GTS was tasked with investigating the structure of a historic bridge crossing the Lackawanna River in northern Pennsylvania. The bridge had stone faced abutments and wingwalls as high as 40 feet, and original construction plans were no longer available.

The bridge was scheduled for reconstruction with an eye towards maintaining its historic character and appearance. However, without detailed confirmation of the supporting abutments and footers, it was difficult to design a replacement structure that could reuse the original materials. Exploratory boreholes had suggested that the stone facing was thin with relatively little structural support behind, which if true, would indicate that the original abutment stonework might not be sufficient for supporting modern replacement structures. GTS proposed using ground penetrating radar (GPR) to penetrate and reveal the materials and structure behind the abutment and wingwalls.

The GPR survey was complicated by an active railroad track 20 feet from the abutment. This meant that hydraulic lifts were impractical to gain access to the upper reaches of the abutment. As an alternative, GTS constructed a self-contained GPR platform for its Mala X3M GPR system, which normally requires an operator to be with the equipment. A backplane was constructed upon which the control computer, batteries and cables could be mounted. The backplane was attached to the integral GPR antenna and GPR transceiver so that all GPR components were a single independent unit. An extensible ladder and pulley was used to lift the GPR up the abutment walls.

The radargrams verified the borehole data that the abutments and wingwalls were composed of solid stone approximately 2.5 to 3.5 feet thick with a backfill of cobbles and boulders filled with a cement matrix. The original cut slope behind the construction materials was clearly visible on the radargrams.

Please click the image to enlarge

The GPR verified the historic bridge's original construction with greater detail than boreholes. Unlike boreholes, GPR did no damage to the bridge and did not disrupt either vehicular or railroad traffic. The non-intrusive, non-destructive value of geophysics is clearly illustrated in this example.

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