6 th International Conference on Earthquake Geotechnical Engineering 1-4 November 2015 Christchurch, New Zealand Liquefaction Induced Negative Skin Friction from Blast-induced Liquefaction Tests with Auger-cast Piles K.M. Rollins 1 and J.E. Hollenbaugh 2 ABSTRACT Blasting was used to induce liquefaction around auger-cast piles extending to 8.5, 12, and 14m below ground at a site in Christchurch, New Zealand. Liquefaction led to negative skin friction and pile settlement. The depth to the neutral plane increased as the pile length increased. Skin friction following liquefaction was compared to pre-liquefaction values based on static load tests. Negative skin friction in the non-liquefied soil was equal to the positive skin friction. Contrary to common design assumptions, the negative skin friction in the liquefied sand was not zero. As excess pore pressure dissipated, the increased effective stress allowed negative skin friction to increase. After consolidation, the average negative skin friction was roughly equal to 50% of the positive skin friction which agrees with previous full-scale tests with driven steel piles. The average unit side resistance for the auger-cast piles was typically 50% to 70% of the unit side resistance predicted by design FHWA equations for drilled shafts. Introduction Frequently, deep foundations extend through potentially liquefiable sand layers near the ground surface and bear on more competent layers at depth. However, when liquefaction occurs, negative skin friction and associated settlement may occur. Many design procedures assume that negative skin friction in the liquefied layer will be zero and use this value to evaluate the consequences of negative friction and pile settlement. However, a full-scale field test indicates negative skin friction after liquefaction may be 50% of the pre-liquefaction skin friction (Rollins and Strand 2006). To investigate the loss of skin friction and the development of negative skin friction, soil-induced load was measured in three instrumented full-scale piles after blast-induced liquefaction at a site in Christchurch, New Zealand. The three test piles consisted of 0.6 m diameter Auger-Cast Piles (ACP) extending 8.5, 12, and 14 m below the ground surface. The piles were instrumented with strain gauges to determine load vs depth in each pile. Geotechnical Site Conditions The test site was located in Avondale near the Avon River in Christchurch, New Zealand. This area experienced significant liquefaction settlement (0.3 to 1.0 m) during the Christchurch earthquake sequence in 2010-2011 and most homes in the area had been condemned. In connection with this study, site characterization, consisting of cone penetration tests, standard penetration tests, shear wave logging, and undisturbed sampling, was performed by Tonkin and Taylor in association with the Earthquake Commission in New Zealand. 1 Prof., Civil & Environ. Engrg. Department, Brigham Young University, Provo, Utah, U.S.A., rollinsk@byu.edu. 2 Sales Operations Analyst, Qualtrics, Provo, Utah, U.S.A., erick.hollenbaugh@gmail.com