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During initial driving process, open-ended steel piles are driven through the soils at their bases. However, shaft friction will gradually develop between the steel piles and soils inside piles at some time after pile driving. The hitting action of driving hammers induces forces to the soil and later it comes to a stage when the inertial forces of inside soils, together with the internal frictional forces exceeds the bearing capacity of soils at pile toes. Consequently, the soil plug formed is brought down by the piles.
It is practically possible to excavate all soils inside steel tubular piles and replace them completely by reinforced concrete. However, as engineers strive to produce economical design the extra cost associated with excavation of soil plug and filling of concrete could be saved in case the soil plug remains in position. Moreover, from the technical point of view it is considered unnecessary to remove the soil plugs because it serves to provide a platform for the placing of on-top infill concrete on one hand and to fill the void space below the infill concrete on the other hand. In addition, the soil plug is considered to be sufficiently compacted by pile driving action and is deemed to be stable during the design life of the piling system.