Engineering and Mineralogical Evaluation of Airfield Subgrades (05-2384)
Imran Majid Ali Syed, Thomas L. Brown Associates, P.C.
Gary K Fuselier, Metropolitan Washington Airports Authority
Thomas L. Brown, Thomas L. Brown Associates, P.C.
Somba V. Ndeti, Thomas L. Brown Associates, P.C.

The Metropolitan Washington Airport Authority (MWAA) constructed Taxiway F as part of the Dulles Development (d2) Program. The purpose of this taxiway is to provide Group V aircraft access to future Tier 2, future Tier 3 and to the Runway 30 Hold Apron at the Washington Dulles International Airport. At approximately Station 734 of Taxiway F baseline, a baggage tunnel, tug tunnel and APM tunnels crosses the Taxiway F. In order to avoid future tunneling beneath the new Taxiway F when it is operational, portions of the proposed baggage tunnel, tug tunnel and APM tunnel were constructed in conjunction with the taxiway construction. The tunnels were founded on bed rock and were constructed with the cut and cover technique. The excavation above the tunnel boxes were filled with substantial amount of controlled backfill soils. The backfill soils were placed around and over the tunnel to act as the subgrade for the taxiway to support the 1.5-million-pound aircraft surface loading. This paper describes the results of the three plate bearing tests that were performed in this project to study the following: • Modulus of Subgrade Reaction, k-value • Estimate Settlements and back-calculate Modulus of Elasticity for the controlled backfill soils. The plate bearing test results indicated that the subgrade support value was highly sensitive to moisture. The k-value nearly dropped by a factor of 4 to account for the moisture correction. A mineralogical evaluation was performed to study the type of clay minerals present in the backfill soils and develop a strategy to overcome the problem. Results of the mineralogical evaluation are presented in the paper.