In the construction of port facilities such as breakwaters or revetments, normally consolidated alluvial clay deposits often found in Japanese coastal area raise difficult problems both of stability and residual settlement. One of the solution for these problems is the deep improvement by means of sand compaction pile method. The authors carried out an extensive study on the bearing capacity of the improved ground by scaled model tests and elasto-visco-plastic finite element analysis.
　All the scaled model tests are carried out in the high acceleration field by PHRI geotechnical centrifuge. Normally consolidated clay ground is prepared by self weight consoliodation in the centrifuge. Installations of compacted sand piles are carried out on 1g floor. And load tests are carried out again in centrifuge under various combination of vertical load component, V and horizontal load component, H.By the test results, failure envelope of the improved ground is constructed in V-H space. The results of numerical analyses by FEM reasonably explain the failure envelope thus obtained.
　These physical and numerical model tests results are reasonably well explained by the practical design technique commonly used in Japan.
　The validity of the test results is also confirmed by a full scale load test separately carried out by the Third Port Construction Bureau at Maizuru Port.