Drilled Shafts for Railroad Embankment

Drilled Shafts for Railroad Embankment

Embankment for the Hudson Line tracks just north of Peekskill, New York was identified by Metro North for slope stabilization. Slide detectors tripped several times a year, necessitating the closure of tracks while an inspection crew was dispatched to investigate. Years ago a steel lattice signal boom supported by cables into bedrock was installed near the water’s edge to help stabilize the embankment. However the boom was steadily corroding due to exposure.

The embankment is cut into the steep rock face of Manitou Mountain, with a 25 foot wide bench supporting two active railroad tracks. The western side of the embankment slopes steeply down more than 100 feet to the bottom of the Hudson River.

To design a solution, we performed a field exploration program consisting of topographic and hydrographic surveys, sub-bottom profiling, underwater dive inspection, and soil/rock borings with acoustic tele-viewing of the rock to determine bedrock joint orientation. Inclinometers were installed in the casings of three land borings for monitoring of the slope for movement during construction.

Our studies found steep thick rock fill over a buried bedrock “valley” feature oriented perpendicular to the tracks. These features contributed to the historic instability of the embankment. Our stability analyses indicated that the factor of safety against embankment failure was below accepted values.

To stabilize the embankment, a drilled shaft wall was designed and installed along the river’s edge 18 feet from the nearest track. Twenty 48-inch diameter drilled shafts at six feet on center, with reinforced concrete panels now retain the embankment. Each shaft consists of a steel casing to rock and a rock socket, shaft concrete and a 24″ deep wide flange steel core beam placed in each shaft. The shafts are supported near the top with a continuous wale and steel bar tiebacks drilled into rock.

Kiewit Infrastructure was the contractor. The project was an ENR 2014 Award Winner.

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