2010 年 59 巻 1 号 p. 26-31
As a foundation engineering technique to deal with soft ground, a rational design method involving the formation of composite ground around piles (mainly using the deep mixing method) and reflecting the increased shear strength as the horizontal resistance of piles was studied and developed. The result of combining piles and composite ground is referred to as the composite ground pile foundation. The scope of improvement for composite ground around piles is set as a range raised to the working gradient of passive earth pressure from a characteristic length of 1/β according to engineering theory. The horizontal resistance of piles is designed by converting the shear strength of composite ground into the modulus of deformation and using the elastic subgrade reaction method. The validity of the design method was confirmed empirically through an on-site horizontal loading test using actual piles. In addition, the dynamic mechanical behavior of piles and composite ground with this method in response to large-scale earthquake motion was verified through a centrifugal excitation test. The results confirmed that piles and composite ground maintained their soundness and displayed the required subgrade reaction under earthquake conditions. Dynamic nonlinear finite element analysis was also conducted on an actual site with this method as a model to confirm that pile displacement and strain were controlled and that the seismic performance of the foundation was improved by forming composite ground around piles. As it was possible to summarize basic data on the validity of this method, plans are now under way to develop guidelines for its effective use on actual sites.