When structures undergo earthquake vibration, they are subjected to various resisting forces from the surrounding ground through interaction (Fig. 1). Because of insufficient data on earthquake damage and constraints in analytical technology, no methods have yet been established to evaluate resisting forces 2. to 5. in Fig. 1. As a result, these forces have generally been neglected in safety considerations, and foundations have therefore often been judged to require reinforcement against large-scale earthquakes. Consequently, the RTRI has proposed a technique to evaluate in detail the interaction between foundations and the surrounding ground, aiming to determine the earthquake resistance of existing foundations more precisely than by conventional methods.
      To this end, the RTRI has developed a technique to model the shear friction resistance at the footing bottom and the front face resistance of footing, pillars and underground beams. This model uses three-dimensional analysis according to ground conditions and the contact between ground and foundation. Applied to calculate the earthquake resistance of bridges and viaducts, the model showed foundation strength to be 20 to 60% higher than the value estimated by conventional methods when the ground conditions were good (Fig. 2).
      An excavation survey of the pile foundation of a viaduct subjected to the seismic motion of the Sanriku Minami (south) earthquake showed that, although shear failure was found in the pillars, the piles were intact, whereas the conventional method predicted that the pile foundation would be subject to shear damage. In contrast, the proposed method predicts that shear failure of pillars will occur, but that the pile foundation will remain intact. This agrees with the survey results.

Fig. 1 Concept of interaction between the surrounding ground and the foundation	Fig. 2 An example of the discussion of shear strength (on a viaduct)

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