Piezometer Performance at Wildlife Liquefaction Site, California

Abstract

In response to an urgent need for field data from instrumented liquefaction sites, the U.S. Geological Survey in 1982 selected and instrumented a site in southern California called the Wildlife site. Two accelerometers (one at ground surface and one at a depth of 7.5 m) and six electrical pore-pressure transducers (five in a liquefiable silty sand layer) were placed at the site. The November 1987 Superstition Hills earthquake triggered sand boils and the desired instrumental response by generating excess pore-water pressure that approximately equaled the initial effective overburden pressure. These records are the first from a field site to trace ground motions and pore pressures through the entire liquefaction process. Because pore pressure continued to rise after most of the seismic energy had propagated through the site, questions about the fidelity of the pore-pressure records have been raised. Because of the importance of the Wildlife records, we reexamine pertinent aspects of the instruments and their placement, review their 1987 response, evaluate and respond to criticisms by Hushmand et al. in 1992, and examine analyses of the records by other investigators that are pertinent to an evaluation of the fidelity of the piezometer records. This review concludes that no data or analyses have been developed that convincingly demonstrate that the pore-pressure piezometers responded incorrectly. Conversely, an analysis by Zeghal and Elgamal in 1994 provides strong evidence that the piezometers responded with a high degree of fidelity.