Abstract:
Freight railroads are classified as critical infrastructure and are an important piece of the United States economy. Railroad tracks cover the entire nation, allowing freight trains to transport goods and materials between ports and other transportation hubs. We have shaped the landscape around us to construct America’ s vast network of railroads. We have flattened mountains, carved into hillsides, filled valleys, and spanned gorges. The subject for this presentation is an earthen embankment fill that supports a freight rail line in southern Virginia and has a long history of slope stability issues.
The embankment is a valley fill between two natural hillside slopes. The embankment fill is over 70 feet tall with steep side slopes ranging from about 1.4H:1V to 1.6H:1V. The embankment has a history of slope stability issues dating back to the 1960s. Over the years, various repairs methods, including grouting, timber piles, and drainage installation, were attempted, but all proved to be ineffective.
Clear signs of deep-seated slope instability were evident in the fill. A subsurface investigation was performed in 2021and included in situ and laboratory testing of the soil and rock materials as well as installation of inclinometers to estimate the depth of sliding in the embankment. Following the subsurface investigation, a slope stabilization repair was designed. The design goal was to increase the factor of safety for global slope stability to at least 1.3 for the long term, post-construction condition.
The slope stabilization repair design consisted of a two-tier system with micropiles along the crest of the embankment and micropiles and tieback anchors along the toe of the embankment. The micropiles and tiebacks were arranged in an “ A-frame” style configuration and were connected with concrete cap beams. The micropiles and tiebacks at the toe of the slope were socketed into rock while the micropiles at the crest of the embankment were embedded in embankment fill soils.
Slope stabilization construction occurred during the first and second quarters of 2022. Some interesting aspects of the construction that will be presented include the challenges associated with installing micropiles in close proximity to an active railroad, use of instrumented micropile load tests to analyze load transfer along the piles and validate the contractor’ s alternate installation method.