مطالعه تحلیلی درباره عوامل اصلی آسیب رساننده به پی های شمعی در زلزله توهوکو در ساحل اقیانوس آرام در سال 2011 Analytical study of the main causes of damage to pile foundations during the 2011 off the Pacific coast of Tohoku earthquake

1. Introduction Strong ground shaking during the 2011 off the Pacific coast of Tohoku earthquake (the 2011 Great East Japan Earthquake) damaged pile foundations over wide areas in Miyagi and Chiba prefectures in addition to causing various other forms of foundation damage through liquefaction-induced ground deformation, slope failure and complete destruction by tsunami. Such damage to pile foundations, which has been commonly observed in past major earthquakes, resulted in building structure tilting even though superstructures remained practically intact. Pile foundation repair and careful jacking-up to level were required to restore the buildings for use. Most of the damaged buildings were constructed before the Japanese code requirements for the seismic design of foundations went into effect in 1985, while several of them were constructed after 1985. According to the damage investigation,1 74% of the damaged buildings were constructed before 1985. Since approximately 26% of the damage occurred in buildings constructed since 1985, the year of construction does not appear to be a crucial factor in the damage to piles. In addition, pile damage notably occurred in precast concrete piles known to have non-ductile deformation characteristics, and the soil in which damage was observed consisted of very soft clay, a liquefiable layer and/or non-homogeneous strata in most cases. These conditions indicate that significant effects of nonlinear behavior of piles and soil materials must be considered when simulating the pile failure process. The authors focused on a case in which pile damage resulted in building tilting although the pile foundation design was based on a simple linear elastic analysis in accordance with the existing design code in 1987. An analytical study was performed using a foundation structure model to simulate the distinctive pile failure modes observed during the 2011 off the Pacific coast of Tohoku earthquake. In the analysis model, a remarkably damaged frame was removed and used as a calculation model consisting of piles with soil springs. Based on a previous study2 on the seismic performance of pile foundations during severe earthquakes, nonlinear flexural rigidity of the pile element and rotational stiffness at the pile head were adopted. The nonlinear characteristics of the soil springs were also adjusted in accordance with soil type and depth (overburden pressure). In addition, the pile group effect was considered depending on the position and spacing of each pile in a group. The seismic response of the building and the ground deformation were evaluated based on a free field ground response analysis using ground motions recorded at an observation point near the site. The inertia force and axial force at the pile heads were estimated based on the results of elastic analysis.