Frictional resistance of expansion-type steel pipe pile in sandy soil ground

Expansion-type steel pipes are used for rock bolts in tunnel excavation work in Europe. The authors are trying to apply the expansion-type steel pipe for the components of the reinforcement soil under small building foundations. The diameter of the steel pipe for rock bolts is expanded from 36 mm to 54 mm by adding inner high water pressure. The authors have carried out the field tests to confirm the reinforcement effect of the expansion-type steel pipe piles. From the compression tests of a piece of the steel pipe before the field tests, it was obtained that the average proportional limit load of the raw pipe pieces was 96.3 kN (stress intensity 316.7N/mm2), and that of the expanded steel pipes was 104.6 kN (stress intensity 344.1N/mm2), and then the strength of the expansion-type steel pipe piles were increased by about 9% by through hardening work. These test results were summarized as follows, 1) In order to examine the soil conditions around the expansion-type piles, piles were placed at three different intervals (300 mm, 450 mm, 600 mm) between the piles, and then SWS tests were carried out before and after expanding the piles at the middle position of each piles. The compaction effect on the surrounding soil of the expanded piles decreased as the pile spacing increased. In relatively loose sand, the compacted area is up to about 8 times (450mm) of the pile diameter. 2) From the results of the competitive axial tensile load field tests (using the expansion-type steel pipes and normal steel pipes with the same expanded diameter) in three sites (Toyonaka, Fukutsu, and Karatsu), the ultimate frictional resistance of the expansion-type piles was found to be about three times higher than that of the normal steel pipe pile. 3) According to the results of ten field tests on the expansion-type piles in the same above mentioned sites, the following equations were obtained for the relationships between the ultimate friction s and the average N value from the SPT test, or the average N' value from the SWS tests using Inada's conversion formula. However, these equations are be based on a small amount of data and a relatively large standard deviation It is supposed that the increment of the normal soil stress acting on an expansion-type pile in sandy soil increases the ultimate frictional resistance of the pile. On the basis of this supposition, the comparisons between the solutions of the ultimate friction based on the cavity expansion theory and the test results were in good agreement.