As the EPC Foundation Engineer of Record, Crux was responsible for oversight of the geotechnical characterization program, as well as foundation design and oversight of installation. Foundation design included both driven pile and drilled shaft foundation alternatives developed to resist the high static structure loads, downdrag loads resulting from deep potentially liquefiable soils, and extreme lateral spread due to the adjacent channel slope.
Foundation design was challenging across the board, but of particular note was the foundation design and construction for Tower T2X, a four-legged lattice tower with a height of approximately 387 feet and a leg span of 60 square feet. The structure was heavily loaded; each tower leg foundation was required to support design compression, uplift, and shear loads of approximately 6770 kips, 3840 kips, and 580 kips following incorporation of required factors of safety. A maximum displacement (vertical and lateral) of 1 inch was permitted under static loading conditions, with a requirement that the new structure not collapse and maintain its ability to carry normal working design loads after an earthquake. In addition, highly corrosive and potentially contaminated undocumented fill soils, environmental sensitivities, and known and unknown underground utilities were expected to be encountered.
Based on project-specific subsurface data obtained prior to construction, and following over a year of technical review, it was established that the design liquefaction depth at T2X was 86 feet and seismically induced lateral displacement was estimated to be greater than 6 feet. This meant the designed foundation system would be required to resist and/or accommodate more than 6 feet of ground movement in the form of sliding towards the Channel due to seismically induced liquefaction of the underlying soils.
Crux collaborated with foundation contractors throughout the design process, entailing iterative soil-structure analyses and working to establish the optimal foundation size based on design requirements, material and equipment availability, and cost. The final design included fully cased, large-diameter drilled shafts at each tower leg connected by a combination grade- and tie-beam system. Full size grade beams were constructed perpendicular to the Channel slope, and tie beams parallel to the Channel face. The system was specifically designed to address the liquefaction induced lateral spread in a four-legged structure, and would result in uniform lateral displacement, enabling the structure to maintain normal working design loads after a design earthquake.