Forty-four miles north of Grand Central Terminal and between a narrow section of a mountain side and the Hudson River, tracks carry daily commuters into New York City via Metro-North and Amtrak. Shored up over the years by short-term temporary fixes, Metro-North Railroad brought us in to examine and provide long-term solutions to slope loss and service interruption.
Over the decades ago, steel boom truss structures were placed as temporary support at the face of the shoreline slope. The steel boom was held with buried cables horizontally anchored into bedrock. The last boom was placed circa 1990 as a shoreline replacement for an earlier one that failed and was lost into the river and the final temporary boom deteriorated, providing a diminishing benefit to the stability of the embankment.
We collected new field information and performed analyses to better understand the nature of the instability, screened various concepts for slope stabilization, and — working closely with Metro North engineers — selected and developed a final design.
The Solutions Unearthed
- A stabilization design consisted of a 120-foot long wall between the track and the Hudson River. The wall consisted of four-foot diameter steel-cased, reinforced concrete shafts at six feet on center, drilled through the rock fill and socketed into the underlying bedrock slope.
- The shafts were braced at the top with tie-backs drilled below the tracks and anchored into bedrock. The spaces between shafts allowed drainage while preventing large rock fill from moving below the river level.
- Concrete lagging was installed above the water. This solution minimized impact to the environment, permitting railroad operations.
- The open-work drilled shaft wall was constructible, allowing location tolerance for difficult drilling through rock-fill into submerged, sloped bedrock surface.
- The project improved the safety of the traveling public with a long-term fix.
Client: Metro North Commuter Railroad