Replacement of the Morgan St Bridge over the Eisenhower Expressway
- 16 Drilled Shafts Imbedded Two Feet into Bedrock
- 73 Drilled Micropiles into Rock Socket
Owner's Envisioned Structure: Two new abutments would support the new lengthened bridge. Located in a congested neighborhood and on a limited site, the new abutments were located just behind the old ones to permit an additional future lane of traffic as the highway neared/exited from the Circle Interchange. With the abutments supported on rock caissons, the base of the excavation was to be 25' below Morgan Street, and a new water main was to be installed under the north abutment.
In many cases, the new caissons conflicted with a new water main, the existing abutment, and/or the piles supporting the old bridge. As a result, all internally braced ERS would need to be in place prior to caisson construction.
Also, two sets of buried high tension ducts were to remain in-service during construction. Prior to construction, these ducts ran underground up to the bridge and then were supported across the highway within the bridge girders. During construction, the ducts were to be suspended over the abutment excavation and then carried by a temporary support beam spanning the highway and bearing on the existing abutments. This set of conditions meant a lot of challenges for a caisson design.
Value Engineering Design: For the caissons, the presence of the internal bracing, the suspended utilities, and the existing abutments meant a drill rig could not work from within the excavation and would have to install the shafts from street grade along one side of the excavation. However, the limited street level work site could not accommodate an oversized drill rig that would have adequate reach, and temporary false work to suspend a smaller rig was complex and expensive.
Our alternative solution was micropiles. Their smaller element size allowed for greater flexibility in the layout, such that conflicts with existing conditions could be readily accommodated. Further, with our pile-and-leads method of installation we could easily reach every pile location from within the prescribed work zone.
Value Engineering Benefit to the Project : The complexities of the site that made the work so difficult for a caisson design were readily addressed by the flexibility of our micropile design - in the end saving time and project cost.