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MASW Survey of Interstate 476 Blue Route

After exploratory drilling failed, an MASW survey identifies the root cause of sinkholes.
  • A night-time MASW survey, when traffic was minimal, provided good quality continuous profiling data-Gannett Fleming

    A night-time MASW survey, when traffic was minimal, provided good quality continuous profiling data.

Pennsylvania Department of Transportation, District 6-0

Montgomery County, Pennsylvania

Our Role
Geophysical Investigation.

  • Identified the root cause of 44 of the 52 reported sinkholes along I-476
  • Findings assisted engineers in selecting the best approach to remedial reconstruction.

The section of I-476, known as the Blue Route, is the primary north-south Interstate corridor through eastern Pennsylvania. The section between the Schuylkill Expressway (I-76) and the Mid-County Turnpike Interchange (I-276), in Montgomery County, Pennsylvania, was constructed in the 1970's and has a history of sinkhole activity. It comprises six lanes and carries approximately 100,000 vehicles daily. 

The highway between mile markers 17 and 19 is underlain by karst terrain prone rock formations. Between 2005 and 2007, sinkholes were found in the north and southbound shoulders.

What We Did

Where traditional drilling and an electrical resistivity imaging survey proved partly effective, our multi-channel analysis of surface waves (MASW) survey linked 44 out of 52 reported sinkholes to geologic conditions that included: enhanced weathering, the transition from shallow, competent rock to enhanced weathered rock, pinnacled rock, a potential fault or fracture, and potential voids. All of these features are expressed as differences in shear wave velocities, which is a measurement of material stiffness.

Key Features

  • MASW survey was conducted on a 3,500-foot-section of the southbound inside lane of the reinforced concrete roadway during the course of three nights 
  • Surveyors worked between 10:00 p.m. and 5:30 a.m., averaging approximately 800 linear feet of continuous data each night
  • The MASW survey explored to a depth of approximately 80 feet below grade, a difficult or impossible task by other geophysical profiling methods.

Sustainability Features & Outcomes

  • MASW is a non-invasive, non-destructive, continuous profiling method.

Similar Projects: Geotechnical: Geophysics, Geotechnical, Transportation: Highways, Transportation