grout plant setup and a concern of column nonuniformity due to the presence of boulders and cobbles within the geo- technical profile. Based on the above concerns and risks, Nicholson Construction (Nicholson) proposed a value engineering (VE) alternate design that would perform in a similar fashion to jet grouting. The proposed alternate design included the use of micropiles to provide the necessary structural support of the B-Cell and would allow access to perform the required excavation for the removal of the contam- inated soils. This proposed method was ultimately accepted but only after under- going a significant design and verification process. A few key factors afforded by the micropile VE design that helped validate the proposed alternate were potential cost savings compared to jet grouting, limited spoils generation, test pilot holes that could be drilled at the same location as the micropiles to verify contamination and limit risk of exposing unknown contaminants during drilling, a solid steel structural element for load transfer and a more efficient equipment setup. Prior to achieving the 30% design stage and continuing through the design phase, several finite element analysis (FEA) studies were conducted to ensure the proposed alternate provided the B-Cell with sufficient redundancy, building code compliance, met project goals and was constructible. Micropile verification load test Three underpinning scenarios were pre- sented to CH2M Hill Plateau Remediation Company (CHPRC), which varied from a less intrusive “minimum total support” system (option 1) to a more elaborate “maximum total support” system (option 3), with option 2 being a middle-of-the- road scenario. Each scenario was evaluated individually for compliance with applicable codes and standards for dead weight and seismic loads as well as footing bearing pressures, vertical displacement, micropile displacements and global building stability. Then, each scenario was evaluated against the others and were scored based on overall structural performance, redundancy, con- structability, cost/schedule and worker safety. CHPRC ultimately selected option 2 as the desired support configuration, which was selected for the following reasons: • Provided a constructible system that addressed worker safety • Met the project requirements for design, constructability and schedule • Provided good structural performance and redundancy • Improved overall constructability by limiting structural modifications to the basement level and eliminated the need for structural modifications to the first- floor level near highly contaminated below-grade pipes • Provided excellent balance between de- sign, structural performance, con- struction, cost/schedule and worker safety, including minimizing exposure to highly radioactive contaminated materials. CHRPC also addressed the concern that additional lateral support would be required in addition to the micropiles to control sloughing and contamination migration during excavation. This concern was addressed by adding chemical grouting to the underpinning scope. Nicholson, as the geotechnical contractor, provided guidance in developing a grouted “gravity block” retaining wall to be constructed using permeation grouting. Once the des ign concept was developed, the geotechnical engineers, Shannon & Wilson, developed a FLAC numerical model to determine the new modulus of subgrade reaction for the permeation grouted soil, lateral soil loads and updated soil spring properties. Atkins, the structural engineer, then utilized this information to update the overall FEA model for the structure and analyzed the construction sequencing and performance of the system. Once approved, the entire underpinning concept (grouting and micropiles) proceeded through 60%, 90% and final design processes along with a verification program to monitor the installation and performance of the grouting and micropiles (to compare to design DEEP FOUNDATIONS • NOV/DEC 2019 • 85
