Harrisburg International Airport Cargo Apron Expansion 

Microtunneling Allows Airport Operations to Continue During Construction of Storm Drainage System

Because of the forecasted increase on cargo freighting, the Susquehanna Area Regional Airport Authority (SARAA) had to expand its cargo parking area within the Harrisburg International Airport (MDT) in Pennsylvania. It was determined that SARAA needed to accommodate four (4) new cargo aircraft parking positions and a new storm drainage conveyance system without disrupting existing systems and the day-to-day operations of the airport.

As part of the Cargo Apron Expansion project, AECOM provided site work for a 52,000 SY concrete pavement expansion of the existing air cargo apron to accommodate four (4) new cargo aircraft parking positions.

An integral part of the expansion was a new 3,500-foot-long storm drainage conveyance system, that included twin 60-inch inner diameter (ID) reinforced concrete pipe (RCP) storm drainage pipelines microtunneled underneath Runway 13-31, 10,001-feet in length by 200-feet in width, which was still in active use by Group V aircraft, some of which weighing over 1,000,000 pounds. To avoid disruption to the airport operations, the crossings of the proposed drainage pipes were installed using microtunneling methods with only 10 feet of cover between the runway and the microtunnel boring machine (MTBM) crown.

An extensive geotechnical investigation on the conveyance system was conducted using test borings, laboratory and in-situ testing, and a geophysical investigation. Based on the findings, AECOM specified pressurized face, fully sealed microtunneling for the installation of two 60-inch ID RCP storm drains crossings under an active runway along with pre-excavation ground improvements’ designs along either tunneling envelope to prevent potential operational delays. Liquidated damages for any delays in operations of the airport’s runways are at $1,500/minute, and therefore settlement/heave/frac-outs were unacceptable in order to maintain the operation of the airports single runway and commercial and cargo aircraft operations. 

The shallow microtunneled drives underneath the runway faced various challenges requiring innovative technical and contractual solutions. Based on sophisticated geophysical surveys, AECOM anticipated that slag cobbles could affect runway settlement. To mitigate the risk, the team specified MTBM methods with cutterhead access and an airlock, developed sharp baselines in the GBR to promote competitive bidding, implemented a continuous monitoring program of the runway using automated mechanized total stations and designed pre-excavation grouting program using microfine cementitious and chemical grout that involved 30 HDD bores underneath the runway to install grout pipes on a 4-foot grid.

AECOM partnered with the specialty subcontractor, Michels, daily to refine the mix design, means and methods to achieve design objectives of injecting as much grout into the targeted zone without heaving the ground or causing inadvertent returns.

Because there was only a foot of clearance between the MTBM crown and an electrical duct bank that served the airports power and communication systems, the team had to avoid any harmful microtunneling impact that could result in a major power outage to the entire airport. AECOM structurally engineered an above grade anchor support to fully restrain the duct bank, supplemented with a full grout encapsulation, thereby, stabilizing the ground using a permeation grouting design. AECOM also mandated provisions to the contractor’s microtunneling means and methods while traversing under the duct bank, requiring the machine to maintain -0.1 bar of pressure (suction) in slurry plenum while crossing under duct bank to avoid heaving.

Finally, due to the airport’s operational status, a significant contractual constraint was negotiated to regulate work within the Runway Safety Area (RSA) during non-operational hours—from midnight to 5 a.m. During design, AECOM successfully petitioned the airport regulator to permit microtunneling 24/7 if the entire RSA was grouted, not solely to the runway’s confines. This flexibility promoted more competitive and innovative bids.

The project was executed with perfection of due diligence and innovation from study phase’s outset, transitioning into the basis for collaborative Owner/EOR risk workshops; into the development of alternatives’ evaluations through detailed design/procurement in the refining of the preferred choice; and into strategic partnering among all contractors to complete the project ahead of schedule and below budget.