Discussion of “Plugging effect of open-ended piles in sandy soil”1

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 (Qm), 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 minor misrepresentation, the figure represents the typical response during driving. The authors’ measurements show that the length of the inside soil column increased throughout the driving of the test piles. In driving, therefore, shaft resistance along the inside of the pipe can be assumed to be mobilized along the full soil column length as indicated in fig. 2. The full picture is a complex combination of shear forces, wave travel, wave reflections, and inertia, which I will not attempt to discuss here. However, the response of the pile to a static force is very different to that shown in fig. 2 and is more similar to what I show in Fig. D1. In static loading, the pipe is pressed down engaging shaft resistance along the outside and toe resistance on the annulus area, the relatively small area of the steel pipe wall. The inside column— the core— ismade to follow the downwardmovement, but the movement meets resistance at the pile toe, which generates a base force that compresses the core and causes a relative movement between the inside wall and the core. That relative movement only acts along a distance represented by the length of the core compressed by the total base force, the length necessary to “spend the force” in a spring actionwith the compression of the core being equal to the toe movement. The actual load values determined in the static loading test reported by the authors are impaired because of the interaction between the outside and inside pipes caused by the welding the pipe together at the lower end. This fact becomes obvious in Fig. D2, which combines the authors’ pile-head load–movement curves for pile 2 (fig. 11) with the loads separated on the outside and inside pipes measured at depths of 1.9 and 3.7 m, respectively (figs. 12a through 12f). The sumof the outer and inner pipes should be about equal to the applied load (curve labeled “Head both pipes”), but they are not. As can be expected from the response of the inner pipe, no change of resistance is likely to have developed between the pile head and the first gage level. In contrast, between the pile head and the first gage level in the outer pipe, an extrapolation indicates that up to 80 kN might have developed as shaft resistance along the outer pipe before the 1.9 m gage level. I believe the indicated about 400 kN difference between the sum of the outer and inner records of load and the 2000 kN applied load is due to the interaction between the two pipes, as follows. At the 2000 kN maximum applied test load, figs. 12c and 12d indicate the loadsmeasured at the first gage level in the inner and