Blast-Induced Liquefaction for Full-Scale Foundation Testing Scott A. Ashford, M.ASCE 1 ; Kyle M. Rollins, M.ASCE 2 ; and J. Dusty Lane, M.ASCE 3 Abstract: This paper describes a pilot test program that was carried out to determine the appropriate charge weight, delay, and pattern required to induce liquefaction for full-scale testing of deep foundations. The results of this investigation confirmed that controlled blasting techniques could successfully be used to induce liquefaction in a well-defined, limited area for field-testing purposes. The tests also confirmed that liquefaction could be induced at least two times at the same site with nearly identical results. Excess pore pressure ratios greater than 0.8 were typically maintained for at least 4 min after blasting. The test results indicate that excess pore pressure ratios produced by blasting could be predicted with reasonable accuracy when single blast charges were used. However, for multiple blast charges, measured excess pressures were significantly higher than would have been predicted for a single blast with the same charge weight. The measured particle velocity attenuated more rapidly with scaled distance than would be expected based on the upper bound relationship developed from previous case histories. Settlement was typically about 2.5% of the liquefied thickness, and about 85% of the settlement occurred within 30 min after the blast. Cone penetrometer test results show that blasting initially reduced the soil strength, but after several weeks the strength had substantially increased. DOI: 10.1061/ASCE1090-02412004130:8798 CE Database subject headings: Liquefaction; Full-scale tests; Pile foundations; Slope stability; Lateral loads; Blast loads; Blasting; Deep foundations. Introduction The lateral load capacity of deep foundations is critically impor- tant in the design of bridge structures in seismically active re- gions. Although fairly reliable methods have been developed for predicting the lateral capacity of piles in nonliquefied soils, there is very little information to guide engineers in the design of piles that are surrounded by liquefiable soils. Certainly, ongoing cen- trifuge studies using small-scale models e.g., Wilson et al. 2000; Dobry et al. 1996 are providing valuable insight into soil-pile interaction in liquefied soil. However, full-scale tests are neces- sary to verify and calibrate these models and to provide ground truth information. To improve our understanding of the lateral load behavior of deep foundations in liquefied soil, a series of full-scale lateral load tests have been performed at the National Geotechnical Experimentation Site NGES at Treasure Island in San Francisco. The ultimate goal of the Treasure Island Liquefac- tion Test TILT project is to develop lateral load-displacement relationships for a variety of individual piles and pile groups in liquefied sand under full-scale conditions. The tests were carried out using a high-speed hydraulic loading system after the sand surrounding the piles was liquefied using blasting techniques. The first step in the testing program was to evaluate the ability of controlled blasting to produce a liquefied soil layer suitable for the testing program. Although blast densification has been suc- cessfully performed for over 50 years in many different soil and site conditions, site-specific studies are generally recommended Narin van Court and Mitchell 1995. Therefore, a pilot liquefac- tion study was designed to determine the optimal charge size, pattern, and delays required to liquefy the soil to a depth of about 5 m and a radius of 5 m surrounding the foundations. Further- more, a second objective of the pilot program was to confirm that the liquefaction testing was repeatable, i.e., that a second blast at the same site would yield similar results. In order to accomplish these objectives, a series of controlled blasts were carried out at three separate locations at the Treasure Island NGES. It is clear that the process of inducing liquefaction through controlled blasting compared to that resulting from seismic load- ing is quite different. In earthquake-induced liquefaction, the porewater pressure is slowly increased by shear waves propagat- ing up through the soil. In contrast, in blast-induced liquefaction, the porewater pressure is almost instantaneously increased by compression waves generated from the explosion. Blasting pro- duces higher accelerations and higher frequency ground motions than an earthquake; nevertheless, the velocity and strain levels are comparable. Field measurements during blast testing Gohl et al. 2001 and analysis Finn 1978 suggest that the residual excess pore pressure may largely be due to cyclic shear strains in both cases. During these experiments, we were not attempting to model the process leading up to liquefaction during an earth- quake, but to model soil behavior once liquefaction had been induced. Site Characteristics Treasure Island is a 160-hectare manmade island immediately northwest of the rock outcrop on Yerba Buena Island in San Fran- 1 Associate Professor, Dept. Structural Engineering, Univ. of California–San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0085. E-mail: sashford@ucsd.edu 2 Professor, Civil and Environmental Engineering, Brigham Young Univ., 368 CB Provo, UT 84602. E-mail: rollinsk@byu.edu 3 Graduate Student Researcher, Civil and Environmental Engineering, Brigham Young Univ., 368 CB Provo, UT 84602. E-mail: jdlane@et.byu.edu Note. Discussion open until January 1, 2005. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on February 6, 2001; approved on June 12, 2003. This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering, Vol. 130, No. 8, August 1, 2004. ©ASCE, ISSN 1090- 0241/2004/8-798 – 806/$18.00. 798 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2004