Design and Construction of Taxiway Realignment over Soft Soils at Philadelphia International Airport

Abstract: Taxiway P at Philadelphia International Airport (PHL) is a parallel taxiway to Runway 9L-27R; however, it does not meet Federal Aviation Administration (FAA) separation criteria for Group V aircraft in low-visibility conditions. Therefore, the taxiway was realigned to meet the separation criteria, resulting in the taxiway footprint being located on normally consolidated, very soft clay soils with standard penetration test blowcounts of zero. Anticipated settlement under the weight of the pavement was anticipated to be up to six inches and take over one year to achieve. To address this intolerable settlement, site improvement was a key aspect of the project design and included surcharging to accelerate consolidation as well as two first-of-their-kind uses at PHL: Low-Density Cellular Concrete (LDCC) and Cement-Treated Subgrade (CTS). Most of the site improvement for the realigned taxiway footprint was completed via surcharge while the existing Taxiway P remained operational. LDCC was utilized at critical utility crossings and within the Taxiway Safety Area (TSA) of the existing Taxiway P after the taxiway was closed for realignment. The final step in the site improvement process was cement treating the subgrade to provide uniform support to the overlying pavement structure and a non-moisture susceptible construction platform. After the site improvement was completed, a 5-inch asphalt base course was placed followed by a 17-inch concrete pavement. This case study evaluates the site improvement and pavement design processes and assumptions against as-constructed conditions via a settlement plate monitoring program, quality control and quality assurance materials testing, and Heavy-Weight Deflectometer (HWD) testing. The results are intended to further the state of the engineering practice for airfield pavement projects constructed over soft ground susceptible to consolidation settlement.