Abstract: This study focuses on addressing the issues associated with the “end-suspended pile” structure in deep foundation pits within soil-rock composite strata by implementing an enclosure structure called “micro-steel tube pile internally connected into concrete pile”. The structure is analyzed considering the “knot effect” between the micro steel pipe pile and the rock, as well as the similar mechanical characteristics between excavated rocks and cracked concrete. The structure is modeled as a stepped variable section pile (beam), consisting of an upper concrete pile and a lower equivalent cracked beam without stirrups. The micro steel pipes serve as axial reinforcements for the equivalent beam, while the rocks represent the cracked concrete of the equivalent beam. The reduction in lateral stiffness is used to quantitatively assess the impact of section reduction and rock mass fractures. An elastic subgrade method based on lateral stiffness reduction is employed, taking into account the dowel action of axial reinforcement to calculate the shear bearing capacity of the equivalent beam. The analysis of a case study demonstrates that the lateral stiffness reduction coefficient β  ranges from 0.85 to 0.12, the bending moment of the upper concrete pile increases by 55.8%, the bending moment of the lower equivalent beam decreases by 48.7%, the shear force of the upper concrete pile decreases by 13.6%, the shear force of the lower equivalent beam decreases by 22.0%, and the horizontal displacement of the concrete pile top increases by 39.5%. Comparing this structure with the “end-suspended pile” structure, it is observed that the horizontal displacement and vertical settlement of the concrete pile top do not significantly increase. The final horizontal displacement and vertical settlement of the concrete pile top are only 43% and 69%, respectively, compared to the “end-suspended pile” structure. The measured horizontal displacement at the concrete pile top is 5.5 mm, which is only 35% of the calculated results, suggesting that the lateral stiffness reduction coefficient can be appropriately adjusted. This study provides valuable insights for the design and protection of deep foundation pits in soil-rock composite strata.

Key words: soil-rock composite stratum, micro-steel pipe pile internally connected into concrete pile, knot effect, dowel action, cracked concrete, bending stiffness reduction