A new composite pile foundation, namely composite foundation of steel pile with soil improvement, is proposed to resistant intensive seismic horizontal force and to reduce the environment impact. Soft soil at the shallow depth is improved by cement mixing method and a cemented soil mass can be formed in order to increase horizontal resistance. Steel piles are constructed through the cemented soil mass in order to transmit vertical loads to bearing stratum. The new composite pile foundation can be expected to have great horizontal bearing capacity as caisson foundation. Two in situ specimens were prepared, and horizontal loading tests of the composite pile foundation were conducted to confirm their behavior. Specimens included two steel piles of diameter 0.6m, in which soil was improved by cement mixing method from surface to 8m depth. It is shown that the cemented soil mass and steel piles worked as a mass under some extent displacement. The horizontal bearing capacity was much greater than that of steel pile only. To apply it to practice foundation, a spring model of the composite pile foundation is proposed, and compared with the results tested. Based on in situ test and analyzed results, it is concluded that the spring model proposed can be used for practice problems.
After the great Hyogoken-Nanbu Earthquake, Japan Highway Association (1995) has revised seismic design specifications. For I site (Rock) and II site (Stiff and Normal soils), the standard response spectra are assigned to 2000gal and 1750gal, respectively. For III site (Soft soil), it is assigned to 1500gal at 0.5–1.5 second period domain. To withstand such intensive seismic force, it may be difficult to design pile foundation at III site due to its low horizontal bearing capacity. Meanwhile, environment of geotechnical engineering becomes more and more strict.