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Measuring Stockpiles with 3D Mobile Laser Scanning


By Richard A. Hisert and Trevor R. Thomas

It is not that often that a new technology comes along that effectively makes the old methods obsolete. The personal computer, Global Positioning System (GPS) and the Internet are a few examples of technology breakthroughs that once they became available, there was no going back. Mobile laser scanning, or what some refer to as LiDAR (think of radar, but instead of radio waves a laser beam is used) is one of those “game-changing” technologies.

Instead of collecting one 2D survey point at a time with laser scanning, you are collecting tens of thousands of 3D points per second. Now mount the laser scanner on a vehicle and drive around a stockpile or quarry and you will have the data needed to develop a highly accurate 3D model of the as found conditions. If you do this on a recurring basis you can then calculate the changes in the size, shape and volume of materials.

As we know, performing physical inventories of material stockpiles and quarries is a task that most 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.

Accuracy
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 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.

Brooks noted, “About three years ago we began to investigate the use of 3D laser scanning.” Instead of collecting 10 to 20 shots on a typical 20,000 CY stockpile using a total station, 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 in.

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 significantly 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. We made the decision to give it a try and have been very impressed with the results,” Brooks commented.

The H2H mobile laser scanning system is easily mounted on the back of a standard SUV. The 3D point data is collected by driving around the stockpile typically at a speed of 5-10 mph with the scanner rotating at 1,800 revolutions per minute. The mobile laser scanning system includes a GPS receiver and IMU (inertial measurement unit) that allows the data to be georeferenced to the exact location on the earth. This insures that each inventory operation will be referenced to the same coordinate system and pile limits each time the inventory is performed.

Faster Turnaround
Speed was the key factor in Vic Coleman’s decision to use H2H’s mobile laser scanning system. Coleman is the CFO at Braen Stone, a New Jersey firm with more than 100 years of involvement in 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. “Using the H2H mobile laser scanning system we can do three sites in two days. The speed is remarkable and we have a high degree of confidence in the results,” Coleman noted. He has seen three or four generations of technology, and is continually amazed at the pace of change. Coleman recalled, “I remember when the field crews had to keep the batteries warm with their hands.”

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 H2H can reduce the 3D data for that same 20,000 CY stockpile and compute the volumes in 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, noted that, “H2H typically produces quantities within a couple of days of the survey. They have been providing an excellent overall service.”

In 2012, H2H visited 130 different sites throughout the Northeast but since the system is so portable they were also able to provide their services in Oklahoma, Missouri and Arkansas as well. In total they surveyed 4,250 stockpiles, averaging 200,000 cu. yd. per site and a total of 34 million cu. yd. inventoried.

Improved Safety and Standards
Site safety is another factor that leads companies to choose mobile laser scanning over other inventory methods. Wall noted, “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 (MSHA) 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 this 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 ASTM – the American Society of Testing Materials International. Originally formed in 1904 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 stockpiles. 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.

Richard A. Hisert, Ph.D., principal and Trevor R. Thomas, P.E., are with H2H Associates LLC, based in Troy, N.Y., a consulting firm that provides geologic, hydrogeologic, construction, regulatory and environmental services to private and public clients. H2H began offering laser scanning services to the construction materials industry in 2008 in order to provide more efficient and accurate measuring of their physical inventories.