The project
Keller provided a design-build underpinning solution and was able to quickly adapt the design and construction methods to address multiple changed conditions.
To address increasing student enrollment, expansion and internal renovation of a historic public school, originally constructed in 1894 and once attended by John F. Kennedy, was needed. The scope of the project included demolition of existing additions and construction of a new integrated gymnasium and other facilities.
The challenge
During the planned demolition, it was discovered that the structure’s bearing walls terminated just below grade rather than being supported on spread footings. The school’s rear wall that would tie into the foundation support for the planned excavation required underpinning prior to commencement of the remaining excavation and construction.
The solution
Given the lack of spread footings, the underpinning pit geometry required modification to increase the anticipated cut height from 9 to 13 ft. This additional height required redesign and field adaptation of the original underpinning plan. Two levels of tiebacks were incorporated for additional lateral restraint. The remainder of the excavation support was designed as cantilevered drilled soldier piles and lagging to allow cuts of up to 20 ft, with more economical temporary soil nailing and shotcrete facing used where property line restrictions permitted.