As a result, lateral loads on the drilled piers that support the core foundation were reduced by using the bedrock’s passive resistance, in conjunction with lateral resistance provided by the drilled piers. It was designed to “kiss” the undisturbed bedrock at 29 feet below street grade and seven feet below the lower-level parking garage floor. Two longer beams would run along the longitudinal axis of the building and be pulled together by four shorter, perpendicular beams. The system included eight drilled piers, each ten feet in diameter and boring more than 100 feet into the bedrock, which supported seven-foot deep, seven-foot-wide tie-beams. Martin/Martin and CTL created a tie-beam and drilled pier system with a footprint of 120 feet by 47 feet. Foundation DesignĪ downhole view of the reinforcing cage and concrete placement for a drilled pier used in the foundation for 1144 Fifteenth Street.īefore the non-conventional process could be attempted, the foundation needed to be designed. After the bedrock surface was cleaned, the reinforcing mesh would be placed and attached to the soldier piles then shotcrete would be applied pneumatically to protect the bedrock and preserve its strength. The excavation at the core perimeter would expose the top of the foundation piers, soldier piles, and bedrock. The process would require the piers to be poured 7 feet below basement grade and the installation of vertical concrete and steel beam soldier piles around the perimeter before excavating. This system would rely on undisturbed bedrock around the perimeter lateral face of the tie-beam system to transmit a portion of the lateral load to the bedrock. Instead of excavating the bedrock to support a conventional drilled pier and thick monolithic reinforced concrete mat core, the team designed a system using a tie-beam connecting and supported by drilled piers. The shallow bedrock could have presented an additional challenge, but CTL, along with Martin/Martin engineers, used it to their advantage. Here the second section is hoisted into place.ĭuring soil testing, the CTL team identified that bedrock was at the elevation of the building’s lower level. Due to the length of the piers, the reinforcing steel was built in two sections that were spliced together. Reinforcing steel cages form the drilled piers used in the foundation for 1144 Fifteenth Street.
The team had to dig deep, but ultimately they came up with a way to get the difficult job done – create a foundation design using a shoring system that allowed lateral loads from the drilled pier/tie-beam core foundation to be transferred directly to undisturbed bedrock. Lujan, it was with a pretty sizable task: design a building foundation for a very visible and very heavily-loaded skyscraper in a tight building envelope and sitting on the area’s sedimentary bedrock. Needless to say, when Martin/Martin Consulting Engineers tapped CTL|Thompson’s geotechnical engineers Marc Cleveland, David Glater, and Benny I. The architectural team needed as wide a base as possible, so the foundation was designed “property line to property line.” Secondly, land is scarce in Denver and the 1144 Fifteenth site – in a prime location – is a tight one, bound by downtown streets on three sides with the equally iconic Four Seasons Hotel on the fourth. Moreover, soil and bedrock conditions can vary significantly within any individual foundation area. As compared to New York, that is underlain by strong bedrock like schist, the comparatively soft rock (intermediate geomaterial) prevalent throughout Colorado’s Front Range cannot withstand the loads induced by very tall buildings, particularly with a dense office use.
There is a reason the buildings of Denver’s skyline are primarily low compared to the skylines of other U.S. North face of iconic 1144 Fifteenth at topping-out ceremony in July 2017.įirst and foremost was the ground itself.
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