Stockpile Analysis


November 13, 2011

3D mobile laser scanning offers quick and accurate inventory measurements

A mobile laser scanning system can be mounted on the back of a standard SUV and allows data points to be collected as the vehicle drives around the stockpile.

Performing physical inventories of material stockpiles is a task that most aggregate companies see as a necessary evil — it is something that has to be done for accounting purposes, but it takes up valuable resources and can slow down production. At the same time, the accuracy and traceability of the methods are important from the accounting side and need to be consistent from inventory to inventory so that the results can be relied upon. After all, these are valuable corporate assets that are being characterized.

Steve Brooks, plant manager for Tilcon in Poughkeepsie, N.Y., wanted to use the most accurate method available to inventory his company’s stockpiles. Tilcon, an Oldcastle Materials Group company, is an integrated materials company with multiple New York and New Jersey operations that include quarries, asphalt plants, recycling plants, water terminals, and a heavy highway construction division.

“About three years ago, we began to investigate the use of 3D laser scanning,” Brooks says. Instead of collecting 10 to 20 shots on a typical 20,000-cubic-yard stockpile using a total station measurement system, with laser scanning on the order of 100,000, 3D points are typically collected for that same pile. The spacing of the laser scanned points on the pile surface is typically on the order of 3 to 6 inches.

Manual surveys cannot begin to match this level of surface detail. In fact, the laser scanning field crew has to be careful not to collect too much data because this can bog down the post processing and data analysis without improving the end result.

“We were using a different laser scanning contractor when we became aware of the mobile laser scanning-based methodology that H2H offers,” Brooks says. “We made the decision to give it a try and have been very impressed with the results.” H2H Associates is a geo-consulting firm based in Troy, N.Y., with expertise in mining, engineering, geology, survey, regulatory, and environmental services.

Once collected, stockpile data can be reduced and computed into volumes within a matter of hours.

Its mobile laser scanning system is mounted on the back of a standard SUV. The 3D point data is collected by driving around the stockpile, typically at a speed of 5 to 10 miles per hour, with the scanner rotating at 1,800 revolutions per minute. The mobile laser scanning system includes a global positioning system (GPS) receiver and inertial measurement unit (IMU) that allow the data to be geo-referenced to the exact location on the earth. This ensures that each inventory operation will be referenced to the same coordinate system and pile limits each time the inventory is performed.

Faster data collection and processing

Speed was the key factor in Vic Coleman’s decision to use a mobile laser scanning system. He is the CFO at Braen Stone, a New Jersey firm with more than 100 years of involvement with the aggregates industry. Braen uses the physical inventory surveys for both internal and external financial audits.

When Coleman first started doing quantity surveys, his crews were averaging two sites per week, but the mobile laser scanning system has allowed crews to assess three sites in two days. “The speed is remarkable, and we have a high degree of confidence in the results,” Coleman notes. He has seen three or four generations of technology, and says he is continually amazed at the pace of change. “I remember when the field crews had to keep the batteries warm with their hands,” he recalls.

Mobile laser scanning not only speeds up the data collection portion of the workflow, but as compared to the use of aerial photogrammetry, it has led to much faster turnaround of the actual physical quantities. Using point cloud software supplied by the sensor manufacturer, the 3D data for a 20,000-cubic-yard stockpile can be reduced and computed into volumes within a matter of hours.

Gary Wall, operations manager for Tilcon Connecticut, a leading supplier of quality crushed stone, hot-mix asphalt, and ready-mix concrete throughout the state, notes that stockpile quantities are typically available within a couple of days of the survey.

Safety and standards

Site safety is another factor that leads companies to choose mobile laser scanning over other inventory methods. Wall notes, “The use of laser scanning, in general, is a safer and more accurate method for inventorying stockpiles.”

Safety can be either prescribed, in the form of government regulations, or desired, in the form of company policies that reduce liability, property damage, and costly injuries. The U.S. Mine Safety and Health Administration has issued a number of regulations that apply to stockpiles and other related facilities such as drawholes, bins, hoppers, and surge piles. These regulations prohibit a worker from standing or walking on a pile when it may expose the person to a hazard. Other regulations require the construction of platforms, staging, or safety lines.

Laser scanning eliminates the need for anyone to walk on a storage pile and can, in the case of mobile scanning, keep the survey crew safely inside a vehicle. When the project requires the use of a tripod-mounted scanner, the survey crew generally has the flexibility to select locations that are not in harm’s way.

The use of mobile laser scanning as a standard method for inventorying aggregate stockpiles in the United States is being addressed by the American Society of Testing Materials International (ASTM). Originally formed in 1904, ASTM Committee D05 on Coal and Coke is responsible for a variety of standards for this industry that have application to aggregate and mineral mining and storage as well.

ASTM D6172 was originally developed to standardize photogrammetric measurement procedures for inventorying coal and coke stock piles. Recognizing the importance of laser scanning techniques, as well as GPS, this standard is now in the process of being updated to include these game changing technologies.

Investing in the future

During a downturn in the economy, forward-looking companies may see the use of new technology as an opportunity to position themselves for future growth. Having up-to-date, accurate inventory information and confidence that it is being obtained in a cost-effective, safe, and defendable manner is critical to overall business operations and a potential source of competitive advantage for aggregate companies.

3D laser scanning is a powerful technology that, from a technical point of view, is ideally suited to the physical inventory of aggregates and minerals. Safety agencies, standards organizations, and owners are beginning to recognize the multiple benefits that this methodology offers. In this rapidly emerging world of 3D technology, it is not a question of if the use of laser scanning will become the preferred method of developing physical inventories of construction materials, it is just a question of when.

Richard A. Hisert Ph.D., principal, and Trevor R. Thomas, P.E. are with H2H Associates, LLC. Based in Troy, N.Y. H2H is a consulting firm that provides geologic, hydrogeologic, construction, regulatory, and environmental services.

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