Abstract: The inner core of traditional stiffened deep mixed piles is typically a prestressed concrete cylinder, relying solely on friction and bonding between the inner and outer cores to transmit load. Field investigations reveal that the bonding force between the soil-cement pile and the tubular pile interface is minimal. When the design is carried out according to specifications, the shear strength of the inner interface is often significantly low. To address this issue, the inner core prestressed concrete cylinder is replaced with a steel pipe pile, and the shear strength of the interface between the inner core prestressed concrete pipe pile and the outer core cement-soil pile is investigated. Additionally, a steel pipe pile with spiral blades on its outer surface is proposed as a new type of inner core pile. Shear tests on the internal interface were conducted on pile section samples with inner cores of non-ribbed steel pipes and spiral blade steel pipes. The factors influencing the ultimate shear force of the internal interface were analyzed, and the quantitative data were characterized. The shear failure mode of the inner and outer cores of the new composite pile, along with the evolution law and calculation method of the shear strength parameters of the inner interface, were determined. The shear strength parameters of the inner interface in the bearing capacity calculation formula of the pile, as specified in the Technical specification for strength composite piles (JG/T 023－2007), were modified.

Key words: stiffened deep mixed pile, steel pipe pile, spiral blade, shear strength of internal interface, model experiment, specification parameter correction