Analytical solution of the internal interface stress in composite piles under confining pressure

Composite pile technologies represented by the stiffened deep cement mixing (SDCM) pile, combining a rigid core pile with a cement-soil ring, offer high bearing capacity and cost-effectiveness. This study investigates the internal interface (IIF) stresses at the boundary between these components under confining pressure induced by the surrounding cement soil. An analytical model based on elasticity theory is developed to derive radial and circumferential stress distributions, treating the pile as an infinitely long composite circular tube under uniform confining pressure. The model’s accuracy is validated through axisymmetric finite element simulations. Parametric analyses using the analytical solution, considering core pile and cement-soil pile diameters, elastic modulus, and Poisson’s ratio, reveal that the pile diameter ratio predominantly governs stress distribution, with material properties exerting negligible influence. Research findings indicate that the stiffness of the core pile is pivotal to shaft stresses, and radial stress at the IIF is approximately 1.2 times the confining pressure for diameter ratios of 1.5–2.5, providing a simplified design parameter for estimating interfacial shear strength via Coulomb’s friction law. While finite element analysis effectively captures complex stress variations near pile ends, where plane strain assumptions break down within one pile diameter, the proposed solution offers a reliable theoretical framework for evaluating interface stresses, enhancing composite pile design and performance assessment, and serving as a basis for future analyses under complex conditions such as non-uniform confining pressure or multi-layered soils.