Point of Beginning

Imaging Goes Underground

November 1, 2009
Essroc decided to use underground caves for temporary clinker storage. Spatial imaging was used to establish baseline information on the empty caves.

The old quarry and cement plant had been a fixture at the south end of Martinsburg, W.Va., since 1884. Acquired in 2002 by Essroc, a leading North American cement producer with headquarters in Nazareth, Pa., the operation began as a limestone mine and started producing Portland cement in the 1920s. When the time came for an upgrade, Essroc got a boost in productivity and flexibility from surveying technology.

Like other U.S. facilities owned by Essroc, the Martinsburg plant mines raw materials and produces cement on a single site. The plant uses three rotary kilns--each over 450 feet long--to produce clinker, an intermediate product in the cement-making process. Clinker is made by heating a mixture of crushed limestone and clay to approximately 2,700 F. It emerges from the kilns in the form of small balls or pellets, usually 1 to 3 inches in diameter. The clinker is stored adjacent to the kilns on the plant site. When needed, clinker is pulverized and combined with other additives to make various types of finished cement.

In 2006, Essroc embarked on a project to replace the three old kilns with a single high-efficiency unit and construct new production facilities, material storage and pollution controls. At a total cost of $500 million, the upgrade will increase the plant capacity from 650,000 metric tons to 1.8 million metric tons per year. At the same time, the new system will cut production time from three hours to less than 30 minutes and reduce water consumption by 36 percent.

“We’ll produce two to three times as much clinker as the old kilns and use only 20 percent more energy to do it,” says Scott Stephens, the plant’s controller. “It’s a big improvement in efficiency and cost.”

The Trimble VX Spatial Station at the lower cave entrance.

Keeping Things Moving

Essroc must keep the plant operating while the upgrades are being built. In planning the project, a key concern was where to stockpile clinker while new storage facilities (a large geodesic dome and two concrete silos) for raw material and clinker were constructed. The company needed a location with easy access that was close to the kilns and production facilities. Instead of looking outward, Essroc looked down.

During the early days at the quarry, limestone mining had created several large underground caverns. The caverns were abandoned in the early 1900s and had been sitting idle on the quarry property ever since. Essroc identified two storage areas in the caverns, known as the upper and lower caves. Each cave is about 60 feet wide, 30 feet high and 200 feet long and can hold about 66,000 tons of clinker. Since the caves would provide the ability to keep the clinker dry and near the production facilities, the Essroc team decided that they would work well for the temporary clinker storage. Instead of conveyors, the team would use wheel loaders and Caterpillar 777F haul trucks to move the clinker to and from the caves.

As part of its production and financial control, Essroc needed to know how much clinker was stored and how much had been moved in or out of the caves. To get this information, Stephens contacted Essroc’s surveying consultant, Tom Stark, PLS, of Marsh and Legge Land Surveyors in Winchester, Va.

Stark looked at how to measure the needed volumes. He could measure the volume of the empty cave, then measure the volume of the cave when filled with clinker. Simple subtraction would do the rest. The measurements could be obtained by taking cross sections using total stations. But Stark rejected that approach. “For that type of survey, conventional methods would be inappropriate,” he says. “There was no cost-effective way to do it using a standard total station.” Having attended a seminar on 3D scanning, Stark realized that it was the best way to approach the project.

An aerial view of the cement plant.

"Just Push the Button"

Although his firm does not own a scanner, Stark knew someone who did. He contacted John Mettee, PLS, who is head of the surveying department at Frederick Ward Associates (FWA) in Bel Air, Md. Established in 1955, FWA has a long history of commitment to advanced surveying technology. For example, it was one of the first firms in the country to have a computerized CAD system. FWA adopted electronic data collection and total stations in the mid-1980s and did the pioneering work to develop their own field-to-finish procedures. FWA’s founder, Fred Ward, now deceased, had often encouraged Mettee to put new technology to work. “He’d tell me, ‘John, just push the button and make it happen,’” Mettee recalls.

As 3D scanning and spatial imaging came into the surveying industry, Mettee kept an eye on it. “We attended conferences to get the basic understanding of the technology,” Mettee says, “and a key client was pushing the importance of scanning for deformation monitoring work.”

In 2007, he made the decision to purchase the Trimble VX Spatial Station. “We liked the VX because it combined scanning and imaging with the ability to do conventional surveying tasks,” he says. And since the Trimble VX uses the same data collector (Trimble TSC2 with Trimble Survey Controller software) as FWA’s other total stations and GPS (Trimble 5600 and 5800 GPS), there was only a small learning curve for the field crews. Mettee says that they use the instrument every day for scanning or conventional surveying work.

Interior surface images meshed onto filtered point cloud data for the upper cave area. The FWA team used traverse data and photographs to register the scans.

10,000 Points Per Day

FWA first visited the caverns in July 2008. Essroc had already started moving clinker into the caves, so FWA needed to quickly establish baseline information on the empty caves. Using ground control provided by Stark’s surveyors from Marsh and Legge, the FWA surveyor carried control into the caves using traditional traverse methods with the Trimble VX. The surveyor then captured photographs using the high-resolution camera built into the spatial imaging tool. The digital images were used to help visualize the caves.

Much of the control is destroyed soon after the surveyors leave due to the heavy-truck traffic. But Mettee says that is not much of a problem. Neither the size nor the shape of the caves change, so absolute control is not critical for the subsequent scans. The FWA surveyor connects the setup points using the Trimble VX to conduct traverse measurements. As a result, all of the scans are tied together, greatly reducing the time needed to register and combine the scans in the office.

Because Essroc’s heavy equipment must stop working when a surveyor is in the caves, it is important to work quickly. Using the VX, FWA is able to complete scans of the caves in a single day using a one-person crew. It takes roughly six setups in each cave to collect the needed data. “Once we are set up and measuring,” Mettee says, “things go very fast.” He has not seen any problems with reflectivity of the clinker or cave walls affecting the measurements.

In the current work in the caves, the FWA surveyor gathers between 5,000 and 10,000 points in each eight-hour visit using a grid density of 3 feet between points. The ability to control the point density is an important benefit because it lets FWA optimize the time spent onsite while gathering information to provide the needed accuracy. The surveyor uses polygonal framing to define the irregular boundary for each area to be scanned. This saves time in both the field and office by eliminating unneeded measurements.

In the office, Mettee uses Trimble RealWorks Survey software to process the data. With the field surveyor sitting at his side, Mettee organizes and combines the scans into a single point cloud. He says the office work is straightforward.

“Registering the scans is easy because all the setup points are tied together in the field,” Mettee says. “I use the built-in tools in RealWorks Survey to compute the clinker volumes. We also take some conventional check shots and measurements. I use those to compute approximate volumes to double-check the values from RealWorks Survey.”

FWA provides results to Stark and Stephens in the form of PDF maps and illustrations together with volume reports. The construction at the Essroc plant is scheduled for completion by the end of 2009, and Mettee expects just a few more visits to Martinsburg to measure the clinker volumes. When the plant construction is finished, any remaining clinker will be removed from the caves and placed in the new storage silos.

Plant controller Stephens is pleased with the results. “The volume reports come quickly,” he says. “They have passed the audit tests and agree with our own truck counts. There have been no surprises from the surveying--which is just the way we like it.”

Sidebar: Thinking Like an Imager

Mettee described his experience in transition from conventional surveying to spatial imaging: “In the field, we learned how to plan our setups to capture the needed data and to augment the scans with photos. For example, we know to set up on both sides of a site for scanning instead of on one side when we were capturing individual points. The survey workflow makes it easy for the crew.”

Learning the office software was a bigger learning curve. “It’s a paradigm shift,” Mettee says. “You have all this data with many points that you may need to remove or ignore. That’s different from the thought process when working with individual topo points. It took a few tries for me to get the hang of it.”

With the Trimble VX now in a mainstream role in FWA’s business, Mettee will continue pushing Fred Ward’s technology button.