ASPHALT MILLING: A Century in the remaking

March 10, 2011

In Oregon, Century Drive Highway is a big deal. How big? It is important enough to merit a Wikipedia entry.


In Oregon, Century Drive Highway is a big deal. How big? It is important enough to merit a Wikipedia entry.

The route winds its way through a dark-green pine forest and leads to the Cascade High Lakes recreational area. In the warmer months, traffic can get quite heavy, including a fair number of bicycle riders enjoying the scenery along the way. Until recently, bicyclists were precariously relegated to the edge of the road while four-wheel traffic dangerously whizzed past on their way to a number of lakes, campgrounds and small resorts at the foot of the Cascade Mountains.

In 2007, the Western Federal Lands Highway Division of the Federal Highway Administration (FHWA) put out a bid to have 10.5 miles of the forest highway resurfaced, as well as widened and realigned in some areas. Because 5-ft-wide paved shoulders were added to accommodate bicycles, the project was jokingly referred to as the $7 million bike path. However, the FHWA would soundly argue that the increased traffic combined with the age of the two-lane road called for the work to be completed, regardless of the enhanced safety for bicyclists. The finished paved width of the highway is 32 ft, including the paved 5-ft shoulders to accommodate cyclists.

Burning the stakes

Tidewater Contractors, based in Brookings, Ore., won the project bid. As the general contractor, Tidewater was responsible for this two-season project, which started in May 2008. The company has been in business since 1978 and has a focus on highway construction with adjunct ready-mix and asphalt paving operations.

Tidewater Contractors’ first step was to check the existing survey control network on the national forest road project. These are the ground surface points that were used for survey staking and grade control.

“We used a Trimble SPS930 Universal Total Station to verify the points and confirm the quantities,” stated Mark Mann, project engineer with Tidewater Contractors. “This is all necessary prep work for creating an accurate 3-D road model.”

The original ground model and staking note data in digital format was provided by the FHWA. The original data and Mann’s data collected at the site were used for creating the 3-D design model for the project.

“I began by building a 3-D road model for the project that we use for both survey stakeout and grade checking—which is something I do with all road projects,” Mann said. “I’ve been working with Trimble survey equipment since 1999 and have been taking the digital design to the field for use directly with the survey equipment and grade checking to construct a project, which provides us with a pretty substantial payoff by itself.”

Additionally, Tidewater examined the scope of the work to be performed and saw beneficial opportunities for machine control. The company developed a “value-engineering proposal” for the project to include the use of 3-D machine control. The proposal, which was accepted by the FHWA, specified that Tidewater and the government would share in cost savings due to the reduced survey staking reaped by the technology.

“We didn’t tear up any pavement in 2008 because it required that we replace it with temporary asphalt, so we spent the first season installing culverts, building walls . . . most of the work outside the existing pavement,” said Mann. “All the major roadwork needed to be completed by Sept. 15, 2009, so that gave us a very short season to complete all the work through the finished paving.”

In addition to facing a short construction season, the work also was being completed during the high recreational period when the road use is at its heaviest.

“We needed to be extremely productive,” said Mann. “To do this, we decided to implement machine-control technology to eliminate a good deal of the survey staking and ensure the highest finished-paving quality—this was the first project where we implemented machine control.”

Precise set of teeth

Mann had been researching 3-D machine control for several years, attending user conferences and studying the trade magazines.

“I attended a session on 3-D milling [in 2009],” stated Mann. “This was right before we were to begin tackling the substantial amount of milling on the South Century Drive project.”

In the session, Mann learned that a grade-control system can be installed on milling machines (cold planers) for highly accurate milling at variable depth and slope without stringlines.

Mann continued, “I spoke with the 3-D milling expert at the conference and described the project we were about to embark on. They felt it would be a good match for the 3-D milling system.”

Local experts agreed that the South Century Drive Highway project was well-suited for “intelligent 3-D milling” technology due to the advantages of variable-depth milling. With a traditional 2-D control system, the machine mills to a constant depth and essentially copies the old surface. With a grade-control system on the cold planer, the 3-D control system varies its mill depth according to the 3-D design. It is designed to match the design profile with ±5-mm accuracy.

Throughout the entire length of the road, the design called for the existing pavement and base to be milled at 5-in. and 8-in. depths.

“We saved a considerable amount of time and additional work by using [a grade-control system] and milling to the design profile at variable depths from 3 to 12 in.,” Mann stated. “It increased our production, since we milled only what was needed. And we used less asphalt during paving, since there were no low spots or unevenness that required filling in.”

Mann added, “With fixed-depth milling, the machine does not cut to design, so further grading work is required and you don’t know where the design line is; you often cut more or less than is required to achieve grade. This calls for another cut/fill process with a grader. On the other hand, with 3-D milling, you’re milling to a design profile and eliminating the cut/fill process by the grader as is required in fixed-depth milling. This is the critical point that without 3-D milling technology would not be practical: We were milling to the design profile, not a fixed depth.”

With the accuracy of the milling, the grading of the subgrade was more efficient, and Tidewater Contractors was able to mill and finish grade up to 12,000 sq yd of road surface at a 3 to 12 in. depth in one day.

The time-saving advantage with the machine-control system helped overcome considerable delays caused by a subsequent problem with a rock source for the project.

“In our testing we discovered that the crushed volcanic basalt rock we were planning to use in the asphalt mix had degraded over winter,” Mann said. “It’s very rare to see this happen, but we couldn’t use the stockpile percentages per the original mix design, which required crushing additional aggregate.”

The issue was discovered when Tidewater found that in its initial test strips they could not achieve compaction.

“This is a national standard Superpave mix design, and maintaining aggregate gradations on the mix design is crucial,” stated Mann. “The FHWA recognized that the aggregate problems were not our fault.” But the snafu did cause delay while Tidewater waited for additional aggregate to be crushed, tested and stockpiled.

“There’s no question in my mind that had it not been for the use of machine control that this project would not have been completed in 2009. It would have slipped into a third year, which would have had a substantial cost impact to the government,” Mann concluded. “We estimate that the use of 3-D machine control for milling and grading saved us 11 days on the project.”

Like magic

With the challenges of a brief season, the variable-depth milling challenge and the issues with the crushed rock, Tidewater Contractors was still able to pull the rabbit out of the hat.

“We finished the last of the paving and the pavement striping in snow flurries that led into five days of continuous snow,” Mann stated. “At the time, the project was behind schedule due to [FHWA] change orders and paving rock issues, but we’re pleased that by using technology we prevented work carryover into another season.”

As Mann looked back on the project, he pointed to several accomplishments: “First, we received a smoothness bonus of $127,397, and the Western Federal Lands Highway Division reported that we achieved the smoothest ride since they started using the IRI [International Roughness Index] method for calculating smoothness. In our case, we had super-accurate milling and consistent, accurate subgrades, which made it possible to finish with our bonus-winning smooth surface.

“I was also very pleased with the support we received from our Trimble dealer. They came out while the project was ongoing and installed the systems and tailored the machines to the project and the work styles of the operators without interruption to the work in progress. Once the mill was calibrated and field-tested, we didn’t have to touch it again. And finally, we created a road with a smooth ride that’s safe for everyone heading up to the Cascade High Lakes area—which is a great feeling.”

About The Author: Winke is with Jeff Winke Consultancy.

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