Use of Drilled Shafts for Railroad Embankment Stabilization Adjacent to Hudson River

The railroad embankment for the Hudson Line tracks just north of Peekskill, New York was identified by Metro North as a location requiring slope stability improvement. The embankment had historically experienced localized slope failures and slides. Slide detectors at that location were tripped several times a year, necessitating the closure of both tracks while an inspection crew was dispatched to investigate. Years ago a steel lattice signal boom supported by cables into bedrock was installed near water’s edge to help stabilize the embankment; however the boom was steadily corroding due to exposure to the elements.

As part of the Metropolitan Transportation Authority’s Capital Program, Metro North undertook an $8.6 million embankment stabilization study, design and construction project beginning in 2011. The stabilization improvement project is currently being constructed.

The existing right-of-way 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.  The nearest access point to the site from land is a mile away.  The site is accessible by hi-rail vehicle with flag protection or by boat.

GeoDesign led the geotechnical-based site exploration with the support of several subconsultants: e4sciences of Sandy Hook, Connecticut; McLaren Engineering Group of West Nyack, New York; Hager-Richter Geoscience of Salem, New Hampshire; KC Engineering and Land Surveying of New York City; and Warren George of Jersey City, New Jersey.  The exploration focused on documenting the existing conditions of the embankment and compiling additional information required to design a stabilizing solution.  The exploration program consisted 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 to allow monitoring of the slope for movement during construction.

GeoDesign reviewed the field investigation data and performed stability analyses of the embankment.  The field studies and engineering analyses indicated that the fill and underlying bedrock are steeply sloped in the area.  Additionally, there is a special subsurface condition of locally thick rock fill over a buried bedrock “valley” feature oriented perpendicular to the tracks.  The combination of factors likely contributed to the historic instability of the embankment at this location.  The results of the stability analyses indicated that the factor of safety against embankment failure was below accepted values.

In order to stabilize the embankment, GeoDesign recommended and provided design for a solution consisting of drilled shafts installed approximately 18 feet from the centerline of the railroad track closest to the river and socketed into rock.  The twenty 48-inch diameter shafts will be six feet on center, with reinforced concrete panels retaining the backfill in the two-foot gap between shafts.  Each shaft will consist of a steel casing filled with concrete and reinforced with a 24″ deep wide flange steel core beam.  The shafts will be supported near the top with a continuous wale and twelve steel bar tiebacks drilled into rock at an 11-degree angle from horizontal.

Kiewit Infrastructure Co. of Woodcliff Lake, New Jersey began construction in the spring of 2013, and installation of the tied-back drilled shafts is expected to be completed by the end of the year. The terrain is very challenging, and most of the construction work is being performed from barges in the river.  The track closest to the river is taken out of service during non-peak hours on weekdays.  Concrete for the drilled shafts is being delivered to the site by hi-rail concrete trucks.  A. DiCesare Associates, P.C. of Bridgeport, Connecticut is providing construction inspection and GeoDesign is providing construction support services for Metro North.  The railroad tracks are monitored regularly for vertical and horizontal movement.  A video camera installed at the site allows all parties a continuous real-time view of the construction on their computers, tablets, or smart-phones.

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