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.