One hundred feet of the cut-and-cover structure crosses under the 39th Street Bridge, specifically piers 6S and 7S. Both the open cut and cut-and-cover structures are nestled between some of the busiest rail lines within the northeast. This essentially made the work site an island with no access other than from above on the 39th Street Bridge and, on sparse occasions, from work trains. Underpinning and Foundation Skanska Inc. (UFS) was tasked with figuring out how to install the support of excavation (SOE) piles within the tight working area, underneath the overhead catenary lines, and within the difficult soil conditions.
Due to the stringent working constraints, the team had two different pile designs for the 250+ piles ranging from 40 to 50 feet in depth. The first type was a conventional solider pile utilizing 1000-mm temporary Bauer casing and a wide flange beam as the main pile element. The UFS team was able to value engineer these piles to allow the entire pile to be installed with low strength flow fill to eliminate any need to chip out concrete when the follow-on crew installed the lagging. The second pile design consisted of an 18-inch diameter minipile installed with an excavator mounted drill rig equipped with a Mincon Spiral Flush ring bit and hammer. The spiral flush system in combination with an excavator mounted machine was selected in order to install these piles within the aforementioned constraints presented by the site and soil conditions. Additionally, this setup minimized the impact of drilling on surrounding infrastructure. The use of this drilling system marked one of the first times that air drilling of this kind was used within an active rail interlocking.