agregate

A G G R E G A T E S

Construction Sand and GravelWilliam Langer

INTRODUCTIONSand and gravel are among the most familiar minerals in everydaylife, and together comprise one of the two principal sources ofaggregate. Crushed stone, the other source, is discussed in a sepa-rate chapter and is mentioned here only in a general manner, suchas when both sand and gravel and crushed stone are being describedas aggregate.

This chapter is intended to present a general perspective on thegeology of sand and gravel deposits, and how they are explored,mined, and processed for aggregate to meet the needs of the con-struction industry. The transportation, marketing, specifications, anduses of sand and gravel are discussed only briefly in this chapter (fora more detailed discussion, see the Lightweight Aggregates chapterin this volume).

Most sand and gravel is used as construction aggregate. Inmany cases, it is preferred over crushed stone for use in portlandcement concrete because its smooth, rounded shape allows foreasy mixing without addition of excess water and cement. Gravelcommonly must be crushed for use in asphaltic concrete whereinterlocking edges add strength. An average six-room houserequires about 82 t of aggregate, and an average school buildingrequires about 14,000 t of aggregate (Langer and Glanzman 1993).Sand and gravel also has numerous applications in an unboundstate. The proportional share of aggregate consumed by each per-son in the United States is about 9 tpy, about 3.8 t of which is sandand gravel.

DefinitionsWide variations exist for the definition of sand and gravel; someterms have different meanings to different users. In addition, someterms have descriptive characteristics that are dependent on localconditions and, as such, serve an important function. The termsused in this chapter generally conform to geologic descriptions.

• Sand and gravel: a mixture of unconsolidated material result-ing from the natural disintegration of bedrock and the subse-quent transport, abrasion, and deposition of the particles byice, water, wind, and gravity. Sand and gravel normally occurtogether and can contain particles ranging in size from clay toboulders.

• Sand: natural granular material resulting from rock disintegra-tion, consisting primarily of particles having a diameter in therange of 1/16 in. to 2 mm, according to the definition of Bates

15

and Jackson (1987). Manufactured sand resulting from theproduction of crushed stone is discussed in the Crushed Stonechapter in this volume.

• Gravel: unconsolidated, natural accumulation of rounded rockfragments resulting from erosion, consisting predominantly ofparticles larger than sand (diameter greater than 2 mm), suchas boulders, cobbles, pebbles, and granules (Bates and Jack-son 1987).

Production and Trade, Substitutes, and Resources and ReservesSand and gravel is the second largest nonfuel mineral commodity inthe United States in both volume and value. About 1.13 billion t ofsand and gravel, with a value of $5.8 billion, was produced by anestimated 4,000 companies from 6,400 operations in 50 states(Bolen 2004). About 25% of the operations produce less than25,000 tpy of sand and gravel. About 3% of the operations producemore than 1 Mtpy of aggregate and account for nearly 30% of thetotal sand and gravel production (Bolen 2002). About 10% of theoperations account for more than half the annual production. About53% of the 1.13 billion t of construction sand and gravel producedin 2003 was for unspecified uses. Of the remaining total, about 42%was used as concrete aggregates; 23% for road base and coveringsand road stabilization; 15% as construction fill; 12% as asphalticconcrete aggregates and other bituminous mixtures; 3% for con-crete products, such as blocks, bricks, and pipes; 1% for plaster andgunite sands; and the remaining 4% for snow and ice control, rail-road ballast, roofing granules, filtration, and other miscellaneoususes (Bolen 2004).

Sand and gravel production, expressed as a percentage of totalaggregate production, varies from state to state depending on a vari-ety of reasons, including geology, availability, ease of obtainingpermits, local or regional specifications, and operator preferences.Generally, in the United States, more crushed stone is producedthan sand and gravel (Table 1). More sand and gravel than crushedstone is produced in 24 states.

The amount of foreign trade of construction sand and gravelis minor. During 2001, about 3.06 Mt were exported, and about3.82 Mt were imported. Most trade is with Canada and Mexico.

Total world production of aggregate (including crushed stoneas well as sand and gravel) is estimated at 15 billion tpy. Productionin the European Union is about 3 billion tpy, representing about two

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160 Industrial Minerals and Rocks

Table 1. Sand and gravel production, by state, for 2002

State

Sand andGravel Value,

$1,000

Sand and Gravel Production,

kt

Sand and Gravel as Percentage of Total

Aggregate Production State

Sand andGravel Value,

$1,000

Sand and Gravel Production,

kt

Sand and Gravel as Percentage of Total

Aggregate Production

Alabama 60,700 13,500 19.8 Montana 94,300 20,200 86.4

Alaska 67,800 11,500 88.5 Nebraska 41,500 12,300 66.6

Arizona 265,000 47,700 83.7 Nevada 168,000 32,300 77.9

Arkansas 52,500 10,400 24.4 New Hampshire 45,200 8,830 65.0

California 1,160,000 157,000 71.1 New Jersey 91,700 15,400 47.1

Colorado 236,000 44,800 72.3 New Mexico 58,500 11,100 67.7

Connecticut 68,400 11,500 52.0 New York 152,000 28,700 32.4

Delaware 13,500 2,320 100.0 North Carolina 52,400 10,300 13.9

Florida 117,000 26,300 20.0 North Dakota 26,700 10,300 100.0

Georgia 28,100 6,750 8.6 Ohio 249,000 48,100 38.4

Hawaii 6,000 500 6.7 Oklahoma 41,300 10,200 20.6

Idaho 60,200 16,900 78.6 Oregon 114,000 19,500 46.0

Illinois 128,000 28,200 26.7 Pennsylvania 121,000 18,700 16.1

Indiana 122,000 28,100 33.0 Rhode Island 10,100 1,280 49.4

Iowa 65,000 14,200 27.8 South Carolina 31,200 9,510 27.5

Kansas 27,400 9,450 30.5 South Dakota 39,500 10,500 63.9

Kentucky 41,000 10,100 15.5 Tennessee 54,900 9,680 14.7

Louisiana 76,800 16,000 100.0 Texas 384,000 76,900 37.9

Maine 44,600 10,900 72.2 Utah 119,000 30,600 78.4

Maryland 76,000 11,000 32.8 Vermont 18,900 4,240 50.5

Massachusetts 90,600 13,900 48.4 Virginia 60,100 10,800 15.3

Michigan 269,000 75,500 64.8 Washington 238,000 43,900 74.8

Minnesota 179,000 45,300 82.3 West Virginia 8,080 1,560 8.8

Mississippi 77,900 14,900 72.9 Wisconsin 139,000 35,500 52.4

Missouri 40,700 9,480 10.6 Wyoming 47,600 9,570 66.2

Total 5,710,000 1,120,000 41.7

Source: Bolen 2002; Tepordei 2002.

thirds of the European total (Regueiro et al. 2002). Aggregate pro-duction rates plummeted in Eastern Europe following the politicalreorganization of the early 1990s. More recently, production rateshave stabilized, resulting in modest increases in demand in countrieswith emerging economies. Approximate production in other majoraggregate-producing countries, in metric tons per year, follows:China, 4.5 billion; Japan, 550 million; Russia, 432 million; and Can-ada, 385 million (Regueiro et al. 2002).

Although aggregate resources, like all nonrenewableresources, are finite, the potential worldwide supply of aggregateresources is so large that “finite” loses its urgency in this context.But natural aggregate of suitable quality for an intended use can bein short or nonexistent supply on a regional or local scale becauseof unfavorable geology, encroachment by incompatible land uses,and the inability to obtain necessary permits.

A number of materials may be used as a substitute for sandand gravel. The most widely used one for gravel is crushed stone;for natural sand, it is manufactured sand. Other substitutes includerecycled concrete or asphalt, slag, and shells.

GEOLOGIC ORIGIN AND MODES OF OCCURRENCEThe distribution and size of deposits of sand and gravel are prima-rily controlled by glacial, fluvial, and marine processes (Langer1988). Wind is an inconsequential geologic agent when consideringgravel. Consequently, sand and gravel is widely distributed andabundant near present and past rivers and streams, in alluvialbasins, along marine or lake shorelines, and in previously glaciated

areas (Figures 1 and 2). The debris from in situ weathering of somebedrock can also be a source of sand and gravel.

Glacial DepositsThe most recent episodes of glaciation took place over the last2.5 million years during which much of the world’s temperate zoneswere alternately covered by glaciers and uncovered during thewarmer interglacial periods. A number of ice sheets expanded fromCanada into the northern tier of conterminous states and Alaska, attimes covering all of New England; almost all of New York; all ofMichigan; parts of Wisconsin and Iowa; almost all of Minnesota andNorth Dakota; and some of New Jersey, Pennsylvania, Ohio, Indi-ana, Illinois, Missouri, Nebraska, South Dakota, Montana, Idaho,Washington, and Alaska (Mickelson and Colgan 2004; Booth et al.2004). These ice sheets persisted until about 11,000 years ago, andremnants occur in Alaska, Canada, and the higher portions of someRocky Mountain states. Extensive ice fields and valley glaciers werefound in the high ranges in all of the mountainous western states(Pierce 2004). These glaciated regions generally contain abundantdeposits of sand and gravel, although glaciated areas exist wheresand and gravel is absent or is covered with sufficient fine materialto make discovery or exploitation uneconomical.

As glacier ice melts, rock particles that had been crushed,abraded, and carried by the ice are further transported, abraded, androunded by meltwater. These deposits are potential sources of sandand gravel that are of great importance for use as aggregate. Theparticle size in the glacial meltwater deposits ranges from boulders

Construction Sand and Gravel 161

to sand, silt, and clay, and can change abruptly, both with depth andlaterally, especially where the material was deposited under the ice(eskers, for example) or on or near the ice (moraines or kames, forexample).

In some areas near the ice margins, large lakes were formedwhen meltwater was trapped between the ice and higher ground.For example, Lake Agassiz covered an area of about 365,000 sq mi(945,000 sq km) in Manitoba, Ontario, and Saskatchewan in Can-ada, and North Dakota and Minnesota in the United States; and theGreat Lakes were once much larger than at present. Deltas formedwhere glacial meltwater streams emptied into these and numerousother smaller temporary lakes. These glacial lake deposits areimportant sources of high-quality aggregates.

Pluvial LakesIn the Great Basin of the United States, a large number of closedbasins exist. During the cooler weather that prevailed in that areaduring glacial times, precipitation was greater than it is today, andwater did not evaporate as rapidly. Many closed basins containedlarge lakes, such as Lake Bonneville (of which Great Salt Lake inUtah is a remnant) and Lake Lahontan (of which Pyramid Lake andWalker Lake in Nevada are remnants). Fifty or more other lakeshave since disappeared. Sand and gravel was deposited along theshorelines of some of these lakes. The old shorelines of formerLake Bonneville are a conspicuous landform at Salt Lake City.These types of beach deposits are a large potential supply of sandand gravel resources.

Marine Beaches and TerracesDuring periods of glacial retreat, much of the glacial ice melted andthe water returned to the oceans, causing the sea level to rise. Someinland areas of today were once coastal beaches or terraces, such asstrandlines along the Atlantic and Gulf coasts. These are importantsources of sand and gravel, although the gravel in some of the olderdeposits has deteriorated due to prolonged weathering.

Marine Offshore DepositsIn Europe, Japan, and elsewhere, marine (offshore) sand and graveldeposits are economically exploited for use as aggregate. Marinesand and gravel deposits are located on the continental shelf ofNorth America along the Atlantic, Pacific, and Gulf coasts. Gravelis most common offshore from Canada, New England, New York,and New Jersey on the Atlantic Coast, and from Canada, Washing-ton, and Oregon on the Pacific Coast. Gravel occurs infrequently insouthern coastal areas of the United States.

Some sands obtained from U.S. federal waters have been usedfor beach renourishment, and sand and gravel has been recoveredfrom dredging projects. Otherwise, marine deposits have generallynot been commercially exploited in the United States.

Alluvial DepositsIn many states, alluvial (river) sand and gravel, either in the chan-nels or floodplains of rivers and streams, or in terraces found along-side the rivers or streams, are the principal sources of sand andgravel. Alluvial sand and gravel deposits are products of bedrockerosion and the subsequent transport, abrasion, and deposition ofthe particles. The availability and quality of the gravel is stronglydependent on occurrence and properties of nearby bedrock sources.

If a river or stream changes gradient and downcuts its channel,the older channel and floodplain deposits may be preserved as riverterraces. Repeated downcutting can result in a series of terraces orterrace remnants above the level of the modern stream base, whichcan be sources of sand and gravel. Older terraces may be exposed to

prolonged weathering, thus weakening the material and reducing itssuitability as aggregate.

Alluvial FansIn mountainous arid and semiarid regions of the western UnitedStates, rock fragments are eroded and transported during stormsdown steep-gradient streams to the adjacent basins. Upon reachingthe basins, the suddenly reduced sediment capacity of the watercauses deposition, resulting in alluvial fans. These deposits com-monly contain thick unconsolidated material ranging from largeboulders to clay-size particles. Generally, the largest material isdeposited adjacent to the mountains and becomes progressivelyfiner toward the downstream edge of the deposits. Over time, fansfrom adjacent valleys can coalesce to form continuous, thick depos-its. Sand and gravel in alluvial fans may be suitable for aggregate,but much of the material is poorly stratified and poorly sorted. Inolder fans the gravel may be highly weathered and not suitable foraggregate. In addition, fan gravels may be cemented with caliche, acalcium carbonate precipitate in the soil, making them difficult toextract and process.

In Situ Weathered RockIn some areas physical or chemical weathering can disaggregaterock in place, resulting in a coarse lag material that sometimes is

Figure 1. Generalized distribution of sand and gravel in the conterminous United States

Figure 2. Generalized sand and gravel regions in the conterminous United States

Glaciated Areas

Areas ofAlluvial Fans

Areas of PaleoBeaches and Terrraces


used as sand and gravel. Rock types that are particularly suscepti-ble to this type of weathering include conglomerate, sandstone,and coarse-grained granitic rocks. In some situations, the lag grav-els may contain considerable fine material and require additionalprocessing.

PROPERTIESSome properties, such as mineralogy, density, and porosity, areinherent in sand and gravel particles. Other properties, such as size,shape, and sorting, are acquired as the result of processing thedeposit. The properties discussed in this chapter are the inherentproperties.

Sand and gravel is always of secondary origin and commonlyreflects the petrology of the local rock types. An exception occursin glaciated terrains where deposits can contain exotic rocks trans-ported long distances by ice. Sand and gravel deposits may be com-posed of materials in which a single mineral dominates or mayconsist of a wide variety of rocks and minerals.

The properties of sand and gravel result from the petrology ofthe source rocks, the method of transport and deposition, and thesubsequent weathering of the aggregate particles (Langer 2001b).Sand and gravel should be durable and strong, which means theyshould support the intended load and resist mechanical breakdownresulting from the action of mixers, mechanical equipment, and/ortraffic. Particles should be sound, which means they should be ableto resist weathering, such as repeated freezing and thawing or wet-ting and drying. Sand and gravel should be composed of clean,uncoated particles of proper size and shape for the intended use.Sand and gravel commonly meets these requirements, because thenatural abrasion processes associated with the formation of mostsand and gravel deposits tends to eliminate weaker particles.

The quality of the aggregate is defined most often by its con-formance to specifications set by the user. Because aggregate com-monly is used in highway construction, those specifications areusually set by state departments of transportation. Most states con-form to the grading and testing standards of the American Associa-tion of State Highway and Transportation Officials (AASHTO) orthe American Society for Testing and Materials (ASTM).

Physical PropertiesThe producer has little control over certain properties of individualparticles in a deposit. These are particle size, shape, strength, spe-cific gravity, porosity, and petrology. Size and shape can be modi-fied if the gravel clasts are sufficiently large and can be processedby crushing and screening. If the properties do not meet local spec-ifications, the deposit probably is not economically exploitable.

Particle Size and Size DistributionAggregate for most construction applications can be prepared fromsand and gravel deposits containing a wide range of particle sizes.Particle-size distribution of sand and gravel deposits can be quitevariable, both laterally and vertically. The larger the gravel-to-sandratio, the better the deposit (Langer and Knepper 1998), except inareas where crushed stone is abundant and natural sand is lacking.Silt and clay occurring either as layers, interstitial material, or ascoatings on larger clasts are undesirable. Silt-sized or smaller fineparticles generated from crushing gravel are also undesirable.

Particle ShapeThree techniques are commonly used to describe particle shapes ofnaturally occurring gravel. One technique, commonly used by geol-ogists, compares the lengths of three diameters of the particle anddescribes them as equidimensional, disk, blade, or rod shaped(Langer and Knepper 1998). Another technique, commonly used by

engineers, utilizes a proportional caliper to determine the ratio of themaximum-to-minimum dimensions. A third technique visually esti-mates shapes and describes them using terms such as rounded, flaky,and elongate (Marek 1991). Particle shapes tend to be beneficialwhen the predominant shape is equidimensional and detrimentalwhen the predominant shape is disk, blade, or rod shaped (Langerand Knepper 1998). Particles can also be described based on theangularity of their edges and can be divided into a number of classesfrom round to angular. The desired roundness or angularity dependson use.

Specific GravitySpecific gravity is the ratio of the mass of a given volume of aggre-gate to the mass of an equal volume of water. Very low specificgravity frequently indicates aggregate that is porous, weak, orabsorptive; high specific gravity generally indicates high-qualityaggregate. Bulk specific gravity is the ratio of the weight of a givenvolume of material, including all voids, to the weight of an equalvolume of water. Apparent specific gravity is the ratio of the weightof a given volume of material, including all impermeable voids, tothe weight of an equal volume of water.

Porosity and Pore StructurePorosity is the percentage of the total volume of a gravel particleoccupied by pore spaces. Pore structure is the size, shape, volume,and interconnectedness of the spaces within an aggregate particle.Pores can be impermeable (isolated, enclosed cavities) or perme-able (interconnected and connecting to the surface of the particle).Gravel particles with high permeability are not desirable for mostapplications because they absorb large volumes of water or saltsolutions, thus reducing soundness. For bituminous mixtures, highpermeability also increases the absorption of binder, thus increasingthe cost of the paving mixture (Barksdale 1991).

An approximate inverse correlation exists between aggregatequality and rock porosity. Porosity affects the strength and elasticcharacteristics of aggregate, and may influence permeability,absorption, and durability. Rock with water absorption of 2% orless will usually produce good aggregate, whereas otherwise suit-able rocks with a water absorption that exceeds 4% may not (Smithand Collis 2001).

Chemical PropertiesThe chemical properties of sand and gravel vary from deposit todeposit and within clasts, and commonly are identified through pet-rographic analysis. They are primarily controlled by the mineralogyof the individual particles. Most sand and gravel is considered to beinert, but it may contain minerals that adversely react with portlandcement or bitumen and affect the product life.

Weathering and ImpuritiesSand and gravel should not be excessively weathered. Weatheringof gravel clasts lessens the strength of aggregate and increases theoverall cost of mining and processing. Once sand and gravel hasbeen deposited, it is subjected to natural weathering processes. Therate and type of weathering depends on the local climatic condi-tions, the geometry of the deposit, the relationship to the watertable, and the properties of the sand and gravel clasts. Weatheringcan range from slight discoloration of the clasts, through the intro-duction of fractures and alteration of minerals, to complete decom-position and disintegration of the clasts. In arid and semiaridclimates, sand and gravel may be cemented with caliche, a calciumcarbonate, and may be difficult to extract and process. In humidregions, sand and gravel may be cemented or stained with iron ormanganese dioxide. Sand and gravel must be free of objectionable


rocks, such as shale, mica schist, coal, and gypsum, and of rootsand other organic material.

PROSPECTING, MINING, AND PROCESSINGSand and gravel must physically be able to be mined and be accessi-ble to transportation systems and to markets. The site must qualifyfor all necessary land use and environmental permits. The operationmust be profitable considering all costs, including acquisition, oper-ation, compliance with regulations, and reclamation (Dryer 1976;Banino 1994). These requirements usually make opening a newoperation a complicated process that involves substantial cost andcan take many years.

Prospecting TechniquesThe U.S. Bureau of Mines Information Circular 6668 (Thoenen1932) was one of the first comprehensive publications to describesand and gravel prospecting techniques. The 3rd edition of Indus-trial Minerals and Rocks (Lenhart 1960) and all subsequent edi-tions contain discussions on prospecting for sand and gravel. Dunnand Cutcliffe (1971) include socioeconomic factors as part of theexploration process. A chapter in The Aggregate Handbook (Dunn1991) and a four-part series of articles in Rock Products (Timmons1994, 1995) describe many aspects of both exploration and charac-terization of aggregate resources.

Exploration for sand and gravel has become more than simplylocating a source of suitable material and commonly is based on theprinciple of the weakest point (Dunn 1991). During exploration, aweakest point exists that should be analyzed before proceeding toother elements of exploration. For example, it may be unwise tointensely study sand and gravel deposits in an area where frag-mented land ownership makes acquisition difficult. It would also beunwise to thoroughly address all permitting requirements beforedetermining some estimate of the quality of a deposit. Thus, explo-ration requires judgment and experience in order to proceed in alogical sequence. Each weak point should be resolved to an accept-able level of risk before proceeding to the next weakest point (Dunn1991).

When assessing preliminary target areas for sand and gravelexploration, economic and social factors such as markets, transpor-tation options, current land use, zoning regulations, and propertyownership should be considered. Public hearings commonly areheld before governmental bodies such as county or town boards,and the public and governmental views toward aggregate produc-tion must be carefully analyzed (Dunn 1991).

Exploration for sand and gravel deposits within target areascommonly begins with desktop studies utilizing existing data.Detailed geologic maps and cross-sections for large parts of theUnited States and elsewhere can be used to help locate sand andgravel deposits. Hydrologic reports of alluvial areas frequentlycontain logs of wells and test holes. In glaciated terrain, geolo-gists commonly can readily identify typical glacial features thatare likely to contain sand and gravel by using topographic mapswith a contour interval of about 5 m or less, and large scale (about1:50,000 or larger) aerial photographs. Geologists can also iden-tify stream terraces, floodplains, alluvial fans, and beach depositsin many nonglaciated terrains using the same type of maps orphotographs.

In developing areas or other locations where existing large-scale geologic information and other map data are inadequate,remote sensing data (satellite or airborne spectral imagery) and air-borne geophysical surveys may be useful for detecting sand andgravel resources. Knepper, Langer, and Miller (1995) reviewed anumber of techniques that have been successfully applied in locating

sand and gravel. These include the use of satellite data, in someinstances in conjunction with aerial photographs, to delineategravel-filled paleochannels (USGS 1981), buried gravel deposits(Peterson, Goodrick, and Melhorn 1975), and calcrete for use asroadstone (Henry 1989). Sand and gravel has been identified usingsatellite or airborne thermal infrared data to detect differences indiurnal (Carr and Webb 1967) and soil-moisture temperatures (Jack-son et al. 1978; Schmuggee, Jackson, and McKim 1980; Sabins1984). Knepper, Langer, and Miller (1995) also describe the applica-tion of airborne very low frequency (VLF) resistivity surveys tolocate exposed gravel deposits (Middleton 1977) and buried graveldeposits (Culley 1973).

Field studies commonly are conducted to check the veracityof desktop analyses and collect new data. These activities mayinclude a comparison of the geomorphology, as observed in thefield, with the desktop interpretations of the topographic and aerialphotographic analyses. Natural and synthetic exposures are investi-gated and sampled to determine the extent of the deposit, sand andgravel properties, and subsurface conditions. A geologist may alsoevaluate the site to identify potential environmental factors and toprepare plans for more detailed site evaluation.

Detailed Evaluation of DepositsIf reconnaissance studies indicate a target area worthy of furtherinvestigation, the next step is more detailed field studies. Acquisi-tion of more detailed information increases costs, and it is appropri-ate to assess the weakest point at this stage of exploration.Economic and social factors, such as current land use, zoning regu-lations, and property ownership, might be investigated in moredetail, particularly because it will be necessary to obtain permissionfrom landowners to access sites for further studies.

Systematic sampling should be conducted to determine theareal extent, thickness, stratigraphic variation, and physical proper-ties of deposits. Test pits, truck-mounted augers, and other truck-mounted drills commonly are used to determine these factors aswell as to obtain samples to find out the properties of the sand andgravel. A power hoe can dig test pits or trenches about 4 or 5 mdeep. Truck-mounted augers have a capacity to drill to depths ofabout 60 m. Auger sizes range from about 6 to 60 cm, with the mostcommon sizes between 10 and 15 cm. Deposits of cobble and boul-der gravel are difficult for augers to penetrate and may require theuse of other drilling equipment.

The spacing of drill holes, test pits, and trenches, and remotesensing patterns are site specific and should be based on the pre-dictability and geologic continuity of the deposit’s critical charac-teristics as determined by qualified personnel experienced in suchprocedures (Dunn 1991; Timmons 1994). Sample spacingdepends on the desired level of detail and needed confidence, andtypically ranges from 30 m in highly complex areas to as much as500 m in large areas of very simple geology. Sampling plans maybe modified based on updated knowledge gained during ongoingsite characterization.

Electrical resistivity techniques were used to explore for sandand gravel prior to the late 1930s (Patterson 1937). Techniques usedtoday include direct current resistivity (DCR) soundings, timedomain electromagnetic (TEM) soundings, ground-penetratingradar (GPR), and seismic refraction.

Properly conducted surface geophysical surveys can provideinformation on the areal extent and thickness of the deposit, thick-ness of overburden, stripping ratios, depth to the water table, andcritical geologic contacts. They are appropriate for a quick locationand correlation of geologic features such as silt and clay lenses.Surveys can be run where closely spaced geological changes might


be undetected by drilling, such as areas of suspected buried chan-nels, and where other elusive, but important, geologic conditionsmay exist.

Geophysical surveys can be conducted to provide subsurfacedetail in areas where digging test pits, augering, or drilling areencumbered because of limited access. Landowners may prohibitinvasive exploration activities, such as drilling or digging test pits,or restrict vehicular access to existing roads. Augering, drilling, ordigging test pits might be unacceptable in some areas such as activecropland or forested land. Auger trucks or drill rigs may be unableto reach sample areas if the ground surface is soft, muddy, or cov-ered with water. The geologic properties of the target deposit mayconfound augering—for example, where deposits contain cobble-sized or larger clasts. Additional problems related to drilling aredescribed by Timmons (1995).

Geophysical surveys can be of value when performed early inthe field exploration program in combination with limited subsur-face investigations. Such surveys, when conducted prior to inten-sive drilling, can be used to help locate auger or drill holes and canreduce the number of test drilling sites. Geophysical surveys canprovide continuity between sampling sites in order to upgrade theconfidence of reserve calculations from probable reserves to provedreserves.

An experienced field geologist can determine whether or not ageophysical survey is warranted and which methods should beused. The spacing of geophysical survey patterns is site specific andshould be based on geologic characteristics of the site and the pre-dictability of continuity of the deposit’s critical characteristics asdetermined by personnel specially trained in such procedures(Dunn 1991; Timmons 1994). Survey plans commonly are modi-fied during site characterization based on knowledge gained duringthe sampling process.

Ellefsen, Lucius, and Fitterman (1998) tested the effectivenessof DCR, TEM, GPR, and seismic refraction using S-waves to char-acterize sand and gravel. Haeni (1995) similarly tested DCR andseismic refraction using P-waves. In most cases, useful geologicinformation, such as sediment thickness, was obtained with allmethods. Shortcomings include the presence of clay-rich soil limit-ing the effectiveness of GPR and the difficulties of P-wave seismicstudies used below the water table.

The results of geophysical surveys are limited because it is notpossible to translate geophysical data without correlative evidencenormally provided by boreholes. In addition, geophysical surveysgenerally provide a very broad guide to the characteristics of a sandand gravel deposit that must be refined by other means. Actual sub-surface information is desirable so that geophysicists have an ideaof what to expect when measuring resistivities. The presence ofconductive fluids in pores and of synthetic conductors such as fenceposts and pipes can create erroneous results.

All critical variations in the geologic characteristics of a sandand gravel deposit should be recorded. Geologic mapping systemat-ically documents field observations and is a valuable tool for plan-ning drilling and sampling locations (Dunn 1991), assessinggeophysical needs, establishing background data, and identifyingpotential environmental impacts associated with aggregate develop-ment. Geologic maps, cross-sections, and accompanying reports ofpotential sand and gravel operation sites commonly show the loca-tion and outline of the deposit, location of sampling sites and geo-physical surveys, thickness of the deposit, and descriptions ofmaterials throughout the deposit. Supplemental maps and reportscan also describe related issues such as the thickness of overburden,depth to groundwater, and geologic hazards. In many states, suchreports must be prepared by a licensed professional geologist.

The difficulty of calculating volumes of deposits has beengreatly simplified with the use of specialized mining and quarryingsoftware, the geographic information system (GIS), and computer-aided design (CAD) software. Usable volumes exclude areas of set-backs, access roads and other infrastructure, maintenance facilities,and so forth. Information obtained during geologic mapping andsampling can be entered directly into these programs, and resourcevolumes can be calculated using various algorithms. For peoplewho choose not to use computers, other methods exist to calculatevolumes of deposits (Peters 1987).

Gross reserve tonnage is calculated by multiplying the unitweight of sand and gravel by the estimated usable volume. The unitweights of sand and gravel varies widely from deposit to deposit andis sometimes arbitrarily considered to be 1.78 t/m3. The in-placedensity of deposits can be determined more accurately by weighingthe contents of an excavated known volume.

Net reserve tonnage is the percentage of the gross reserve thatcan be made into usable products. Excluded are overburden, finesand other waste, and uncrushable oversized boulders.

There are numerous methods for appraising sand and gravelpits depending on the purpose of the appraisal and the status of thepit (Paschall 1998; Evans 1995), but for industrial minerals such assand and gravel, economic and legal issues usually are far moreimportant than the geologic aspects of a deposit. Companies per-form detailed reserve estimates and appraisals for a variety of rea-sons, including sale or purchase of undeveloped resources orreserves, acquisitions and mergers, valuation of a mining property,accounting, financing, taxation, condemnation, mineral conserva-tion, and for internal planning. Customers or investors can precipi-tate an analysis of reserves by demanding proof of the existenceand quality of reserves before committing to long-term purchaseagreements.

MiningSite preparation starts with grubbing and stripping sufficient over-burden to access the resource. Often required by regulation, topsoilcommonly is separated from the overburden and stockpiled for rec-lamation activities. Overburden may be used to construct berms, bestockpiled, or sold. Soil or overburden should not be stockpiledover parts of the deposit where future extraction is expected, andfinal need for reclamation should be considered. Site preparationalso includes construction of access roads, fences, berms, haulroads, drainage ditches, culverts, settlement ponds, processing andmaintenance facilities, and other plant infrastructure.

Sand and gravel is mined from open pits and dredged fromunderwater bodies. In upland areas, such as high terraces and someglaciofluvial deposits, sand and gravel may occur as unsaturateddeposits. Where sand and gravel mining does not penetrate thewater table, the aggregate can be extracted by using conventionalearth-moving equipment.

In some areas, such as low terraces and glaciofluvial deposits,the pit may extend into the water table. In some geologic settings,wet pits can be dewatered by collecting the groundwater in drainson the pit floor and pumping the water out of the pit, or by isolatingthe pit from the groundwater through the use of slurry walls. Insome situations where the sand and gravel pits penetrate the watertable, such as floodplains or low terraces, the pit may be allowed tofill with water, and the operator may extract the material by usingwet mining techniques such as draglines, clamshells, bucket andladder, or hydraulic dredges.

Sand and gravel can be excavated directly from stream chan-nels or from the edge of stream channels. During times other thanflooding, aggregate can be skimmed from bars in channels or from


active floodplains. Stream channels may be temporarily or perma-nently diverted to allow for aggregate extraction using conventionalearth-moving equipment in the natural streambed. A variety ofmethods can be used to divert the channel, including the construc-tion of a new diversion channel, split-channel mining, and the con-struction of harvest pits (Colorado Division of Minerals andGeology 1998). In arid areas, some stream channels or washes onlyoccasionally have stream flow, and sand and gravel can be minedfrom the channels using conventional excavation equipment.

ProcessingAfter extraction, sand and gravel occasionally is used as-is, which iscalled bank-run or pit-run gravel; most frequently, it is processedbefore use. Processing may be as simple as a portable crusher andscreens or can be a highly automated, sophisticated procedure. Thereare numerous ways to configure equipment depending on the charac-teristics of the sand and gravel deposit and its final use. Figure 3 is aflow diagram showing the generalized processing steps.

Material to be processed commonly is transported from the pitface to the processing plant by conveyor or haul truck. The materialmay be processed to remove lumps of clay, which can plug process-ing equipment and contaminate the final product. Deposits contain-ing boulders may be fed through a “grizzly” screen to removeboulders too large to go through the crusher. The material is storedin a surge pile. A gate at the bottom of the surge pile releases a con-trolled amount of sand and gravel to a screen where the sand is sep-arated from the gravel.

Gravel follows one path through the process; sand followsanother. If the gravel is larger than about 1 to 1.5 in. (2.54 to3.81 cm), it is crushed and screened to separate properly crushedparticles that go through the screen (“throughs”) from those that goover the screen and back into the crusher (“oversized”). Throughspass over a series of screens to divide the gravel into specific parti-cle sizes. Gravel may be washed to remove fine particles. Convey-ors move the sized gravel to separate stockpiles awaiting sale. Sandis sent to a classifier where it is tailored for its final use (such asasphalt, portland cement concrete, or masonry sand). Waste finesare sent to a settling pond.

Procedures must be followed carefully when stockpiling andhandling the final product. The processing of aggregate requiressignificant effort and cost to prepare a product that meets exactingspecifications for grading, freedom from contaminants, and otherrequirements. Mishandling of aggregate can result in significantdegradation of the product (Rollings and Rollings 1996). Whenaggregate falls too far from conveyors onto coned stockpiles, whenit is dumped from trucks down a slope, or when equipment pushesit over long distances, the material can separate from a well-blended product into individual size fractions. Improper handlingcan also result in contamination of the aggregate with foreign mate-rial from underneath the stockpile.

Upon sale, the stockpiled material may be sold as a single-sizeproduct, or two or more materials may be blended to make a new,graded product. For example, sand and properly sized gravel maybe mixed in specific proportions to be used with cement to makeconcrete.

REGULATORY AND ENVIRONMENTAL CONSIDERATIONSRegulationsSand and gravel mining is permitted or controlled at the federal,state, and local levels of government by numerous governmentalagencies (Arbogast 2002). All states are subject to federal law,whether land within state boundaries is federal, state, or privately

owned. Indian tribal lands are considered sovereign and are subjectto federal law, but without state jurisdiction or taxation.

Approximately one third of all land in the United States, mostof it in the westernmost conterminous states and Alaska, is ownedby the federal government. The agencies managing most of thoselands are

• Bureau of Indian Affairs• Bureau of Land Management• Bureau of Reclamation• National Park Service• U.S. Army Corps of Engineers

• U.S. Fish and Wildlife Service

• U.S. Forest Service

Figure 3. Diagram showing typical sand and gravel processing

Excavation

Haul orConveyor

PrimaryCrusher

Oversized

LandscapeBoulders

Throughs

GrizzlyScreen

SurgePile

Washing &Scrubbing

WetClassifying

FineScreeningDewatering

WashedSand

Products

Sand

SizingScreen

WaterSpray Oversized

ScalpingScreen

Product

Throughs

Oversized

SecondaryCrusher


The General Mining Act of 1872 encouraged exploration offederal lands for mineral resources, including common minerals. In1955 Congress removed common construction materials (termed“salable minerals,” including sand and gravel) from the mining lawand made them available through a contract or bidding process atthe discretion of the land management agency (Arbogast 2002).

The Water Quality Act of 1965 and the Federal Water Pollu-tion Control Act Amendments of 1972 (retitled the federal CleanWater Act) were enacted to provide efficient programs of water pol-lution control. Every major point source (a confined conveyancesuch as a pipe, drain, or ditch) from which pollution is dischargedinto U.S. waters requires a joint federal–state permit (Arbogast2002).

The Air Quality Act of 1967 (amended by the Clean AirAmendments of 1970) gives states and local governments theresponsibility to develop and implement plans to address airbornepollution at its source. Sand and gravel mining activities that arecovered include dust and exhaust emissions (Arbogast 2002).

Other federal regulations indirectly control the production ofaggregate resources through numerous acts, including the Fish andWildlife Resource Management Act, the Fish and Wildlife Coordi-nation Act, the Migratory Bird Treaty Act, the Endangered SpeciesAct, including the Rivers and Harbors Act, the Coastal Zone Man-agement Act, and the National Environmental Policy Act. Theoperation of sand and gravel mining is controlled at the federallevel by the Mine Safety and Health Administration (MSHA).

Each of the 50 states has a different process to obtain permis-sion to extract minerals. Some states issue permits for aggregatemining; others leave that authority with local agencies. The result isinconsistent policies among states and between the states and thefederal agencies. Commonly, state agencies enforce federal regula-tions such as air quality, water quality, diversion or impoundment ofsurface water, and withdrawal of groundwater.

In a review of various state mining laws, Arbogast (2002) foundthat state laws related to mining vary greatly. At least 37 states regu-late non-coal surface mining on a statewide basis. Thirty-fivestates require some sort of bond or security from the operator. Atleast 26 states provide for public comment at permit review. Somestates have no legislation to regulate sand and gravel mining. Inother states, sand and gravel mining is regulated at the local level.

The federal government encouraged state and local govern-ments to create zoning codes through the Standard State ZoningEnabling Act of 1922. The authority of governments to enforcezoning regulations was upheld by the Supreme Court in 1926 in thelandmark case of Village of Euclid, Ohio v. Ambler Realty. Sincethen, every state has enacted zoning legislation. Many, but not all,states transfer zoning authority to county or municipal authorities.Some rural areas in the United States, however, remain unzoned.

The United States is made up of more than 3,000 counties andmany more local governmental authorities. Counties and other localgovernments may or may not have final permitting authority forsand and gravel mining. Even where local governments do not havethis authority, they commonly influence the final land-use decisionby controlling land-use activities such as ground disturbance, grad-ing, noise, traffic, aesthetics, storm water, erosion and sedimenta-tion, land use, building codes, hours of operation, and regulation ofutilities. Approval of sand and gravel mining is frequently contin-gent on these issues (Arbogast 2002).

Environmental ConsiderationsTwo reference documents dedicated to the aggregate industry thataddress environmental impacts from aggregate mining are byBarksdale (1991) and Smith and Collis (2001). Five comprehensive

collections of individual papers that describe many issues related toaggregate development are the International Association of Engi-neering Geology (1984), Kelk (1992), Lüttig (1994), Bobrowsky(1998), and Kuula-Väisänen and Uusinoka (2001). These papers, aswell as information from other journals, are summarized by Langer(2001a). Langer, Drew, and Sachs (2004) describe potential envi-ronmental impacts from aggregate extraction and methods to con-trol those impacts. A study of these reports provides anunderstanding of the many different environmental impacts relatedto aggregate mining and also gives a historical perspective of theissues.

Sand and gravel deposits commonly occur in areas that are alsofavorable for other land uses. Frequently, urban growth occurs with-out any consideration of the resource below or any analysis of theimpact of its loss. Prime aggregate resources are precluded fromdevelopment if permanent structures such as roads, parking lots,houses, or other buildings are built over them. The value of theimprovements probably will permanently prevent any further devel-opment of aggregate at those locations. Such a situation is referredto as “sterilization” of the resource. New aggregate operations mayhave to be located long distances from the markets, and the addi-tional expense of the longer transport of resources must be passed onto consumers. Also, the new deposit may be of inferior quality com-pared with the original source, yet it may be used to avoid theexpense of importing high-quality material from a more distantsource. Any savings for aggregate may be offset by increased pro-cessing costs or decreased durability of the final product.

In addition to encroaching on aggregate resources, urbangrowth often threatens established aggregate operations. Some resi-dents in the vicinity of pits and quarries object to the noise, dust,and truck traffic associated with the aggregate operation. Many citi-zens do not support mining, in part because they do not recognizethe dependence of society on aggregate. Personal use of aggregateis very little, if any, and individuals may not recognize aggregatemining as a necessary land use, even though the need for the com-modity is constant. For these and other reasons, citizens commonlyprefer that stone and sand and gravel not be mined nearby (Langerand Glanzman 1993). This “not in my backyard” syndrome mayrestrict aggregate development. Furthermore, governments requirepermits, impose regulations, or establish land-use zones, some ofwhich may preclude mining.

Poulin, Pakalnis, and Sinding (1994) concluded that steriliza-tion, permits, and regulations restrict development or expansion ofaggregate production in established areas more than any actual lim-itations of suitable resource availability. The failure to plan for theprotection and extraction of aggregate resources often results inincreased consumer cost, environmental damage, and an adversarialrelation between the aggregate industry and the community.

Because sand and gravel mining is an extractive industry, itcannot be obtained from the landscape without causing environ-mental impacts, which are commonly engineering related. Theseimpacts, which include conversion of land use, changes to thevisual scene, loss of habitat, erosion, sedimentation, noise, anddust, generally receive the greatest public attention. Many of theseimpacts, however, are restricted to the mining site, start whenextraction begins, occur only as long as extraction and processingtake place, and are easy to predict and control using standard engi-neering techniques.

The extraction of sand and gravel from river and stream ter-races, floodplains, and channels commonly attracts additionalattention because it may conflict with other resources such as fish-eries, aesthetic and recreational functions, or with the need for sta-ble river channels. The floodplains and channels of many rivers and


streams can accommodate the removal of some portion of sand andgravel without creating adverse environmental impacts, providedthat the mining activities are kept within the limits set by the naturalsystem. The principal cause of impacts from in-stream and near-stream mining is the modification of natural characteristics beyondwhat the system can tolerate.

ReclamationMining sand and gravel resources creates economic wealth, and thefacilities made from aggregate improve the quality of human life.Reclaiming the mine site and associated areas may also create addi-tional wealth and improve the quality of life. Reclamation is amajor step in environmental stewardship.

Arbogast, Knepper, and Langer (2000) provide a review of theliterature for actual and proposed reclamation sites. In the expand-ing suburban areas of today, mined-out aggregate pits and quarriesare converted into second uses, such as wildlife habitat, recreationalareas, agricultural areas, parks, school grounds, high-quality lake-front housing sites, and a myriad of other land uses. A plan forreclaiming the disturbed land and its ecosystem should be a part ofevery plan to mine natural aggregate.

There is a growing appreciation for the reversionary value ofsand and gravel operations. Reclamation is becoming a major factorin sustaining the environment and in creating habitat biodiversity.And the need for specific postmining land uses such as water storageis becoming an important consideration in mining. For example,local municipalities have condemned alluvial land and leased theland to aggregate operations for gravel extraction, with the end goalof creating water storage reservoirs. The postmining land use isbeing offered as justification for gravel extraction permits.

OUTLOOK AND FUTURE TRENDSThe U.S. Geological Survey (USGS) calculates future sand andgravel production using a conservative value of 0.5% increase peryear. At this rate, sand and gravel production would exceed 1.25 bil-lion tpy by 2025. Even with an increasing market, the USGS antici-pates an increasing gap between crushed stone and sand and gravelproduction with a move toward crushed stone (Tepordei 1997).

In the last few decades, the process to permit new sand andgravel reserves has become increasingly difficult, lengthy, andexpensive. Regulations, more than actual resource availability,restrict development or expansion of aggregate in established areas(Poulin, Pakalnis, and Sinding 1994). This trend, fueled byincreased citizen opposition to disruptive land-use practices, islikely to continue. One area of particular concern is the in-streammining of sand and gravel because of its potential for widespreadenvironmental impacts; opinions differ on which impacts are theresult of mining or other land uses.

The industry trend is toward large businesses with large opera-tions and large output. Acquisitions of such companies are expectedto continue, especially as a means to obtain their permittedreserves. Increased costs, combined with local opposition to aggre-gate operations in populated areas, will likely force the location ofnew operations into more remote localities. These large centralizedplants will utilize rail, barge, or ship transport and will service oneor more urban distribution centers. The automation of processingplants and bulk material handling systems will allow for larger pro-duction rates at reduced costs.

Imports from Canada and Mexico are likely to increase, butwith strong environmental controls to offset the concern of export-ing environmental problems from the United States to Canada andMexico.

Integrated software for permitting, reserve calculations, minedesign, mine operation, stockpile management, scalehouse opera-tions, and reclamation have become available at reasonable costs.Machine guidance systems are available that use global positioningsatellites to determine the position of excavating equipment withcentimeter-level accuracy.

Silicosis is a potential environmental health issue that facesthe sand and gravel industry. This disabling, sometimes fatal lungdisease afflicts workers that are overexposed to respirable crystal-line silica. Silicosis is an occupational disease that is not known toaffect the general population. Possible measures to address theissue, in addition to dust control, include employee training on sil-ica hazards, periodic medical exams of potentially silica-exposedworkers, and exposure monitoring (Bailey and Sharpe 2003).

There is a slow but inexorable move toward implementation ofsustainable resource management principles and best managementpractices by sand and gravel producers, particularly large U.S. andmultinational companies. Sustainable aggregate resource manage-ment can be achieved in a practical sense by adopting the followingsimple principles (after Plant and Haslam 1999).

Planning, project design, approvals assessment, and site con-ditions should

• Maximize the economic value of the resource, by extractingas much material as possible from the disturbed area and forthe most economically valuable use it can accommodate

• Minimize waste of the resource, by avoiding high grading(picking the best parts of the resource and spoiling the abilityto extract the remainder) and by finding uses and markets forall of the disturbed material (e.g., turning crusher fines into“manufactured sand,” thus reducing the need for natural sandsources in more environmentally sensitive areas; and blend-ing of lower-quality with higher-quality material as long asproduct specifications can be met)

• Minimize social and environmental impacts, by planning thatprotects important resources from urban encroachment andprotects growing communities from the nuisance impacts ofpoorly designed, poorly located, and poorly managed aggre-gate operations; by using best-practice designs and operationsto control the effects of blasting, noise, dust, sediment erosion,and visual scarring in extractive and transport operations; andby providing for conservation of natural areas by managementof buffer areas that maintain or enhance vegetation, wildlifehabitats, and corridors

• Maximize rehabilitation of disturbed areas, by allowing forreclamation as part of the quarry/pit design process beforeextraction begins; by starting rehabilitation from day one; andby being flexible enough to allow advances in technology andfor changing local needs

• Maximize community engagement, by involving the localcommunity in planning activities through open visit days andcommunity awareness and educational activities. (This maylead to a measure of community acceptance and a “sociallicense to operate,” which can be just as important as theofficial legal permits.)

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