What It Takes (Geostructurally) to Extend the Plane Train Tunnel Under the World's Busiest Airport-PPT Presentation

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The presentation describes geotechnical work for a 600-foot westward extension of the Plane Train tunnel at Hartsfield-Jackson Atlanta International Airport. The extension will allow additional trains and reduce the time between stops by 13 seconds. Construction takes place beneath existing domestic-terminal areas, where overhead clearance is limited and the tunnel crown sits at least seven feet below terminal footings. Groundwater is also high near the construction shaft.<br /><br />Keller’s scope included underpinning 19 terminal columns, dewatering part of the tunnel alignment, and supporting excavation for egress and ventilation tunnels. The underpinning system used micropiles, steel beams, and grillage. Design options included different casing sizes and reinforcement for compression and tension loads. Seven column pairs were supported on beams spanning the tunnel; other columns were carried by grillage or cantilever beams.<br /><br />Micropiles were installed through existing slabs and footings, with drilling spoils collected by hydrovac. Tight access and sensitive equipment required careful installation. Verticality testing confirmed the piles met the specified tolerance. A production tension pile was proof-tested in compression, reaching 370 kips with 0.165 inches of deflection. Some micropiles were intentionally designed to remain exposed during tunnel excavation.<br /><br />Column loads ranged from 166 to 215 kips. Loads were transferred to the underpinning frames, then returned to the footings after excavation; the steel was fireproofed afterward. Excavation support used braced micropiles and lagging at the tunnel breakthrough, and braced steel sections and lagging for the egress and ventilation tunnels.<br /><br />Dewatering covered about 150 feet near the shaft, where the soil profile was deeper. Parallel rows of pumped wells were spaced 10 feet apart and extended below the tunnel invert. The project illustrates how underpinning, groundwater control, and excavation support can be coordinated within a constrained, operating airport environment.

Keywords

geotechnical engineering

Plane Train tunnel extension

micropiles

underpinning

dewatering

excavation support

airport construction

Hartsfield-Jackson Atlanta International Airport

groundwater control

tunnel excavation