New 737s Roll Out on New Taxiway at Renton Municipal

Author: 
Robert Nordstrom
Published in: 
May-June
2015

Although Renton Municipal Airport/Clayton Scott Field (RNT) is classified as a small general aviation reliever airport, its traffic is unusually taxing on airfield pavements. Every 737 produced at the adjacent Boeing factory takes its maiden flight off RNT's single runway; and to access the runway, the heavy aircraft must traverse the airport's 3,500-foot Taxiway B.

After decades of use by Boeing and general aviation aircraft alike, the airport recently replaced it. These days, nearly 40 new 737s roll off Boeing's local assembly line and onto the new pavement each month. Lined up nose to tail, the factory's annual output would extend almost 12 miles. That translates into a lot of wear and tear on a taxiway classified for general aviation use, points out Airport Manager Jonathan Wilson. 

"The taxiway was old and its load bearing capacities were not sufficient to support the 737 loads operating on the taxiway," says Wilson, explaining that the taxiway was designed for 15,000-pound King Airs rather than 90,000-pound 737s. 

"The pavements were coming apart, the taxiway didn't have a crown to shed rain water and it was time for replacement," he details. 

The trick was designing and building a taxiway to meet the needs of both general aviation and Boeing operations. Funding was a critical issue because the FAA views RNT as a general aviation airport that needed a taxiway to support lighter aircraft - Group II and under.

"It was tough," Wilson recalls. "We had to make the case that we're different; we need

factsfigures

Project: Taxiway Reconstruction/Rehabilitation 
Location: Renton (WA) Municipal Airport/Clayton Scott Field
Cost: Approximately $10 million 
Funding: FAA (90%); airport (10%)
Engineering Consultant: Reid Middleton 
Prime Contractor: ICON Materials
Electrical Engineering: Elcon Associates 
Geotechnical Engineering: HWA Geosciences
Pavement Quality Assurance Testing: General Testing Services
Survey Monument Installation: Peterson Survey Monuments
Electrical Contractor: Colvico; Innovative Electric
Pavement Milling: Ground Up Road Construction
Thermoplastic Markings: AirMark, by Ennis-Flint 
Installed by: Hicks Striping & Curbing
Hydroseeding: Country Green Turf Farms
Fencing: C & P Fencing
Construction Survey Staking: Baseline Surveying; Apex Surveying
Quality Control/Materials Testing: Construction Testing Laboratories
Concrete Paving: Salinas Construction
Recognitions: American Council of Engineering Companies 2015 Bronze Award; Washington Asphalt Paving Association 2013 Special Merit Award
Of Note: Although airport is classified as a small general aviation reliever, it is used daily by Boeing 737s from local assembly plant; so taxiway needed to be designed & built accordingly.

a bigger section of pavement with a better subsurface that will carry more weight. We were looking at a $10 million-plus project for an airport with only about $2.5 million in revenue a year. Without FAA grants, it would have been impossible to construct the taxiway, or maybe it would have taken us another 20 years. We had to convince the FAA that this taxiway was unique and worth funding beyond Group II standards."

Keeping Pace With Production

Before RNT's recent project, much of the existing taxiway's infrastructure dated back to the 1940s. As a result, the list of design and engineering objectives for the new surface was extensive:

• upgrade pavement load ratings

• bring the taxiway configuration up to FAA standards

• improve stormwater conveyance systems

• alleviate in-field ponding and improve water quality

• replace the fire water line system

• update edge lighting and signage with LED technology

• add an emergency generator to ensure operations during power outages

• standardize taxiway name designation to improve operational safety

Despite the numerous improvements needed, the airport had to keep costs within FAA grant allocations and finish construction within a tight schedule in order to minimize disruption to flight operations. In fact, one of the two runway access points needed to remain open throughout the entire project.

To address the challenges, planners divided construction of the new taxiway into two projects: the 1,700-foot north end, which only affected general aviation traffic, and the 1,800-foot south section, which affected Boeing activity and some general aviation operations.  

Construction began in April 2013 on the north end. Because this section had a 25-foot-wide footprint designed for lighter general aviation aircraft, it required a less complex phasing and construction schedule, Wilson explains. 

As the north project neared completion, the airport eagerly awaited word about funding for the larger, more complex south project - a 50-foot-wide section for heavier aircraft. Timing was critical, because crews needed to begin the south project immediately after the north project was complete in order to finish the entire taxiway during the 2013 construction season. As it turned out, funding was finalized and confirmed just weeks before crews finished the smaller portion of the job. 

"The grant allocation came down to the wire, but we were able to break ground on the south end in early July," recalls Kurt Addicott, principal engineer for Reid Middleton, the prime design and engineering consultant on the project.

"Getting the south project done in 2013 was critically important because Boeing's production rate was increasing. If we didn't get it done in 2013, the production schedules were going to make phasing even more complicated in 2014."

Boeing maintains nine single-aircraft workstations along the south end of the taxiway, where new 737s undergo final preparations before their maiden flights. In general, no more than one workstation was shut down at a time during taxiway construction. 

Only small sections of pavement needed to be closed for rehabilitation and drainage improvements during the three-phase north project. On the south end, work was divided into seven phases; and crews had to take more sections of pavement out of commission. After the pavement was reconstructed or rehabilitated, sections were reopened one by one. 

Boeing also maintains workstations on private property on the east side of the Cedar River (Apron D), which requires aircraft built there to cross a bridge to access Taxiway B. At one point during the taxiway project, aircraft movement across the bridge had to be shut down.

"As we worked through the phasing, we had to be mindful of Boeing operations across the river," recalls Wilson. "If you consider that Boeing had about 440 aircraft deliveries in 2013, shutting down access to the runway from Apron D for 14 days was going to be a problem for them. To get around this problem, Boeing staged aircraft at some underused workstations on the west side of the field before bridge access was shut down." 

Addicott underscores how important phasing was from the start of the project. "It helped tremendously that the airport and Boeing representatives had a good working relationship throughout the project," he reflects. "We worked closely with Boeing to try to develop a phasing schedule that would meet their operational and our construction needs."

Underground Issues

Insufficient subgrade conditions and underrated pavement strength were significant issues during the project, especially on the south end where Boeing operated. Addicott notes that several factors exacerbated RNT's already challenging subgrade conditions: poorly functioning stormwater conveyance pipes; an old, leaking fire water line; and high water tables, due to the proximity of Cedar River and Lake Washington and heavy clay layers that hold groundwater below the pavements.

"A significant part of the airport's footprint was built over marshland that had been hydraulically filled with unstable materials pumped in from Lake Washington back in the 1940s," he explains. "This was a problem, particularly on the south end, where the pavement loads were considerable."

A patchwork of existing pavement in varying depths and sections further complicated rehabilitation efforts. Some areas included asphalt on top of the taxiway's original Portland cement concrete slabs; other areas had crushed rock and asphalt directly on poor subgrade. 

The simplest solution would have been to rip everything up, establish a uniform foundation and build from there, Addicott informs. Time and cost, however, ruled out the "easy option." 

Instead, geotechnical sub-surface investigations were performed to determine the quality of subgrade soils throughout different areas of the taxiway. In some places, pavements were reconstructed by treating the subgrade soils with cement to establish a firm foundation for asphalt top layers. In other areas, pavements were rehabilitated with asphalt overlays to achieve pavement strength goals and reduce the duration and cost of construction. 

Above the Surface

Updating the taxiway's cross-sectional configuration to meet current standards was another important objective. FAA design standards require crowned pavements to disperse water to each side of the taxiway; RNT's previous taxiway shed water across the entire width of the pavement.

"The existing pavement was not crowned," Wilson explains. "All the water was supposed to move across the pavement from one side to the other. Over time, though, the pavement had settled, creating ponding and puddling issues."

To mitigate Boeing's concerns about more stormwater being directed into its storm system, the airport installed a slot drain along the length of the taxiway. The system captures water from the new crowned taxiway and moves it through an oil/water separator before draining it into the Cedar River.

The airport also replaced paint markings on the south end of the taxiway with new thermoplastic markings - a key element for the project because of the airport's traffic, Wilson emphasizes.  

"The brand new Boeing jets need to leave our field in perfect condition," he explains. "Because the engines are so close to the ground, loose glass beads from the painting process become FOD (foreign object debris) that can potentially be sucked up into the engines. Boeing is nervous about that, and the thermoplastic helps to reduce potential problems."

Although thermoplastic markings are initially more expensive than paint, they reduce maintenance costs over time, he adds. 

Lessons Learned

Reflecting on the airport's new taxiway, Wilson breathes a sigh of relief: "It's all done; the product is first rate; we couldn't be happier with the outcome."

With nearly five years between initial planning for the project and its ultimate completion, the importance of upfront planning is still paramount in his mind. "Once you get to construction, it's too late - especially when the project requires complicated phasing and construction methods," he relates. "One of my takeaways from this project is to spend more time in the planning process to make sure I have all the little details dialed in before final design." 

Shorter, Longer Jet Blast Deflector

At 22 feet tall, the previous blast deflector at Renton Municipal Airport/Clayton Scott Field (RNT) was about 7 feet too high for the non-precision approach procedures airport officials wanted to implement. It was also old and due for replacement, note personnel from URS, the project's engineer of record.  

"It was originally designed for Boeing 727s, which have three engines: one on each side and a higher one at the rear of the aircraft," informs Andy Carpenter, URS' project manager for the blast wall. "The 737s taking their maiden flights out of RNT feature engines lower to the ground and do not generate blast as high as the 727s."

The new wall, by Blast Deflectors, stands about 13 feet at its highest point in the

factsfigures

Project: Blast Fence
Location: Renton (WA) Municipal Airport/Clayton Scott Field
Cost: $1.2 million 
Funding: FAA (90%); airport (10%)
Engineer of Record: URS (now part of AECOM)
General Contractor: Merlino Construction Co.
Manufacturer: Blast Deflectors
Of Note: World's largest continuous radius jet blast deflector

center and slopes down to about 10 feet at both ends. Constructed primarily of steel, it has a red and white angled top cap to provide prominent visual markings for approaching pilots.

"The galvanized steel was powder coated with gray, red and white paints to resist corrosion and prevent potential contaminants from spilling into the surrounding area," adds Don Bergin, sales director with Blast Deflectors. 

The previous blast wall also served as a security fence along West Perimeter Road due to its shape and location. To maintain site security and jet blast protection for people and vehicles south of the runway, the new wall was constructed between the existing blast wall and runway before the old wall was removed. Though the new wall is closer to the end of the runway, it is small enough to not be considered an obstruction. At 264 feet long, it is the largest continuous radius jet blast deflector in the world, says Bergin. 

"The old blast deflector was too tall," reports Airport Manager Jonathan Wilson. "It was a technical obstruction and needed to come down. The new one has a smaller footprint but provides the same benefits. And, it's much nicer to look at!"  

Subcategory: 
Runway/Ramp

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