P&Q University Lesson 2- Site Selection & Plant Design

By |  May 16, 2015
When selecting a site for a new aggregates operation, many factors must be considered. Photo: iStock.com/Lisa-Blue

When selecting a site for a new aggregates operation, many factors must be considered.
Photo: iStock.com/Lisa-Blue

Our nation’s roads, bridges, buildings and other construction projects are created with aggregates. Natural aggregates are composed of crushed stone and sand and gravel. Igneous, sedimentary, and metamorphic rocks are used to make crushed stone and are extracted by open pit and underground mining techniques. Sand and gravel can be mined by open pit, cut and backfill, or dragline methods. Aggregates are encapsulated by cement and asphalt, and then utilized for construction purposes in asphalt and concrete for roads, bridges, buildings and dams. Non-encapsulated aggregate is used for a variety of products, including road base, erosion control stone, railroad bed stone and armor stone (rip rap for shorelines). Other aggregate uses include high calcium limestone for flue gas desulfurization, cement, roofing products, agricultural use and other specialty products.

Selecting a greenfield site (unpermitted quarry or sand and gravel location) depends on one key factor – the location. Several other factors make a location desirable, but most importantly it depends on the current market’s supply and demand needs for construction materials. Transportation costs are vital to supplying a market area. Transportation is not economically viable over great distances and must be evaluated against appropriate geological materials, competitor locations and permitting restrictions for greenfield sites. A professional geologist with aggregate exploration experience is necessary in locating and proving a deposit for aggregate use.

All stone or sand and gravel underlying a property may not be usable for aggregates. Some rock types do not meet the required physical specifications. For example, the stone could be too soft or break down due to freeze and thaw conditions. In addition, the overburden (soil and waste rock) overlying the stone could be too thick to mine economically. Deposits underlying potential sand and gravel properties could have too much clay or silt to be utilized as an acceptable aggregate. Therefore, understanding the geology and the end-use of aggregate products is essential.

Lastly, permission to mine a property is regulated by numerous agencies. Most city and/or counties have zoning restrictions, which require changing zoning classifications to accommodate a mine site. Additionally, permits for air, water and land disturbance from state agencies are typically required for mining and several other permitting issues require federal approval. For example, the U.S. Army Corps of Engineers and EPA regulate wetlands. In addition, the U.S. Fish and Wildlife Service list threatened and endangered species.

IDENTIFY MARKET AREAS

While local market factors are the primary driver for demand of aggregate materials, the local geology is of significant importance. There are many places within the U.S. where the geology is ideal, with plentiful reserves and material that can meet a variety of construction material specifications. However, if such a site is located far from a market it will be difficult, if not impossible, to develop an economically viable site. Conversely, there may be a market for construction materials, but if available potential mining sites are underlain by geology that will produce marginal materials, customers will likely seek out alternative sources. An additional aspect in identifying sites for aggregate mining is determining the demand for the different types of construction materials. For example, if the market area is in a residential construction boom then there will be increased demand for concrete aggregate. Concrete requires a component of sand and if no natural sources are locally available then it will be necessary to use manufactured sand, increasing processing costs. Chemical grade stone, high in calcium or magnesium, is sourced from carbonate rocks (limestone and dolostone). Identifying carbonate rock sources that meet stringent chemical-grade specifications will severely constrain the number of potential sites for such a resource. Aggregate used in asphalt requires a certain threshold of skid resistance. For that kind of material, rocks with harder minerals such as quartz make a better skid-resistant aggregate. Softer carbonate rocks are not necessarily ideal for such applications, but aggregate testing may find that these materials meet the necessary specifications for secondary roads or aggregate base.

Population growth and future demand projections are useful to determine what areas may demand a future aggregate supply.  Many of these estimates are available from state commerce departments along with the U.S. Census Bureau. Typically larger growth is along major interstate corridors and expansion of existing cities. State Departments of Transportation utilize these estimates to plan for future road improvements and additions. Transportation Improvement Programs (TIPs) for states are typically projected for 10 years and can be utilized to identify needs and shortages for future aggregate demand. Identifying existing aggregate producers and their estimated life is critical to determine future demands, as well, and whether the market can accommodate another quarry.

REGIONAL EXPLORATION

Exploration is the most essential phase of the site selection process after the market area has been identified. This evaluation determines the viability of the site for development as an aggregate resource. Much of the exploration phase involves research on geologic formations, deleterious mineralogy, hydrology and any potential quality issues – all of which can influence the suitability of the aggregate material. These data, along with parcel and zoning information from county tax offices, can be compiled in a geographic information system (GIS) that can rapidly assist in finding ideal aggregate sites.

Photo by Kevin Yanik

Photo by Kevin Yanik

GEOGRAPHIC INFORMATION SYSTEMS

Identification of new sites for aggregate mining has been made easier due to the availability of digital mapping datasets, most of which are available online. This data consists of mapping products such as roads, political boundaries, parcels, waterways, aerial/satellite photography and, yes, geological information. The utility of these datasets is that they are georeferenced. In other words, there are geographic coordinates assigned to the data. So if you have several datasets, all of which are georeferenced, you can overlay them on one another and they will be positioned to the correct location based on their geographic coordinates. Software that allows this type of layering of geographic data is referred to as GIS. The information portion of GIS provides much more than just the geographic location. It also provides data about that object. A road network would not only have lines georeferenced to real-world spatial coordinates, but there would be information assigned for each road segment. This could be as little as the highway designation (Interstate, U.S. Highway, etc.) or contain detailed information such as road width. With such information in a GIS database users can then query or search the database for geographic information that meets a set of criteria. This becomes a powerful tool when you seek to identify aggregate resource sites that need to meet several geographic criteria. Throughout the next section several examples of using GIS data are illustrated. There are a variety of GIS software packages, some commercial and some following an open-source model where the software is available for minimal or no cost. Some of these software packages have fairly steep learning curves, so additional educational resources may be necessary to navigate such specialized software. The industry standard, ArcGIS, is utilized by most government agencies and there is a great deal of online support and technical help that is available for this package. Other commercial GIS software includes TECHbase, Manifold GIS, MapInfo and AutoCAD Map. Examples of open source software that have wide community support are GRASS GIS, SAGA GIS and QGIS. Any of these GIS software packages have the basic functions necessary to construct a GIS database for aggregate resource exploration.

Assessment of potential aggregate resource sites requires the assembly of several different types of GIS data. This type of information is critical in identifying candidate sites for exploration and possible development.

GEOLOGY

Geology is a primary driver in determining the location of an aggregate resource. Obviously, some rock types are preferred over others depending on the end-use application of the aggregate material. A regional understanding of the geology of the market area of interest is a good “first pass” at rapidly determining what areas will be suitable with regard to aggregate quality. Most states in the U.S. have published geologic maps at the state scale. These maps are good for understanding the rock types for a region, but should be used with caution at scales that are more localized. Many of these state maps are at a scale of 1:500000 in which one inch is approximately eight miles. Higher detail geologic mapping is always desired and scales of 1:24000 (1 in. = 2,000 ft.) are generally ideal for exploration work. There may also be surficial geologic maps published, which document the types of surface unconsolidated sediment overlying bedrock. Exploration programs that seek sites for sand and gravel pits would benefit tremendously from such surveys. The problem is that 1:24000 scale mapping is spotty for much of the country and what is available is dependent on the location of the market of interest. In some instances, there may not be detailed mapping available and coarser-scale geologic maps may be the only resource available. In such examples, field work is a critical component of the exploration process and may require significant field time to assess the geology at the local scale. Beyond the geologic map, valuable geologic data may be available in state geologic survey reports, scientific journals and federal geological survey assessments. Fortunately, Internet searches can quickly identify such resources. Some of these may be available online, but many of the older reports may have to be ordered as paper copies. In other cases, the reports may be out of print and a visit to a university library or the state geological survey office will be the only way to access those data. Ideally, there may be digitized geologic mapping data that can be directly imported to a GIS. If access is limited to paper maps, scanning and digitization will be a necessary part of building an exploration database.

In some states, there may be drilling information available that could be helpful to exploration efforts. Depending on the state, there may be bedrock drilling information that contains data on the rock types and depths encountered. Alternatively, many states retain logs from domestic water-well drilling. The format of these logs may be as simple as paper copies in an archive or the logs may have been scanned. In other instances the drilling data may be compiled in a database that could be loaded into a GIS.

Identifying a potential aggregate greenfield site also requires assessment of natural hazards. Many of these hazards may not be apparent during examination of geologic and topographic maps, but will require detailed site evaluations. One important hazard is that of slope stability problems. Areas with extensive faulting/fracturing and dipping foliations or bedding combined with topographic relief could result in slide hazards that will interfere with site development. Digital elevation models (DEMS) derived from topographic maps and LIDAR data (light detection and ranging) will allow determination of the topographic slope. This information, coupled with geological data collected in the field, will help to identify any areas of potentially hazardous slide conditions. Another geologic hazard that needs to be evaluated is the presence of any karst features such as sinkholes. This is an issue in areas underlain by carbonate rocks of limestone and dolostone. If, for example, a site for chemical-grade limestone is being evaluated, it is important to review any possible impacts to the groundwater system. If sinkholes are documented as a hazard then it is likely state geological surveys have reports on karst and may even have geospatial databases with the GPS locations for such features. Hazardous mineralogy may not seem like a significant issue, but naturally occurring asbestos should be in the forefront of any exploration effort if the local geological setting is conducive for its formation. Thorough review of geologic maps and reports is an essential first step in determining any risk for asbestiform minerals. Field reconnaissance should be mindful of the potential for these minerals as occurrences could be localized and not previously documented in the literature. In addition, exploratory drilling may encounter zones of asbestiform minerals in the subsurface. A careful logging of core and examination of cuttings will identify areas of concern for further analytical evaluation. Sand and gravel resource areas that drain exposed bedrock with the potential for asbestos are problematic as fibers could be disseminated throughout the deposit.

Photo by Kevin Yanik

Photo by Kevin Yanik

AERIAL PHOTOGRAPHY

Aerial photography and satellite imagery can provide an overall view of land use within a potential market and rapidly identify areas that are not currently developed. The proliferation of free aerial imagery resources has been rapid and probably the best examples are Google Earth and Google Maps. Low- to no-cost satellite imagery is easily accessible that is useful for regional-scale studies. The Landsat program is probably the best known, along with ASTER and SPOT imagery and other sources of multispectral imagery. For site-specific needs, higher-resolution options are available from commercial vendors such as DigitalGlobe. Municipalities may have periodic aerial photo acquisition programs and such updated imagery is useful for determining the current level of development in a particular area. The imagery can also be used to delineate potential natural hazards for further research.

PARCEL DATA

Parcel data that defines the location, size and ownership of properties is a very useful dataset to have available. These data are usually on a county-by-county basis, so for some areas you will still need to visit the county courthouse to review records. If you are fortunate enough to have parcel data online, it is of even more useful if the parcel layers in GIS formats can be downloaded or purchased. The value of having access to the GIS layers is that you can query the database and rapidly identify parcels that meet certain size or location characteristics. This is especially useful as parcels of 100 to 150 acres are the minimum area for development of an aggregate resource as there must be room for the pit, processing facilities and storage areas for saleable product, waste and equipment.

RESTRICTIONS

An ideal site location with regards to market, geology and available property may still not be viable if there are ordnances or restrictions that limit mining activities. Zoning regulations may require mining be limited to industrially zoned areas. In many cases, mining will fall under a special use permit in which an application must be submitted for approval to the local zoning board. Like the parcel data, zoning maps may be available as a GIS layer. The zoning in conjunction with the parcel information can further identify parcels that meet the zoning requirements that allow mining. Regardless of the mapping products available, thorough knowledge of local zoning is necessary. Within an individual parcel, there may be environmental and cultural regulations that will serve to limit any disturbances. Wetland areas can be identified with the National Wetlands Inventory. This provides a GIS data layer of recognized wetlands that are protected through conservation efforts. Other bodies of water such as lakes, rivers and small streams may require significant setback distances, which will decrease the amount of mineable property within the parcel. Furthermore, regulations with regards to endangered species should be reviewed for the area of interest.

UTILITIES

The location of power, gas, data and water/sewer lines should be identified prior to any site development. In some cases utilities may be relocated, though at significant cost, to facilitate exploration and mining efforts. There may be some public databases that contain the location of utilities, but security concerns have limited access to such information.

Photo by Zach Mentz

Photo by Zach Mentz

ROAD AND RAIL

Transport of aggregate materials to customers requires access to roads, rail or barge While most aggregate material is moved by truck over roads, rail or barge can transport aggregate at a lower cost. Such cost savings may allow for the aggregate resource to be developed at a farther distance from the market that would otherwise be uneconomical. The road and rail networks are usually available as GIS data layers from state departments of transportation. The roadway GIS layers will often have the different classes of roads assigned within the database, so you can determine whether the road is a two-lane county road or four-lane divided highway.

Site-specific exploration

Regardless of the availability of geologic maps, aerial photography and topographic information, there is no substitute for an initial, detailed field investigation. Once a candidate site has been identified and access to the property arranged with the landowner, field examination can begin. The equipment necessary for basic fieldwork is that for any geological field study: GPS unit, Brunton compass, basemaps, field notebook, hand lens and sample bags. The GPS unit is necessary to record the location of sites where surface samples are collected. You may even be able to construct a detailed geologic map of the property, depending on the quality of bedrock outcrops or exposures of surficial materials. The Brunton compass will allow for taking structural measurements of any bedding, foliation and faults that may be exposed. If surface exposures seem suitable and collected materials meet the basic aggregate quality specifications, a drilling program can then be planned to better understand material quality in the subsurface.

Before embarking on the subsurface phase of the exploration process, it should be stressed that drilling, trenching or geophysics should be done on appropriately spaced intervals. The spacing depends on the consistency of the formation and the characteristics required of the end products. For example, exploration drilling typically is done on wide-spaced centers and tighter drilling patterns are necessary for developmental drilling projects after permitting is completed. Tighter spacing is required to obtain accurate stripping volumes or specific whole-rock chemistry for selective mining.

TRENCHING

Trenching can be employed to locate shallow rock exposures for mapping and sampling, especially if selective mining is required for varying rock types. In addition, it can be used to identify overburden depths to the top of rock, to determine sand and gravel thickness, or to sample various lithologies for quality testing.

EXPLORATION DRILLING

Auger drilling is a method utilized to identify overburden thickness or sand and gravel thickness. It is a fast and economical method to evaluate a site’s viability. However, auger drilling for quarry sites alone is not recommended. Standard auger drilling corkscrews into the ground. turning the soils out for inspection. Hollow stem augers can be utilized with a split spoon sampling tube to collect soils in place to determine exact sample depths and thicknesses.
Sonic drilling is another method used to collect samples in place. This drilling method is utilized to sample unconsolidated soils and rock. It is very useful in sand and gravel exploration to obtain undisturbed samples with larger-diameter gravels.

Core drilling is a method to collect rock samples for rock-type confirmation, mineralogy, geologic structure and rock-quality testing. A diamond-impregnated bit cuts the rock, which is then transferred into an inner core barrel for collection with a wireline system. Core is then extracted and can then be evaluated by a geologist for weathering thickness, rock quality, rock types, mineralogy, fracture patterns, rock-quality index (RQD), recovery percentage and any other notable quality issues.

Air rotary drilling can be performed quickly and economically to evaluate overburden depths and rock types. This type of drilling pulverizes rock into small chips. Rock cuttings are usually useful in identifying rock type contacts. However, they are too fine to test for most typical DOT coarse aggregate test methods.

Example of a cross-section generated using the refraction seismic method. (Image courtesy of ESP Associates.)

Example of a cross-section generated using the refraction seismic method. (Image courtesy of ESP Associates.)

GEOPHYSICS

In some situations, drilling may be prohibitively expensive, or it may be difficult to get equipment into areas of the property of interest. Geophysical methods use techniques such as seismic, electrical resistivity, microgravity and ground-penetrating radar to image the shallow subsurface. The primary goal of these techniques in the aggregate resource assessment is largely to identify the thickness of overburden material and the location of zones of intense faulting or fracturing. It is also useful in identifying zones of deep weathering or contacts between varying rock type compositions for selective mining purposes.

Aggregate testing methods

Core samples for logging and sampling. (Photo courtesy of Vulcan Materials Co.)

Core samples for logging and sampling. (Photo courtesy of Vulcan Materials Co.)

Several physical tests are required to determine suitability as an aggregate resource. State Departments of Transportation have standard specifications for aggregate test methods and required test results. Core or stone samples should be tested for the following:

■ Los Angeles Abrasion or L.A. test, which tests the hardness or durability of the stone.

■ Magnesium or Sodium Sulfate Loss tests replicate the freeze/ thaw cycle of seasonal temperature changes.

■ Absorption is useful in determining the suitability for an aggregate source with asphalt. Typically, a stone with a high absorption suggests a porous stone, which usually has a high sulfate loss.

■ Specific gravity is required to calculate the tonnage of a proven resource. Lower specific gravity rock is preferred by concrete and asphalt producers due to the lower volume required for the same higher specific gravity rock, which makes it more economic for the producer.

■ Alkali-Aggregate Reactivity is a test that determines the suitability in high alkali cement for concrete use. Reactive minerals can still be utilized in concrete aggregate, but will most likely require fly ash or other additives to mitigate the alkali-aggregate reaction.

■ Flat and Elongated particle shape determines how the stone is situated within concrete and asphalt. Typically, a flat particle shape tends to float, giving a preferred orientation and lower concrete strengths. This method requires hand samples or quarried samples. The geologist can evaluate the core to determine if this might be a problem.

■ Petrographic Examination is required to evaluate a stone for mineralogical composition. This examination can assist with the identification of minerals that can contribute to alkali-aggregate reactivity potential and other mineralogical issues that could cause problems in aggregates or health issues relative to exposure to the public.

■ Compressive & Tensile Strengths are useful to aid in identifying rock-crusher liner types and concrete strengths.

■ Chemical analyses are useful for characterizing the chemical composition of resources that may serve as sources of chemical-grade stone. Customers may also require chemistry data for any aggregates supplied as a matter of standard protocol.

■ British Polish Test or other skid-resistance test methods determine the polishing characteristics for use in surface-treatment asphalt or concrete.

■ The presence of any shale, clay lumps or friable particles should be evaluated. Specifications are often very strict on the content of these materials in aggregate.

SPECIALTY PRODUCTS

Photo by Zach Mentz

Photo by Zach Mentz

Coarse aggregate from sand and gravel deposits should also be tested for the physical properties listed above plus bulk density and gradation. Concrete sand requires a specific gradation measured on percentages retained on various screen sizes. If too high a percentage of fines are present or if the sand is not well-sorted, then it may not be suitable for use in concrete.

Chemical analysis is a necessary evaluation for limestone to determine the calcium and magnesium concentrations.  Customers will have required specifications with regards to the proportions of calcium, magnesium and silicon. Chemical analyses can identify if the product is suitable for usage in soil stabilization, aglime, asphalt, cement, quicklime, hydrated lime or flue gas desulfurization. If end use includes aggregates for base, concrete or asphalt, then the stone should be tested for the physical properties listed above, as well. In addition, loss on ignition (LOI) will quantify the presence of any impurities.

Rip rap or armor stone should be tested for all of the standard tests mentioned above for aggregate usage plus the freeze/thaw test.

PERMITTING AND ZONING ISSUES

Finding and opening a greenfield site is difficult to accomplish. This is owing to zoning restrictions, which most counties have in place and will necessitate rezoning the property for mining. This usually requires a public hearing and is costly due to the various environmental and other types of impact studies required. Unfortunately, most zoning rulings are not decided based on technical merit alone. Zoning changes can be denied owing to significant public opposition compared to permits, which typically are based on technical merits only. Existing mine sites that do not have zoning restrictions and already have existing operating permits are obviously much easier to continue to develop.

Mining is perhaps the most regulated industry in the United States. Permits for air, water and land disturbance from state agencies are required for mining and several other permitting issues require federal approval. Before a company can begin mining, it must go through the rigorous process of obtaining a mining permit. The permit application process includes collecting baseline data to characterize the pre-mine environmental condition of the permit area. This work includes surveys of cultural and historical resources, soils, vegetation, wildlife, assessment of surface and groundwater, climatology and wetlands. In addition, reclamation plans are a required part of the process. The permits and required studies can be very costly and time consuming. Potential delays for acquiring a mine permit should be identified early in the process to limit unnecessary time and expense. Many states require prospecting and exploration permits to perform any exploration drilling.

Example of a glacial till gravel deposit collected utilizing a sonic drill, typically used for sand and gravel deposits. (Image courtesy of Cascade Drilling.)

Example of a glacial till gravel deposit collected utilizing a sonic drill, typically used for sand and gravel deposits. (Image courtesy of Cascade Drilling.)

State regulatory agencies require mining and reclamation plans with a generalized mine plan, a detailed sequencing for mining operation, and end-of-life land use. Sediment and erosion-control plans are required to identify how sediment will be prevented from polluting streams or other surface waters. Groundwater studies are required to determine if any offsite impacts are expected due to the mining operation.

The EPA issues National Pollutant Discharge Elimination System (NPDES) permits for the discharge of wastewater. Surface discharge permits are required for water discharge. Groundwater and surface waters that collect in mines then must be eventually discharged into nearby stream and drainages.

The Corps of Engineers regulates the disturbance of wetlands. It is best to create a mining plan with no or minimal wetland disturbance. Wetlands can be disturbed, however, there is a fee assessed per unit area of disturbance and typically requires mitigation. This involves creating more wetlands habitat of a greater acreage than was originally disturbed. If too many wetland acres exist, then it may not be economical to open the site. Therefore, evaluating the site for wetlands early in the exploration program is highly recommended.

The U.S. Fish and Wildlife Service regulates the Endangered Species Act. An endangered species existence can prohibit or restrict the site from mining. Therefore, research should be performed prior to significant site evaluation. If an endangered species exists on the site, the chances of permitting the site for mining are low, unless it is constrained to a small area of the subject property.

Another permitting issue includes investigation for historical land use. A State Historical Preservation Office may require a Phase I Archaeological Survey. Other studies such as blasting and traffic studies are typically required for rezoning.

Photo by Zach Mentz

Photo by Zach Mentz

FINAL EVALUATION

The viability for a potential mine site is determined by all of the factors listed above. Obviously, the most important aspects are the economics of the site location and aggregate quality. A financial model is necessary to determine the site’s economic viability, which addresses all of the above collected data and projects a rate of return on the investment. This model should be run and edited throughout the process, especially prior to attempting any rezoning. All aspects of the model should be shared and verified by all persons involved with input to ensure an accurate model.

ERI

Aggregate resource developers looking to exploit sand and gravel deposits may be working with less-than-optimal information if they rely too heavily on traditional sampling programs involving test pits and percussion drilling. This is particularly the case when it comes to complex granular deposits.

Percussive drilling does not always yield representative samples, as the return rate of the sample to surface varies, samples are highly disturbed, and may have been contaminated with drilling fluid. Test pits can give a good picture of the undisturbed resource, but they can rarely reach more than 20 to 25 ft. below the surface.

Perhaps the biggest difficulty with these sampling techniques is that they produce point measurements, and so are reliable only if there is good continuity through the deposit. More drilling and digging can increase accuracy, but this can get expensive – as well as being intrusive owing to noise, vehicle traffic, impacts on sensitive features and disturbance to existing land uses such as agriculture.

If extraction goes ahead based on plans generated by sampling alone, it may be difficult to extract the resource wisely and provide a reliable, predictable product to customers. Often, operators end up “chasing” the resource instead of planning around it.

HOW ELECTRICAL RESISTIVITY WORKS

A core drill in an active quarry. (Photo courtesy of
3D Dycus Diamond Drilling.)

Because of the limitations of test pits and drill samples, it may be wise to increase the accuracy of the aggregate depositional model and extraction plan, through a technique that can fill in the gaps.

A geophysical tool known as the electrical resistivity imaging (ERI) method has been used over the past few decades for aggregate resource evaluations, and is now a mature and widely accepted technology. It is important to understand its advantages and limitations for aggregate resource evaluation and planning.

ERI measures the level of resistance to electrical current of the subsurface to infer rock/soil types, stratigraphy and soil conditions. This is possible because:

■ Water that infiltrates the ground allows electrical current to flow through it more easily as it is highly conductive, whereas the minerals themselves are highly resistant to electrical flow.

■ Resistivity can also help indicate the degree of interconnectedness of the pores – poorly interconnected pore space means that the current must pass through the grains, so measured resistivity may be higher.

■ Resistivity is generally lower in materials where the grains themselves are polarized (like miniature compass needles), as is the case with clays.
These combined factors mean that higher resistivity generally indicates the grains in the soil are larger (as may be the case with loose sands and gravels), and lower resistivity indicates smaller-size grains (such as silt, clay or marl). Experience with various types of soil, and understanding the range of resistivity typically found with each material, helps trained specialists develop a picture of the depositional environment of the resource.

Electrical resistivity usually uses a four-electrode setup. Metal rods are inserted into the soil from the surface. In one of the most popular configurations, called the Wenner method, the distance between each electrode is the same. Current is passed between two of the electrodes, and the change in the potential field is measured between the other two electrodes.

If there is only a short distance between electrodes, the image of the subsurface that is generated will be shallower than it is if the electrodes are further apart. So, testing may start with short-distance measurements. Then, readings are taken from the same location with the electrodes further and further apart to build a more complete picture of the subsurface. Most modern ERI systems use arrays of 56 or more electrodes.

Two- or three-dimensional modeling software, depending on the type of testing completed, is used to convert the apparent resistivity profile, called a pseudosection, to “true” model resistivity profile. The result is information that, when combined, can be used to interpret soil conditions and to plan extraction.

LIMITATIONS TO ELECTRICAL RESISTIVITY TESTING

Members of the aggregate industry need to understand the limitations and advantages of the ERI method, and how to use it for effective planning. Four of the main factors influencing reliability of ERI results are:

■ Depth of deposit: Like trying to look through fog or mist – distant objects are hard to interpret – the resolution and accuracy of ERI data declines as the depth increases. This is because while electrodes that are further apart can provide measurements deeper into the deposit, the resolution is averaged over a longer distance. This means that changes in resistivity are less likely to be resolved at lower depths.

■ Transition zone of saturation: In the transition zone above the water table, electrical resistivity changes are mainly in response to changes in moisture content, rather than changes in grain size distribution. ERI works well above and below the transition zone, but is less accurate in the zone where the transition occurs. The thickness of the transition zone depends on pore size – the larger the pore size, the thinner the transition zone.

■ Clean, well-drained sands: Fine-grained sands that are “clean” in that they do not have large amounts of clay or silt can sometimes show high resistivity values that would otherwise be found in coarser-grained materials, possibly leading to misinterpretation.

■ Cultural features: Metal fences, buried scrap metal and underground pipelines close to the ERI test line can influence the results. It is important to map these “cultural features” and check their influence on the ERI results.

APPLYING ELECTRICAL RESISTIVITY

Photo by Kevin Yanik

Photo by Kevin Yanik

Being able to accurately map and then plan for extraction is particularly important in aggregate deposits that are complex. A seven-stage plan is recommended that has proven to be effective in greenfield site development, as well as in expansions of existing operations:

1. Test lines to ensure the method works at the site.
2. Electrical resistivity imaging/profiling program.
3. Limited drilling program.
4. Grain size and moisture content analysis.
5. Modeling the resource.
6. Defining areas of favorable extraction ratios and tonnage estimates.
7. Incorporation of results into a mining plan.

The integration of this data into a mining plan is the ultimate goal of the study, should the resource exist. When this occurs, the information should be used in conjunction with all historic data from previous site investigations. This can help determine blending requirements for areas with marginal resources, as well as for areas likely to have main extraction potential. This helps avoid future high grading of the resource on site. The ERI method has also been used effectively in sensitive due diligence type settings to provide potential purchasers with some level of confidence, prior to entering into a deal, that the resource is actually there. This is particularly useful in cases when showing up with a drill rig is not feasible at the initial stages of negotiations.

Wise planning, based on sound use of site data including information from the ERI process, can mean more accurate prediction of available resources to reassure corporate management, as well as external stakeholders, that the resource is there and will be used wisely. It also results in better and more profitable mining plans – and fewer surprises. Perhaps most importantly, tools such as ERI promote good resource management, so that issues such as high grading for short-term financial gain can be eliminated in favor of longer-term extraction planning that provides greater return on investment and good resource stewardship.

Plant design

There are many plant-engineering options to consider, both when setting up an aggregates plant from scratch, and when evaluating an existing plant in terms of its efficiency and profitability. Using quality components from the start often prevents problems later.

PROPERTY LOCATION

The purchase or lease of a new piece of property is one of the key factors in whether a project is going to be successful. A short list of some of the things that should be considered when searching for the property are:

■ Are the minerals rights available?
■ What type of mineral is on the property?
■ Quality and quantity of mineral.
■ Access to main roads, such as interstate highways, or railroad service.
■ Taxes.
■ Neighbors (who are they, how close are they, etc.)
■ Local labor available.
■ Union or non-union labor.
■ Availability of water.
■ Water table.
■ Easements.
■ Possible archeology site?
■ Markets – How close is the property to populated areas?

PERMITS

Photo by Zach Mentz

Photo by Zach Mentz

Once a piece of property has been located and it meets or exceeds expectations, it must be permitted for mineral processing. Getting a permit may be the most difficult step in setting up a plant.

The type of plant that is to be built on the property will have a direct impact on the permits that are required. That is to say, permits for a mobile crushing plant may be easier to obtain than permits for a stationary plant.

During the permitting process, there are various environmental agencies at the local, state and federal levels that will be involved in the process. There are various air, water and noise-quality standards that must be met. There are also many and varied state and federal zoning requirements. There may also be local governing bodies that have laws governing plant operating times and other restrictions, such as how high a face can be blasted (for a quarry) or how high a bank can be dug (for a gravel pit). If there is a stream, river or lake, there will be issues with run-off water. These are only a few things to keep in mind during the permitting process.

MARKET REVIEW

While searching for property, producers typically do a “market review” to see what the potential market is. Once the decision has been made to pursue the property, a formal market review needs to be undertaken. The information collected during the formal market review will be used in all aspects of the design of the new plant. Some of the pieces of information that need to come from the market review are as follows:

■ What products are desired (both dry and washed)?
■ How much of each product can be sold (defined by time of year)?
■ What are the local product specifications?
■ Are all sales by customer truck, or should delivery services be offered?
■ Can products be shipped by rail?
■ Selling price for all products.
■ Competition in the market area.

MINING PLAN

Another important step that is often overlooked is a “mining plan.” This mining plan can be simple or very complex, but it is a vital piece in the process of designing a new processing plant.

Photo by Zach Mentz

Photo by Zach Mentz

A formal mining plan forces an operation to look at a number of items that are sometimes overlooked, which could cost considerable money to fix in the future. Some of the items a mining plan should address are:

■ Survey of all material on the property.
■ Where is the best material located?
■ Is there a need to mix materials to produce a quality product?
■ Quarry/pit plan for the next three to five years.
■ What quarry/pit will look like after mining.
■ In-pit crushing or fixed primary?
■ Suggested locations for plant.
■ Access for customer trucks.
■ Access for rail.
■ Settling ponds (if required).
■ Water table.
■ Mining depth.
■ Location of high voltage power lines.

Once the mining plan has been established, it will also have to be determined if in-pit crushing is feasible. In-pit crushing can be used for quarried materials, as well as sand and gravel plants.

MINERAL CHARACTERISTICS

The mineral characteristics also play an important part in the decision as to whether or not the property is suited for aggregate production. The mineral characteristics will have to be analyzed to determine whether you can produce the materials your market review calls for. These characteristics will also help define the type of processing equipment that can be used. Some of the material characteristics that are required for making the proper decisions are listed below:

Photo by Kevin Yanik

Photo by Kevin Yanik

■ Material name.
■ Material source (quarry or gravel pit).
■ Specific gravity.
■ Bulk density.
■ Absorbtion.
■ Hardness.
■ Friability.
■ Compressive strength.
■ Chemical composition used to determine abrasiveness of material (requires at least the following: silica, iron oxide, aluminum and magnesium oxide).
■ Percent clay, dirt, etc.
■ Tramp material.
■ Moisture.
■ Feed material top size.
■ Feed material size distribution.

If more rather than less information that can be gathered, better decisions can be made regarding the selection of processing equipment. It also provides a better insight as to the potential for the material to produce a quality product.

One way to collect some of the above information is by having the material tested. The selection of the samples for initial testing is very critical. The samples that are taken should be representative of the formation. If there is knowledge of variances in the formation, samples should be taken from several areas so that these differences can be analyzed. A sand and gravel deposit is a good example of material that changes greatly in gradation at various levels. Samples should be taken at various depths to confirm the gradation through the various levels of the deposit.

TYPE OF PLANT- STATIONARY, SEMI-FIXED OR MOBILE

The type of plant selected will generally depend on how long the reserves will last. There are basically three types of plants.

Stationary plant: This type of plant is generally associated with quarried materials where the expected life of the quarry is more than 10 years. It should also be noted that large sand-and-gravel plants will fall into this category. Some of the characteristics of this type of plant are as follows:

■ Requires substantial civil work.
■ Primary surge pile.
■ Bins feeding crushers.
■ Large stockpiles.
■ Complete plant automation.
■ Large clearances around equipment for servicing.
■ Truck and/or rail loadout systems.
■ Concrete and steel structures.
■ Stationary conveyors.
■ Very difficult to relocate.
■ Lowest cost-per-ton.

Semi-fixed plant: This type of plant is associated with both quarried materials and sand and gravel where the expected life is less than 10 years. Some of the characteristics of this type of plant are as follows:

Photo by Kevin Yanik

Photo by Kevin Yanik

■ Requires little civil work.
■ May or may not have primary surge pile.
■ Bins feeding crushers.
■ Large stockpiles.
■ Complete plant automation.
■ Somewhat less clearance around equipment for servicing.
■ May or may not have some type of truck and/or rail loadout system.
■ Steel skid.
■ Stationary or skidded conveyors.
■ Can be relocated with some effort.
■ Relatively easy to modify.

Mobile plant: This type of plant is associated with both quarried materials and sand and gravel where moving from site to site is essential. It should be noted that mobile plants can be either wheeled (rubber-tired) or track-mounted. Some of the characteristics that both types of mobile plants have in common are as follows:

■ Require no civil work.
■ Generally no primary surge pile.
■ May or may not have portable bins feeding crushers.
■ Smaller stockpiles.
■ May have some degree of plant automation.
■ Generally little clearances around equipment for servicing.
■ Mobile chassis.
■ Mobile conveyors (either in-pit or over-the-road).
■ Can be relocated with ease.
■ Easy to modify.
■ Transportable over the road or in plant.
■ Easy to set-up.
■ Low move-in, move-out cost.
■ Highest cost-per-ton.

The basic difference between a wheeled plant and a track-mounted plant is that with a rubber-tired plant a semi-tractor is needed to move it from location to location, either in the quarry/pit or over-the-road. With a track-mounted plant, it can be moved from place to place in the quarry/pit under its own power. To move it over-the-road to the next site, you drive it onto a low-boy.

Depending on how the property will be mined, a track-mounted plant has the ability to follow the face and allows an operation to transport the crushed material by conveyor to the remainder of the plant, which eliminates the need for haul trucks.

Generally, mobile plants are used because of their mobility. They can be moved from location to location where their stay may be as little as several days and as long as several years. They can be used for shorter-term jobs where resale of the equipment is important at the completion of the job (dam projects, pipelines, etc.).

They are also used in place of stationary/semi-fixed plants and at times as additions to these plants because of the easy set-up and the different regulations that apply to them.

PROCESS FLOW

The process flow can be defined as a combination of several machines and several techniques that are used together to produce the required products. The process flow is one of the most important parts in defining how your new aggregate plant is going to operate to make the required products as economically as possible.

All of the information that has been collected will be used in the development of the process flow. This information will help with the selection of the proper crushers, screens, feeders, etc. that will be used in the aggregate plant.

All aggregate plants have what are commonly called crushing sections/stages. Each crushing section/stage can be defined as a part of the plant that takes an input material of a given size and reduces it to a smaller size for the next crushing section/stage. Most crushing sections/stages contain screens to remove the finer material before it goes into a crusher.

Also, in some cases, there exists a screening/sizing section/stage in the process flow. This is where material is only screened and not reduced in size by a crusher.

The selection of an appropriate processing circuit for a specific material is one of the most important decisions in the design of a processing plant. The importance is related to the fact that the capital and operating costs for the crushing/screening portion generally represent the major portion of the plant costs.

The number of factors that will have to be considered for any project will depend, among other things, on the type of project. For instance, if the project is an entirely new project, then most if not all of the following factors identified will have to be considered. If the project is an expansion of an existing operation, then the existing operation will influence the choice of factors or design restraints that need to be considered.

NEW PLANT

The more information that can be obtained, the more accurate the process flow will be the first time around. From the data collected it may be determined that there may be several approaches to the process flow, but the final analysis involves practical considerations, as well as economics, with the final result being a plant that can produce a quality product at a reasonable cost.

Photo by Kevin Yanik

Photo by Kevin Yanik

It is helpful to have an understanding of current or local practices in a given area, to find out why others did what they did, and learn from the experience they gained. This information may not be easily gained, but any information that is gathered should be passed on to the person(s) doing the process flow. It can be used as a guide when developing the process flow to help avoid costly mistakes. Remember, the process flow will be only as good as the information that is used to develop it.

Once the category of crusher has been selected, then the crushers for each stage are further defined by the size of feed they can accept and the capacity of the crusher at a defined discharge setting. It needs to be pointed out that there may be several types of crushers that meet this criteria.

Other things that need to be considered when determining the crusher to be used are as follows:

■ Ease of replacing wear members.
■ Ease of setting adjustment to both compensate for wear and to meet product requirements.
■ Quality of product.
■ Serviceability.
■ Parts availability.
■ Ease of operation.

Along with selection of the proper crusher, the proper screens and feeders need to be made. Some of the information required to select the proper screen or feeder are listed below:

■ Feed size.
■ Feed gradation.
■ Tons per hour.
■ Type of screening media (wire cloth, urethane, etc.)
■ Wet or dry process.

There are many other types of equipment that will be used in the new aggregate plant and basically all the same selection principles apply.
The process flow starts with the plant feed material and ends when all of the products have been produced. A lot of work will be required to define all of the process equipment that is required for the new aggregate plant. Once the initial pieces of equipment are selected, process-flow calculations need to be done to see if all of the equipment selected is correct. This is a long and tedious process when doing it manually, however, several manufactures have computer models that let an operation calculate process flow.

PLANT LAYOUT

After the process flow has been initially defined, it’s time to turn attention to fitting the equipment into the designated area. At this point, surge-pile capacities, stockpile capacities, minimum clearances under conveyors, minimum clearances around crushers and screens and how stockpiles are going to be maintained and other considerations will have to be defined.

Photo by Kevin Yanik

Photo by Kevin Yanik

During this process, the conveyor lengths are determined, and the heights of all structures are defined along with the location of each piece of equipment.
During this stage, it is common to make many changes. This is typically because of information that came to light during the process of defining what the new aggregates plant is going to look like.

Other equipment to be considered for effective materials processing at an aggregates plant include:

■ Drilling and blasting equipment, although many times this process is contracted out.

■ Loaders, haul trucks and excavators. Use of “rolling stock” will be determined by factors such as the distance between the quarry face and the primary crusher, the cost of cycle times and other factors. Rigid-frame haul trucks or articulated trucks can be considered.

■ Secondary breakers. Material-size variations at the muckpile may require the use of a secondary-breaking hammer either after the blast, or when the material is loaded into the primary crusher.

■ Log washers and classifying tanks. Fine material separation is also a consideration for some plants.

■ Automation components. Electronic material-processing and data-communications equipment allows some plants to gain maximum operational efficiency using a minimum of employees.

■ Loadout and weighout. Trucks scales, automated ticketing and data retrieval are important considerations.


 

Sources:

Contributors to this chapter include the following, in alphabetical order:

David Hanratty
Associate and Senior Geologist
Golder Associates
dhanratty@golder.com

Jay Lukkarila
Senior Process Engineer
3M Industrial Minerals Products Division

Brett McLaurin
President
Subhorizon Geologic Resources
mclaurinbt@subhorizonresources.com

Christopher Phillips
Associate and Senior Geophysicist
Golder Associates
christopher_phillips@golder.com

Jim Stroud
Vice President
Subhorizon Geologic Resources
stroudjr@subhorizonresources.com

 


Lesson 2 Quiz

1. What do you call software that layers geographic data?

2. What is the usefulness of parcel data?

3. While most aggregate material is transported by truck over roads, what are lower-cost means of transportation?

4. Are permits easier to obtain for a stationary plant or a mobile crushing plant?

5. What type of plant tends to last longer than 10 years and is most difficult to relocate?

6. Which type of vehicle can move from place to place in a quarry under its own power: a track-mounted plant or a wheeled plant?

7. What does ERI stand for?

8. What is the purpose of trenching?

 

Click here for the quiz answers.

 

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