Theoretical analysis of screw pile under individual thread shear failure pattern

To address the problems of calculating the critical pitch of screw piles and the ultimate bearing capacity provided by threads under an individual thread shear failure pattern in high-speed railway subgrade engineering, the locally unfolded region of screw piles was taken as the research object. Research on the individual thread shear failure pattern under ultimate load was carried out. Based on the structural stress characteristics of the thread, a theoretical failure model was established by dividing the surrounding soil into an elastic compaction zone, a passive failure zone, and a transition zone. Through analysis of stress-deformation compatibility relationships in different regions, the ultimate bearing capacity provided by the thread was derived. Corresponding calculation methods for critical pitch and ultimate bearing capacity were developed according to the proposed theoretical failure model of the thread. The validity and accuracy of the model were verified through comparative analysis with existing failure models and large straight shear experiments. Research results show that the individual thread shear failure pattern proposed by the research demonstrates better agreement with the actual failure mode of the thread compared to the Meyerhof failure pattern. The critical pitch of the screw pile is primarily determined by the height of the thread and the internal friction angle of the soil, exhibiting nearly linear variation with the height of the thread and approximately exponential growth with the friction angle. The bearing capacity provided by the thread under the individual thread shear failure pattern surpasses that of the cylindrical shear failure pattern, with the improvement magnitude mainly governed by soil failure surface geometry and shear strength. During screw pile design, appropriately increasing the height of the thread and pitch while ensuring the structural safety of the thread can optimize the surrounding soil to achieve the individual thread shear failure pattern, thereby maximizing the ultimate bearing capacity of the screw pile. The research results provide a theoretical reference for the ultimate bearing capacity optimization design of screw piles in high-speed railway subgrade engineering under silt and clay geological conditions.