Stress Development Inside Large Diameter Pipe Piles Using a Soil Plug Forcing System

It is well known that soil plugging inside tubular steel piles will only appear in rather small diameter piles during impact driving. Therefore, large open ended steel piles, which are often used for the fixation of offshore buildings, such as wind farms, are highly unlikely to develop an internal soil plug. To take advantage of a soil plug, where a significant rise in the piles’ bearing capacity generally appears, a large diameter pipe-pile with an inner steel ring was designed by the third author. The location of the steel ring was determined by the soil formation in situ. To avoid increasing pile driving energy, the internal ring should dip into dense soil conditions only for the last few decimeters of driving. In October 2010, a full scale test was performed in the harbor of Hamburg, using two tubular piles with an outer diameter of 1220 mm. One pile was equipped with an inner steel ring as described above. The second pile was a typical tubular pile without any attached systems. To better compare the results, both piles were driven next to each other. Both piles were equipped with internal total stress and pore water pressure sensors at the pile tip to investigate the radial stress development during and after installation. Acceleration and strain at the pile head were measured to predict the bearing capacity. Using the numerical analysis program CAPWAP (Case Pile Wave Analysis Program) [15], the distribution of shaft and toe friction can be determined additionally. Furthermore, the internal soil movement was surveyed during driving. The results of the measurements showed, that when using an inner steel ring, a significant rise in internal radial stresses and the piles’ bearing capacity occurs. To better understand the stress development inside and outside the two investigated piles during driving, a numerical back-calculation of the recorded measurements was performed. The results of the full scale and numerical simulations, with a particular focus on the use of an internal steel ring to force the soil to plug behavior in large diameter pipe piles, is presented in the following paper.