The reinforcement of existing airports to resist earthquakes has been investigated along with the recognition of their importance. Especially, the re-consideration and optimization of soil improvement area has been recently required on the basis of the performance-based specification in the situation that whole of liquefiable area has been improved by the conventional descriptive specification.
　In this study, the effect of partial soil improvement against liquefaction by the chemical grouting method is examined. At first, the undrained tri-axial test for the both of cyclic shear and monotonic shear that was conducted in order to investigate the characteristic of liquefaction resistance of non-improved soil and improved soil, and volume change. Moreover, the both of model-scale and full-scale experiments were conducted in order to confirm the amount of effects on the restraint subsidence. Finally, total consideration was done.
　Conclusions are summarized in the following three points:
　1) The improved soil has the feature which is hard to be influenced in the change of soil skeleton structure by the propagation of the excess pore water pressure and the cyclic shear in it.
　2) In the case of ground under runway, the partial soil improvement ratio reaches 70-100%, when the chemical grouting method is applied as countermeasure for liquefaction.
　3) It is important to grasp the characteristics of volume change by carrying out model-scale experiment etc, when the numerical analysis methods from the seismic response to the consolidative deformation are performed.