In Situ Dynamic Testing of Pore Pressure Transducers at Treasure Island, California using "Caltech Piezometer"
Liquefaction in saturated sandy deposits is one of the most dramatic causes of damage to structures during earthquakes. The onset of liquefaction has been observed through the occurrence of sand boils, building settlen1ents, lateral spreading, and other phenomena during numerous earthquakes. The most recent reminders of liquefaction and its destructive effects were observed in Kobe, Japan during the 17 January 1995 Hanshin earthquake. A thorough understanding of liquefaction phenomena, and formulation and calibration of analytical techniques to predict liquefaction occurrence and its effects, require data from instrumented sites where in-place soil properties have been appropriately established, and where ground motions have been measured for an earthquake strong enough to have generated significant increases in the pore pressures. So far, among few potentially liquefiable sites around the world that have been permanently instrumented with pore-pressure transducers and accelerometers, only the Wildlife Site in Imperial Valley, California has liquefied during an event. Acceleration and pore-pressure data were recorded at the Wildlife Site during the magnitude 6.6 Imperial Valley earthquake of24 November 1987 which resulted in liquefaction of the site. However, some unusual aspects were observed in the data, including long rise times of the pore pressures and a time lag between the strong ground shaking and maximum pore pressure development. Conditions associated with installation of the pore-pressure transducers might have been the cause of the anomalies observed in the data. In a con1prehensive experimental study under a grant from the U.S. Geological Survey (USGS), the authors performed in-situ dynamic inspection and calibration of the USGS piezometers installed at the Wildlife Site versus a reference pore-pressure transducer carefully installed close to the existing transducers. The field and the laboratory tests performed as part of this research provided valuable information on overall response of the USGS transducers and on the techniques to in1prove the installation and subsequent in-situ inspection of piezometers at any future site. Following the Wildlife field study a prototype unit of a new pore-pressure measuring probe, designed at the California Institute of Technology (Caltech), was fabricated and used in a field testing program at Treasure Island, San Francisco, at the National Science Foundation (NSF) U.S. Geotechnical Test Site to develop standard field techniques for installation and in-situ calibration of pore-pressure transducers. A large number of tests was performed using the "Caltech Piezometer" and several types of USGS pore-pressure transducers at the Treasure Island site in February 1994. The piezometers were all installed at sin1ilar depths, and pore pressures were generated either by dropping a 1300 lb. (590 kg) weight on the ground surface or by detonating explosives at some depth below ground surface at equal horizontal distances from the piezometers. The new Caltech probe performed well in all the tests conducted, and provided a ready means of introducing a deaired saturated probe into the ground to any required depth in the field. On the other hand, the performance of the USGS piezometers deployed was quite variable. The ease of on-site preparation, calibration, and installation of the "Caltech Piezometer" suggests a number of possible uses for it such as a) long-term installation and monitoring of pore pressures during earthquakes, b) testing piezometers at existing instrumented sites, c) rapid deployment and measurement of pore pressures during aftershocks, d) monitoring dynamic compaction of hydraulic or liquefiable natural fills, and e) in-situ field measurement of liquefaction potential of granular soils.
Copyright © 1996 Elsevier Science Ltd. The above research study was funded by the U.S. Geological Survey under Grant No. 1434-92-G-2169. We are grateful to Michael Bennett of the USGS for his assistance in the field tests, and to Brian Rague for organizing and setting up the data acquisition system at Treasure Island. Richard Faris of Western Division, Naval Facilities Engineering Command, San Bruno, California was instrumental in obtaining permission for the tests to be conducted at the U.S. Naval Facility on Treasure Island.