Princeton University planned to deepen an existing, 4-foot crawl space beneath a 100 year-old, 3-story masonry build- ing in order to create a full-depth basement and mechanical room opening on to a future areaway. However, excavation would extend below the underside of the existing, wall and column footings. To preserve the integrity of the aging building during the work, structural engineer Keast & Hood Co. of Philadelphia, PA designed an underpinning system consisting of:
- Conventional continuous concrete underpinning beneath wall footings, and
- Installation of a temporary needling and jacking system to allow existing interior columns and footings to be safely removed so that new columns and footings could be installed at
the required lower level.
When invited to review the scope and procedures involved, along with other companies bidding the project, Moretrench was able to draw on considerable previous experience with this type of underpinning to demonstrate its capability. On this basis, Keast & Hood selected Moretrench for the work.
The exterior wall was underpinned first, after which openings to the future walkway were cut, providing entry to the crawl space. A work area already congested with active utilities, and limits placed on excavation prior to needling and underpinning, added to the challenge of maneuvering and handling steel assemblies weighing 1,600 pounds.
The steel needle system was pre-fabricated in-house by Moretrench to aid final field assembly. At the first location, temporary concrete footings were placed nine feet off the existing column, and the needling system was installed. A hydraulic jacking system with steel grillage above was then installed, and a load applied between the needle system and the existing first floor to allow the load of the column to be transferred to the needle system. Monitoring of the operations was performed before, during and after the loads were applied and removed.
Once the load was supported by the needle system, the existing column and footing were removed, a new, lower concrete footing was placed, and the new steel column was installed. The load was then transferred from the new column/footing to the existing column above.
The temporary needle system was then disassembled and moved to the next location. Following underpinning at all four interior column locations, Moretrench underpinned and formed an adjacent interior wall using conventional continuous concrete techniques.