispersion. Exchange capacity refers to the quantity of exchangeable cations in a soil; not all cations are exchangeable. They increase with the acidity of the soil crystals. Corrosion of iron or steel embedded in soil, in the presence of moisture, increases with soil acidity.

Relatively stable material in the presence of waters Platy but lumpy Low, except properties are radically altered by intense drying. Therefore, when halloysites are used as embankment material, testing procedures should duplicate field placement procedures. The crystal shape is used to identify the clay type see, e. In the diffractometer a powdered mineral sample mounted on a glass slide is rotated at a fixed angular rate in an X-ray beam.

A pick-up device, such as a Geiger tube, rotates about the same axis, detecting the diffracted beams. The impulse is transmitted and recorded on a strip chart. Differential Thermal Analyzer: It measures the temperatures and magnitudes of exothermic and endothermic changes occurring in a sample when it is heated at a uniform rate. The curve forms are characteristic for various clay types.

Formation Topsoil is formed as plant life dies and becomes fixed with the surficial soils. Well-drained granular soils above the water table, poor in minerals other than quartz, develop very thin topsoil layers, even in humid climates. Thick topsoil layers develop in mineral-rich soils and humid climates, particularly where the soil is cool. This phenomenon is evident in tropical countries where the topsoil is usually thin, except where drainage is poor. Rootmat forms in marshy regions Section 3. Peat is fibrous material with a sponge-like structure, composed almost entirely of dead organic matter, which can form to extensive thickness.

Organic silts and clays form in lakes and estuarine environments, where they can attain a thickness of 25 m or more. Occurrence Although surface deposits during formation, organic layers can be found deeply buried by alluvium as shown in Figure 1. Characteristics Organic deposits are characterized by very low natural densities, very high natural water contents, a loss in mass upon ignition, and substantial shrinkage upon drying.

A general summary of the engineering properties of the various soil groups is given in Table 1. Gravels and Sands Hydraulic Properties Permeability: Gravels and sands are free-draining materials with large storage capacity, acting as aquifers or natural reservoirs, providing the sources of water flowing into excavations, or through, around, and beneath dams.

Capillarity: Negligible. Frost heaving: Essentially nonsusceptible. Liquefaction and piping: Potential increases with increasing fineness. Loose fine sands are most susceptible; gravel is nonsusceptible. Failure criteria: General shear failure of shallow foundations does not occur because compression occurs simultaneously with load application, and a deep failure surface cannot develop.

Failure occurs by local shear, the displacement around the edge of a flexible foundation, or punching shear, i. In slopes, failure is relatively shallow in accordance with the infinite-slope criteria. Collapse of soil structure occurs in lightly cemented loose sands. Deformability Response to load is immediate as the voids close and the grains compact by rearrangement. Deformation is essentially plastic, with some elastic compression occurring within the grains. The amount of compression is related to gradation, relative density, and the magnitude of the applied stress.

Crushing can occur in grains of shell fragments, gypsum, or other soft materials, even under relatively low applied stresses. Strength is destroyed by saturation or drying. Upon saturation, collapse may occur in lightly cemented formations, such as loess. Deformability Slow draining characteristics result in some time delay in compression under applied load.

Compaction in fills, either wet or dry, is relatively difficult. Clays Hydraulic Properties Permeability: Clays are relatively impervious, but permeability varies with mineral composition. It is used as an impermeabilizing agent in drilling fluid for test boring or in a slurry trench cutoff wall around an excavation. Kaolinite, with void ratios of about 1. Capillarity: It is high, but in excavations evaporation normally exceeds flow. Frost susceptibility: Many thin ice layers can form in cold climates, resulting in ground heave. Liquefaction susceptibility: Nonsusceptible. Piping: It occurs in dispersive clays.

Rupture Strength Consistency provides a general description of strength identified by the relationship between the natural moisture content and the liquid and plastic limits and by the unconfined compressive strength. Failure occurs by general shear, local shear, or punching shear. Collapse upon saturation or under a particular stress level occurs in certain clays from which minerals have been leached, leaving an open, porous structure.

Deformability Compression, by plastic deformation, occurs in clays during the process of consolidation. Overconsolidated, fissured clays, however, deform in a manner similar to in situ rock, i. Expansion is a characteristic of partially saturated clays in the presence of moisture. The amount of expansion varies with mineral type and swelling pressures, and volume changes can reach substantial magnitudes.

Characteristics of Geologic Materials and Formations: A Field Guide for Geotechnical Engineers

Not all clays or clay mixtures are susceptible. Organic Soils Hydraulic Properties Permeability of peat and rootmat, primarily fibrous matter, is usually very high and, for organic silts and clays, is usually low. In the latter cases, systems of root tunnels can result in k values substantially higher than for inorganic clays. Rupture Strength Peat and rootmat tend to crush under applied load, but shallow cuts will stand open indefinitely because of their low unit weight, as long as surcharges are not imposed.

Organic silts and clays have very low strengths, and generally the parameters for clay soils pertain. Embankments less than 2 m in height placed over these soils often undergo failure. Deformability Organic materials are highly compressible, even under relatively low loads. Fibers and gas pockets cause laboratory testing to be unreliable in the measurement of compressibility, which is best determined by full-scale instrumented load tests.

Compression in peat and rootmat tends to be extremely rapid, whereas in organic silts and clays there is a substantial time delay, although significantly less than for inorganic clays. Rootmat undergoes substantial shrinkage upon drying. Corrosivity Because of their high acidity, organic materials are usually highly corrosive to steel and concrete.

Mixtures Sand and silt mixed with clay commonly assume the properties of clay soils to a degree increasing with the increasing percentage of clay included in the mixture. The plasticity chart see Figure 3. They do not provide the nomenclature to describe mineral type, grain shape, stratification, or fabric. Current Classification Systems A general summary of classification systems defining grain size components is given in Table 1. Bureau of Public Roads system dating from , which is commonly used for highway and airfield investigations.

It was developed by A. Army Corps of Engineers and has been adopted by the U. Bureau of Reclamation, and many other federal and state agencies. It is not universally used, but is applied in the northeastern United States, particularly for the field description of granular soils, for which it is very useful in defining component percentages. MIT Classification System Presented by Gilboy in , the MIT system was the basic system used by engineering firms for many years, and is still used by some engineering firms in the United States and other countries.

Summarized in Table 1. Field Identification and Description Important Elements Field descriptions of soils exposed in cuts, pits, or test boring samples should include gradation, plasticity, organic content, color, mineral constituents, grain shape, compactness or consistency, field moisture, homogeneity layering or other variations in structure or fabric , and cementation.

Significance Precise identification and description permit preliminary assessment in the field of engineering characteristics without the delays caused by laboratory testing. Such an assessment is necessary in many instances to provide data of the accuracy required for thorough site evaluation. Granular soils: Undisturbed sampling is often very difficult, and disturbed sample handling, storage, and preparation for gradation testing usually destroy all fabric.

Test results, therefore, may be misleading and nonrepresentative, especially in the case of highly stratified soils. Clay soils: Unless a formation contains large particles such as those found in residual soils and glacial tills, precise description is less important for clay soils than for granular formations because undisturbed samples are readily obtained.

Sands: more than one half passing no. Silts and clays can be identified by the smallest diameter thread that can be rolled with a saturated specimen as given in Table 1. Field Determinations A guide to determining the various soil components on the basis of characteristics and diagnostic procedures is given in Table 1. Field Descriptions The elements of field descriptions, including the significance of color, and nomenclature for structure and fabric, are given in Table 1. The importance of complete field descriptions cannot be overstressed, since they provide the basic information for evaluations.

The first group from the left consistent with the test data is the correct classification. The A-7 group is subdivided into A or A depending on the plastic limit. NP denotes nonplastic. Depending on percentage of fines fraction smaller than No. After Burmister, D. Sample AH-3 2 C 2. From Burmister, D. Fineness depends on clay content and mineralogy Thread diameter when saturated vs. PI and identification given in Table 1. Barton, N. Bieniawski, Z.

IIA, , pp. Burmister, D. Casagrande, A. ASCE, , —, Core Logging Comm. Cornell Univ. Coon, J. Deere, D. Gibbs, H. Grim, R. Hamlin, W. Hartmann, B. Hoek, E. Jaeger, C. Jennings, J. Lambe, T. Leonards, G. Mohr, E. Morin, W. Osipov, V. Pirsson, L. Serafim, J. Balkema, Boston, , pp. Simpson, B.

Skempton, A. Turner, F. Memo No. Wahlstrom, E. Wickham, G. Rapid Excav. Tunneling Conf. Dennen, W. Kraus, E. Rice, C. Travis, R. Vanders, I. The original mode of formation with a characteristic structure Section 1. Deformation with characteristic discontinuities 3. Development of residual stresses that may be several times greater than overburden stresses 4. Alteration by weathering processes in varying degrees from slightly modified to totally decomposed Rock-mass response to human-induced stress changes are normally controlled by the degree of alteration and the discontinuities.

The latter, considered as mass defects, range from weakness planes faults, joints, foliations, cleavage, and slickensides to cavities and caverns. In the rock mechanics literature, all fractures are often referred to as joints. Terrain analysis is an important method for identifying rock-mass characteristics. Structural features are mapped and presented in diagrams for analysis. Original Mode of Formation Igneous Rocks Intrusive masses form large bodies batholiths and stocks , smaller irregular-shaped bodies lapoliths and laccoliths , and sheet-like bodies dikes and sills.

Extrusive bodies form flow sheets. Sedimentary Rocks Deposited generally as horizontal beds, sedimentary rocks can be deformed in gentle modes by consolidation warping or by local causes such as currents. Cavities form in the purer forms of soluble rocks, often presenting unstable surface conditions. Metamorphic Rocks Their forms relate to the type of metamorphism see Section 1.

Except for contact metamorphism, the result is a change in the original form of the enveloping rocks. Plastic deformation is caused by steady longterm stresses resulting in the folding translation of beds and some forms of cleavage, and occurs when stresses are in the range of the elastic limit and temperatures are high. Creep, slow continuous strain under constant stress, deforms rock and can occur even when loads are substantially below the elastic limit, indicating that time is an element of deformation. Short-term stresses within the elastic limit at normal temperatures leave no permanent effects on masses of competent rock.

Rupture and fracture occur at conditions of lower temperatures and more rapid strain, resulting in faults, joints, and some forms of cleavage. Other Forces and Causes Tensile forces occur during cooling and contraction and cause the jointing of igneous rocks. We provide complimentary e-inspection copies of primary textbooks to instructors considering our books for course adoption. Stay on CRCPress. Preview this Book. Add to Wish List.

Close Preview. Toggle navigation Additional Book Information. Description Table of Contents. Summary Properly understanding and characterizing geologic materials and formations is vital for making critical engineering decisions. Identifying and classifying rock masses and soil formations allows reasonable estimation of their characteristic properties. Comprising chapters from the second edition of the revered Geotechnical Engineering Investigation Handbook, Characteristics of Geologic Materials and Formations provides a basis for recognizing, identifying, and classifying the various rock and soil types.

With clear, concise, and hands-on guidance, this book describes these rock and soil types in terms of their origin, mode of occurrence, and structural features in situ and presents the typical characteristics that are of engineering significance. It also explains the elements that affect surface and subsurface water engineering in terms of controlling floods, erosion, subsurface flow, and seepage, as well as for water conservation. Supplying important correlations used to estimate engineering and geologic properties, the book presents correlations for intact rock, rock masses, and soil formations throughout the chapters and condenses this information into a convenient summary table in an appendix.

Eliminate the need to search through narrow volumes or large handbooks with Characteristics of Geologic Materials and Formations: A Field Guide for Geotechnical Engineers, a convenient and complete guide to the techniques you need. Request an e-inspection copy. Share this Title.