Abstract:
The Henry Hudson Bridge, built in 1935, was the longest plate girder arch bridge in the world at the time of its construction. The bridge carries six [6] lanes of traffic over the Harlem River between northern Manhattan and the Bronx. Adjacent to the main span is a two-level viaduct structure on both the Manhattan and Bronx approaches.
The bridge exhibited severe corrosion due to an Alkali Silica Reaction (ASR) in the concrete substructure, threatening the safety of ~65, 000 vehicles traveling on the bridge each day. The entirety of the concrete piers and foundations in the Manhattan approach had to be replaced with minimal impact to the traffic above. To accomplish this, extensive support of excavation (SOE) and temporary shoring towers were required to support the structure. The SOE for the towers illustrated in the contract drawings was driven steel H-piles and timber lagging with external supports (rakers). Limited headroom and laydown space, noise restrictions and quality control issues presented significant challenges, so the SOE was re-designed for a more constructible solution using steel micropiles internally braced with grouted anchors. During construction it was discovered that much of the existing foundations were built on poor quality weathered rock. The presence of the weathered rock required protecting the existing bridge piers from collapse, and the re-design of the permanent bridge foundations and the temporary support foundations. Construction paused in this area as the appropriate field solutions were identified. Traditional underpinning was considered impractical. Instead, to protect the existing structure and the temporary supports, the construction team drilled additional braced SOE encasing the area around the weathered rock. This SOE installation provided the necessary lateral restraint to restrain the existing material that exhibited properties that behaved somewhere between granular soil and bedrock. This presentation explores the challenges of designing, fabricating, and constructing the temporary works in the densely packed work site, and highlights the teamwork required between the owner, engineer of record, and the contractor/sub-contractors to complete the project successfully.