Application of the Pile Toe Test to Cast-in-Place and Precast Piles(ADSC1996)

H.Ogura(GEOTOP)・M.Sumi(GEOTOP)・H.Kishida(Science University of Tokyo) and T.Yoshifuka

■掲載誌:JFOUNDATION, pp.23-28

Some explanation is needed for the readers who are not familiar with Japanese methods, terminology, and requirements, which are different than in the Vnited States. The conventional load test in which the load is applied from the top, pushing the pile down is called a "head load test". When the pile is pushed up from the bottom with the Osterberg Load Cell, it is called a "toe load test". The load cell is most often called a "jack". In Japan and in most of the rest of the world, cast-in-place con-crete piles and driven piles are both called "piles" and are dis-Unguished from each other as "bored" piles and "driven" piles. However, in Japan driven piles are virtua]ly banned due to noise pollution. Precast prestressed concrete piles are installed in pre-bored holes which are grout-ed. Piles are tested using the Japanese Geotechnical Society Standard which requires that the load test be made by using suc-cessively increasing loads with a minimum of four cycles as seen in Figure Three. The purpose of the tests described in this article is to determine whether using toe load tests give the same results as the head load tests. The first series of tests were made on drilled shafts which were trem-ied. A test was made on a shaft were the side friction was known to be smaller than the end bear-ing capacity by pushing up from the bottom until ulUmate resis-
tance in friction was reached. A reaction frame was then used to hold the top of the shaft fixed while the load cell pressure was increased until failure was obtained in end bearing. In the head load test on an adjacent shaft, the shaft was tested to ulU-mate load and the end bearing was calculated using readings from strain gages placed in the ed by Jonj Osterberg, and has been applied to more then forty load tests in the (JSA', including cast-in-place concrete piles (driue(1 shafts) and steel pipe piles. Some of the test examples have been descr~(1 in a paper by Osterberg, "A New Load Testing Device Tests Driven and Bored Piles by Pushing up from the Bottom". h this article, the writers ~1e pile toe test method has the merit of doing away wtth reac-tion piles and loadjng firames. This results in edonomy due to the drastic reduction in the time required to set up the load test. For this reason the pile toe test is consjdered to be expedient and promising for quality control in pile foundatjons. shaft. The load-settlement curves for end bearing from the two tests are shown in Figure Four and load-settlement curves for the top of the pile (measured in the top loaded test and calcu-lated from the bottom loaded test) in Figure Five. In another series of tests on grouted precast concrete piles, a load cell placed at the top and another at the toe, with the top of the pile held fixed while the toe cell tested the end resistance. Then in another test nearby at the same site, the pile was top loaded, pushing the pile down to measure the side friction. The results are shown in Figures Seven and Eight. The tests show that the side fricUon is virtually the same whether loaded from the top or bottom. (Jorg Osterberg)