Static and cyclic lateral load testing of a full-scale pile group during blast-induced liquefaction

A research study on full-scale lateral load behavior of deep foundations in liquefied sand was conducted at the National Geotechnical Experimentation Site (NGES) in San Francisco, California. The goal of the project was to develop load-displacement relationships for driven piles and pile groups in liquefied sand under full-scale conditions. A series of lateral load tests were carried out while the soil surrounding the piles was liquefied. A pilot liquefaction study was performed at the site to determine the optimal charge size, pattern, and delays required to liquefy the soil to a depth of 5 m and a radius of 5 m surrounding the foundations. The site, consisting of loose saturated sand placed by hydraulic filling, was known to have liquefied in the 1989 Loma Prieta Earthquake. The effects of the controlled blasting were measured by pore-pressure transducers, settlement monuments, and portable seismographs. Each test blast consisted of between one and 16 charges placed at a depth of 3.66 m, with charge masses between 0.5 and 0.75 kg. A lateral load test was then performed on a full-scale pile group to evaluate pile-soil-pile interaction effects. The 30 pile group at 3.3 pile diameter spacing was driven opened-ended into a profile consisting of loose to medium dense sand, underlain by clay. The load carried by each pile was measured and strain gauges were attached to determine bending moment. A single pile test was conducted for comparison. Group effects significantly reduced lateral resistance of all rows relative to single pile behavior. Trailing rows carried less load than the leading row. In contrast to tests in clay, lateral resistance was also consistently lower for middle piles within each row. Back-calculated p-multipliers were 0.8 for the front row and 0.4 for the trailing row piles, which is consistent with results from previous centrifuge tests and full-scale tests where different installation methods were used.