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New Technologies Are Now Making It Easy And Affordable To Accurately Measure The Amount of Material In Stockpiles.

By Timothy N. Troy

Stockpile inventory measurement and management – critical quarry production-flow components – can be a heap of trouble for producers if not done accurately. There are also growing safety concerns regarding measurement practices.

But the developers of newer methods now deploy global-positioning systems (GPS) and three-dimensional laser technology to pave a smooth road to future success. The speed at which the work can be completed, and the precise results that can be obtained, can offer surprising returns on investment.

Measuring stockpiles used to mean risky ascents and physical surveys of the piles themselves, or perhaps a “fly-over” by plane using photography as a basis for projecting volumes. In either case, results could take weeks to turn around and include a lot of guesswork, representing a serious infusion of cash. But in the bold new world of technology, things are changing.

“Compared with traditional inventory methods, such as fly-over photography and survey crews occupying the stockpile, the new systems provide more accurate measurements in a more timely and safer manner,” said Carrie A. Beveridge, consulting geologist and measurement solutions specialist, Vancouver, Wash.-based Rellum Co. “Fly-overs can be very costly. Fly-over volume estimates are less accurate and take a great deal of time to calculate. Having your crew crawl around on stockpiles unnecessarily exposes workers to hazardous conditions for a process that is inherently fraught with error.”

A Need for Advanced Technology
The need for accurate measurement is important to materials producers for a variety of reasons, says Selma, Texas-based Firmatek, a provider of professional services to the worldwide mining industry. In order to control inventory in an aggregate operation, it says, the producer must understand production, storage and sales. “Inadequate control in any one these areas leads to inventory uncertainty and poor information management,” the company said.

In the blast cycle, volume is typically measured by multiplying the drill pattern used by the average bench height by the number of holes. However this approach is really just a rough estimate, due to uneven bench heights, terrain variability and material build-up at the quarry face that make it difficult to calculate actual tonnage produced.

Sand and gravel are usually calculated by counting haul-truck loads or loader bucket loads, Firmatek said. The problems here are similar to those in blasting as load sizes vary. Also dredged materials are often removed below water and are difficult to count. The result, the firm says, is the need for “different technologies … to determine excavated volume.”

Aggregates stockpiles are another area where accurate measurement is vital. Keeping tabs on inventory is a key to speedy turns of product and profitability.

According to Firmatek, the available measurement\techniques are:

  • Manual approximations (using a wheel and tape).
  • Conventional surveying techniques (using cross-sectioning or global position).
  • Aerial photography (“fly-over” techniques).
  • Modern laser-scanning techniques.

Furthermore, three major areas of error exist in stockpile inventory management. The first is a poorly defined base, which Firmatek defines as “critical” in that even the smallest of errors in base measurement multiply geometrically into further calculations of total volume. The second area is insufficient data on the top of the stockpile. Stockpiles are quite tall, and as such, surveyors often don’t know what the top of the piles look like. And in all measurement methods, modern laser scanning included, operators should rely only on a computer-generated model of the entire pile. A third major source of error is the inclusion of extra material that lies below the measured base.

All of these areas of error make it essential to use the best technologies, the firm points out.

“The use of inaccurate methods can result in large errors in apparent volume, necessitating large inventory corrections from inventory to inventory,” Firmatek concludes. “The most accurate method, laser scanning combined with specialized volumetric software, can produce results reproducible to within a few percent.”

Inside the New Gadgets
While measuring stockpiles has in the past been speculative, time-consuming and difficult, technologies are now on the market to provide improved results while taking into account myriad materials – be they gravel, rock or dirt. Quarries of various sizes have some options.

Choosing between buying measurement devices and outsourcing the services is a first decision. Suppliers of new technologies sketch an outline in broad strokes. Smaller firms might prefer an investment in buying equipment and training personnel. Larger firms that require a third-party to verify results to stockholders perhaps are better served by purchasing services on an out-sourced basis.

Suppliers stress high accuracy and speedy returns on investment in both venues. Rellum’s Beveridge says the firm can provide third-party services, but a good chunk of its business comes from marketing its systems and providing on-going training.

Its system consists of the TruPulse 360B laser range finder and MapSmart software package from Laser Technology Inc., and the TDS Recon Pocket PC from Tripod Data Systems. The laser range finder feeds points to the windows-based mobile PDA, uses the software package to offer a three-dimensional calculation of stockpile volumes and tonnages in real time.

“The TruPulse system is easy to use, only requires one operator, and can be learned in a day,” Beveridge said. “The complete system can be expected to pay for itself within one to two inventory cycles, and you have the freedom to collect inventory on an as-needs basis.”

The TruPulse stockpile inventory system’s average error has been specified to lower than 5 percent of actual volumes. Additionally, all data is stored in the system's hard drive and can be transferred to a PC at any point. The data may be archived and referenced at a later date, which is crucial for accounting departments under audit or to appease stakeholders who need to know what their assets are on the ground at any given time. An additional feature is the optional coupling of a GPS unit to the recon- enabling 3D mapping into real world coordinates.

“I have found that users tailor their systems to their specific needs,” Beveridge said. “The TruPulse can be used to range any number of objects and has been routinely utilized to calculate overhang and utility heights, topographic heights, road grades, slopes, and angles of repose. With the TruPulse, you can determine the distance between two remote objects, the height of an object such as a telephone pole, or the top of a hill, the slope of a hill, the straight-line distance, direction or angle between any number of objects. The sky really is the limit.”

Geologic-Hazards Evaluation
As a consulting geologist, Beveridge has found a variety of uses for the standalone TruPulse rangefinder. One example is a geologic-hazards evaluation for which she was contracted.

The analysis involved precise mapping of steep to near-vertical cliff faces within a pre-specified horizontal distance to a group of residences below the cliffs in danger from falling rocks. The TruPulse was helpful for determining the horizontal distance between the residences and the base of the slope.

“High-risk residences were singled out based on their relative proximity to the cliff walls,” she said. “In these areas, the overhanging rock faces were mapped with the instrument and sections of the slope that exceeded a critical slope value were mapped as hazardous and in need of mitigation.”

Further specialized applications for the system include the “Face Profiler.” This is a version of the MapFinder 3D software that enables the user to map rock wall faces for mining and blasting applications, Beveridge said.

About 80 percent of Troy, N.Y.-based H2H’s business is performed as a service to firms needing third-party analysis of stockpile inventories. H2H utilizes the latest in GPS-based mobile 3D laser scanning equipment that quickly collects hundreds of thousands of survey grade points per stockpile.

It also uses state-of-the-art software and computers to reduce data and calculate inventory quantities. “The deliverables that we produce allow the client to see the results in three dimensions creating a highly transparent, reproducible, and defensible result every time,” said Trevor Thomas, manager of technical services.

Much of the need for an accurate physical inventory by H2H’s clients is driven by external and internal accounting procedures. Many clients are required by these procedures to provide an accurate account of the material in stockpiles at varying frequencies ranging from monthly to quarterly to yearly. Others, meanwhile, simply use them to bolster efficiency.

“In some cases our clients do not have specific reporting requirements,” Thomas said, “but they use our services to gauge production, operation performance, equipment efficiencies, and loss prevention.”

Thomas said the major driving factors for these technologies are accounting, safety, accuracy and efficiency. Recent Mine Safety & Health Administration regulations limit how and when stockpiles can be accessed. Three-dimensional laser scanning drastically reduces the amount of time field surveyors spend outside vehicle and adjacent to stockpiles.

“Historically, the method used to measure stockpile inventory is largely based on the accuracy required,” Thomas said. “Prior to the advent of laser scanning equipment a more accurate survey was performed with a theodolite or ‘total station.’ These methods, while producing accurate point-coordinate information, produced only minimal shots per pile, subsequently introducing large amounts of error due to the highly irregular shape of most stockpiles.

“If an accurate survey is required, there are no advantages of the older systems relative to the technology available today,” he said.

Timothy N. Troy is a Cleveland-based freelance writer.

Cost Comparison
(Assuming a mid-sized quarry)

  • Traditional Survey Cost: $4,000 to $5,000.
  • Aerial Survey Cost: $5,000 to $10,000.
  • 3D Mobile Laser Cost: $2,000 to $3,000.

 Source: H2H, Troy, N.Y.