The authors have proposed a new ground reinforcement method using expansion steel pipe piles, which expand diameter from 36 mm to 54 mm by injecting high pressure water into a steel pipe deformed into the heart shape and sealed the both ends. The proposed method aims to increase the bearing capacity of the small building foundation, and to prevent the differential settlement of a small building on poor sandy ground.
In continuation of the research, this study focused on using the expansion steel pipe piles in composite ground, and vertical loading tests were carried out at both an artificial site and two natural sites composed of sandy soil.
The ground at the artificial site was prepared by backfilling sandy soil into a pit with dimensions of 1500 mm width, 4250 mm length and 2300 mm depth. In addition, two natural sites consisting mainly of sandy soil were selected and loading tests were conducted at five locations in each.
At the artificial site, comparison experiments were conducted in order to investigate the settlement behavior and bearing capacity of composite ground containing simple steel pipe piles with the same diameter as the expansion steel pipe piles. At the natural sites, the effect of ground reinforcement was confirmed by changing the size and shape of the foundation and the number of expansion steel pipe piles. In addition, FEM analysis using the artificial site as a model was performed and the effect of the expansion steel pipe piles on improving the ground bearing capacity was investigated. The findings obtained in this study are as follows:
1) At the artificial site (average converted N-value of 2.6), results from the loading tests on composite ground containing simple steel pipe piles show that the ultimate bearing capacity of the composite ground was equal to the sum of the ultimate bearing capacity of the piles and that of the ground. However, in the case of composite ground containing expansion steel pipe piles the ultimate bearing capacity was 15% higher than the sum of the ultimate bearing capacity of the piles and that of the ground.
2) At the natural sites (average converted N-values of 6.5 and 5.4), the ultimate load was not reached during loading tests on composite ground containing expansion steel pipe piles, however after the initial stage of subsidence the bearing capacity of the composite ground was almost equal to the sum of the bearing capacity of the piles and the bearing capacity of the ground.
3) From the results of the 3D FEM analysis, which used the loading tests at the artificial site as a base model, it became clear that the effect of expansion of the expansion steel pipe piles on the compaction of the surrounding ground was greater for ground with a smaller Young’s modulus (or converted N-value), and that this in turn gave a greater increase in the bearing capacity of the composite ground.
4) From the above results of the loading tests and FEM analysis, it can be inferred that expansion of the expansion steel pipe piles in composite ground with a converted N-value of approximately 3 or less will lead to an increase in the bearing capacity of the ground.
