Liquefaction Mitigation by Vibro Stone Columns for Hamner Bridge Project

The planned bridge approach embankment located at the Hamner Avenue Bridge over the Santa Ana River in Norco, CA, is located in a high-risk seismic area. The embankment will be constructed on fill overlying young alluvial river deposits. Under the site design earthquake of magnitude 7.7 and peak ground surface acceleration of 0.57 g, the embankment site will be liquefied and potentially trigger lateral spreading. Treatment by Vibro Replacement Stone Columns was adopted to mitigate the site liquefaction and lateral spreading hazards below the planned embankment retaining wall footprint. To densify the liquefiable sands, the specialty geotechnical contractor performed liquefaction analysis and determined the stone column replacement ratio and treatment based on the soil fines content and pre-treatment CPT tip resistance. The contractor used a real time data acquisition system to monitor and record the stone column installation, including the vibrator penetration depth, electric current, and air pressure, which allowed the contractor and engineer to interpolate stone column diameter as a function of depth. This system provided a reliable field quality control record to the design engineer and the client. As part of the project QA/QC process, 20 post-treatments CPTs were performed and analyzed, which confirmed all the design densification criteria achieved. This paper discusses the project design, construction, and quality controls. The soil densification effectiveness was evaluated statistically by the comparison of the pre- and post-CPTs as a function the CPT Ic values.