The project involved ten drilled shaft foundations to support tubular steel poles for new 345 kV lines just outside downtown Chicago. Planned structures ranged in height from 90 to over 240 feet, and foundations would be subject to baseline reactions as high as 72,000 kip-ft overturning moment and 415 kip shear. Geotechnical conditions consisted of uncontrolled fill and soft- to medium-stiff clays underlain by highly weathered to moderately weathered limestone. Site-specific borings indicated variable bedrock depths ranging from 50 to 85 ft.
Value Engineering Opportunity
Proposed bid-level designs specified drilled shaft diameters between 7 and 17 ft with depths down to 82 ft. This included three 17-ft diameter shafts to be installed in rock, two with 17-ft rock sockets and one with a 31-ft rock socket. Looking at these designs, MJ Drilling (MJD) concluded that construction would entail approximately 540 cu yd of rock excavation and would require a 24-hour work schedule to meet deadlines. MJD suspected bid-level designs were conservative and engaged Quanta Subsurface (QS). After a brief review, QS agreed several of the foundation sizes could be reduced.
QS began by establishing its own geotechnical design parameters through review of existing geotechnical data and local geologic maps. Using this data, minimum foundation diameters were iteratively selected with consideration to anchor-bolt layouts until an optimized solution was developed. Foundation designs were also tailored to MJD’s equipment and tooling when potential cost or schedule savings were present. Overall, the proposed Value Engineering (VE) alternate reduced expected rock-socket lengths by 46%, rock excavation by 60% and concrete volumes by 16%.
Involving the owner early on in the VE conversation was key to its successful implementation. Bid level designs had been permitted for construction in advance, and there was concern that the time required to permit new designs would prevent the project from meeting a strict completion deadline. Upon review, ComEd determined the efficiencies the VE option provided would more than make up for any re-permitting time.