Constructing Free Flow Lanes At Maryland Tunnel

By Pierce Flanigan IV and Phil Blake | September 28, 2010

The Fort McHenry Tunnel in Baltimore, MD, is the world's only eight-lane underwater tunnel for vehicular traffic. Its toll plaza is currently undergoing pavement rehabilitation and installation of two new express toll lanes. As part of Interstate 95, the tunnel crosses Baltimore's harbor under the Patapsco River and links the southern and eastern areas of Baltimore city. As the successful bidder of the Maryland Transportation Authority project, P. Flanigan & Sons have implemented a multiphase construction sequence that ensures minimal traffic flow interruption.

The tunnel opened to traffic on November 24, 1985, and has since become a major artery for city travelers. For 2007, traffic count checked in at 45,214,391. Average daily traffic in the summertime is estimated around 130,000 vehicles. The Fort McHenry Tunnel project involves several sub-projects spanning 11 phases with a dollar value of $10,869,485. The number of people working on the project ranges from 25 to 35 workers.

Project Objectives

The two main goals of the project are to reconstruct and pave the entire plaza and to combine two lanes in both the northbound and southbound direction to create one single lane designated as a Free Flow Lane (FFL).

The new FFL will permit cars with an EZ Pass device to travel through the toll booth at a faster speed than is now safe in the current toll lanes. Current posted speed is 10 mph; after project completion, it will be 30 mph. It is predicted that FFLs, the left-most lanes through the plaza, will contribute to significantly more efficient traffic flow.

To accommodate the new lane configurations, P. Flanigan will also install new storm drain systems, remove three existing toll booth islands, and perform other related work involved in closing off access to these islands. Construction of new barrier walls and new median barriers is also necessary to isolate the FFLs from other incoming traffic.

P. Flanigan will also partially reconstruct the continuously reinforced concrete pavement (CRCP) in the toll plaza totaling 15,865 square yards and then pave the plaza with 4 inches of hot mix asphalt (HMA), complete with new pavement markings. Approximately 6,500 tons of 12.5-millimeter base asphalt will be placed in a 2-inch lift, and 7,500 tons of 12.5-millimeter gap graded asphalt will be placed for the surface course.

Objectives not directly related to either the creation of the north and southbound FFLs or the comprehensive asphalt pavement plan include removal and reconstruction of a section of existing canopy over toll lanes 9–12 that was mandated because the existing columns were located within the new travel lanes.

Construction Sequence

The Fort McHenry Tunnel project is comprised of 11 phases. Phases one through 10 all involve reconstruction of areas of CRCP paving. Generally, work will progress across the plaza one lane at a time, thus the large number of phases. This permits traffic to flow almost uninterrupted.

Phase two is the largest and most complicated phase. It is during this phase that the two new FFLs are actually configured. However, prior to these lanes being completed, the new storm drain systems, canopy reconstruction and barrier wall construction will have to take place. The HMA will be placed along with the pavement markings during Phase 11. Allowed contract time is 545 calendar days, and approximately 30 percent of the time will be expended for the work in Phase two alone.

Phase one and 10 (reconstructed sections of the CRCP paving) are finished and Phase two is well under way, with about one-third of it completed. The Maryland Transit Authority (MdTA) allowed P. Flanigan to rearrange phasing to take advantage of work being performed by other MdTA contractors involved in the maintenance of the tunnel.

Currently, progress is being made on the construction of foundations for the new canopy and the installation of storm drains. The foundations for the new canopy columns are supported by micro-piles, which are small in diameter, about 5 inches, and are drilled 100 feet into the ground before being grouted. These piles are uncommon but ideal for use in areas of restricted space. In this case, the limited headroom under the canopy, as well as the close proximity of the piles to the toll plaza access tunnel, prevented the use of a larger pile.

The team is also working to close off the access tunnel under the toll plaza where the original booths were removed. Once Phase two is completed, Phases three through nine will resume what Phase one started and continue to reconstruct sections of the CRCP paving.

Not all work will be able to be accomplished in the phases outlined. The construction of the exterior terminal joints and main line concrete patching must be completed out of phase in an expedited manner. Out-of-phase work requiring tunnel closures involves planning as much as two weeks in advance, as permits must be granted for all closures. Permits ensure that no other contractors or maintenance workers will be impacted. In such cases, a realistic and flexible short-term schedule is important to make sure work progresses in one area if it is unable to in another.

Unlike other highway construction that prohibits weekend work, Friday and Saturday nights provide 12 hours of construction time for the Fort McHenry Tunnel project — the longest period of time for tunnel or lane closures. The project is also unique in that the many construction phases are designed to avoid major interruptions in traffic flow through the plaza for the duration of the project. To achieve this, P. Flanigan coordinates closely with the toll plaza administration and the toll system contractor who operates the electronic toll equipment. This working partnership is crucial to guarantee that the toll lanes are always operating properly and inconvenience to the traveling public is minimized. Precision scheduling, regular meetings and continual communication foster this working relationship.

The Fort McHenry Tunnel project began on January 7, 2008, and is scheduled for completion on July 4, 2009.

Editor's note: Pierce Flanigan IV is vice president of plants and equipment, and Phil Blake is a project manager with P. Flanigan & Sons, Inc., a Baltimore-based heavy construction and infrastructure builder.