What is the mechanism of formation of soil plugs in marine driven steel piles with reinforced concrete infill?

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 exceeding the bearing capacity of soils at pile toes. Consequently, the soil plug formed is brought down by the piles. Reference is made to M. J. Tomlinson (1977).

Note: A soil plug is a column of soil formed at the bottom portion of marine pile type of steel tubular piles with reinforced concrete infill.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.