A field study on the behavior of three destructive piles in soft soils subjected to axial load was presented.All the three piles with different diameters were base-grouted and installed with strain gauges along the piles.The complete load transfer behavior of the base-grouted pile was analyzed using measured results.Moreover,the thresholds of the relative pile-soil displacement for fully mobilizing skin frictions in different soils were investigated,and pile tip displacements needed to fully mobilize tip resistances were analyzed.The results of the full-scale loading tests show that the skin frictions are close to the ultimate values when the pile-soil relative displacements are 1%-3% of pile diameter,and the pile tip displacements needed to fully mobilize the tip resistances are about 1.3%-2.0% of pile diameter.The load transmission curve of the soils around the pile tip corresponds to a softening model when the pile is loaded to failure.
The increase in capacity of displacement piles with time after installation is typically known as soil/pile set-up. A full-scale field test is carried out to observe the set-up effect for open-ended concrete pipe piles jacked into mixed soils. Both the total capacity and the average unit shaft resistance increase approximately linearly with logarithmic time. The average increase rate for unit shaft resistance is 44% per log cycle, while the average increase for total capacity is approximately 21%. A review on case histories for long-term set-up indicates an average set-up rate of approximately 40%. Based on this, the mechanism of pile set-up is discussed in detail and a three-phase model is suggested.
