Discussion of “Plugging effect of open-ended piles in sandy soil”

Abstract

The authors have presented an interesting account of a fullscale study of the response of a pipe pile driven with an open toe to the presence of an inside soil column in the pile (Ko and Jeong 2015). The tests consisted of dynamic measurements during the driving and static loading tests some time after the driving on double-wall pipe piles. Strain-gage measurements were used to determine the load distribution.The double-wall test piles were fabricated with the inside pipe centered in relation to the outside pipe. The steel cross section of the outside pipe was about 10% smaller than the inside pipe. The about 40 mm "gap" or void between the outside of the inner pipe and the inside of the outer pipe was welded closed at the pile toe to prevent soil from entering the "gap". The welding resulted in a firm and fixed connection between the two pipes, ensuring that the lower ends of the two pipes moved in unison in the tests. I understand that no such fixed connection was made at the pile head. However, the lengths of the pipes were the same, which means that a plate placed on the pile head rested on both pipes enabling the upper end of both pipes to be engaged approximately simultaneously. Presumably, the dynamic gages (accelerometer and strain-gage pairs, I assume) were placed on only the outer pipe and the force delivered to the pile by the hammer impact was determined from the sum of the two pipe areas as based on the assumption of perfect connection to the pile head plate. Because the test pile was made up of a pair of pipes welded together at the lower end, I would expect that the dynamic gages will have recorded considerable reflections during the tests. I would not have expected a good correlation between the CAPWAPdetermined capacities (Table 2) and those determined by the offset limit method from the static loading tests ( fig. 11). It would be interesting if the authors could provide details of the pile and soil models employed in the dynamic analyses.As reported by the authors, development of an inside soil column and plugging during the driving of open-toe pipe piles has been addressed by several researchers. All depict the forces acting on the pipe pile during driving as similar to that shown in the authors' fig. 2, i.e., with upward-pointing shear force vectors both along the outside and inside of the pile and the inside vectors shown along the full length of the inside column. That is, the vectors indicate the forces as acting on the pipe and not on the core. The suggestion is that the open-toe pipe is forced down over the inside core.The force vectors also show soil forces acting on the steel pipe pile both at the base (Q b ) and along the inside of the shaft (Q m ), but the response cannot be both, it must be one or the other. That is, if the pile experiences a toe resistance, it has a rigid plug and there is no inside shaft shear (but for along a very short length of that rigid plug). If the pipe slides down over the core, there is inside shaft resistance, but no toe resistance. Apart from this mino...

Reply to the discussion by Fellenius on “Plugging effect of open-ended piles in sandy soil”

“…The authors would like to thank Bengt H. Fellenius for his discussion of our paper (Fellenius 2015).…”

“…D2 in Fellenius (2015), there is misleading information. When the analysis was rerun, the increase of axial load for outer pile TP-2 was extrapolated, and is estimated to be 120 kN not 80 kN.…”