Soil plug in marine piling system of steel tubular pile with reinforced concrete infill

At the time of initial driving process, open-ended steel piles are driven through the soils at their bases. Though shaft friction will steadily develop between the steel piles and soils inside piles at some time after pile driving. The hitting action of driving hammers persuades forces to the soil and later it comes to a stage when inertial forces of inside soils together with the internal frictional forces surpasses the bearing capacity of soils at pile toes. Thus soil plug formed is brought down by the piles. 

It is almost possible to excavate all soils inside steel tubular piles and replace them entirely by reinforced concrete. Though as engineers strive to produce economical design extra cost associated with excavation of soil plug and filling of concrete can be saved in case the soil plug remains in position. Furthermore from the technical point of view it's considered unnecessary to remove the soil plugs since 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 instead. Additionally soil plug is considered to be satisfactorily compacted by pile driving action and is deemed to be stable at the time of design life of the piling system.