Model Tests on Compressive Bearing Properties of Thorn Piles in Sand

[Objective] Traditional piles with uniform cross-sections have inadequate properties of the bearing stratum at pile tip or difficulties in penetrating into rock when used in soft soil areas or karst regions. Based on the concept of transforming end-bearing piles into mid-bearing piles, this study proposes a novel discontinuous variable cross-section thorn pile, aiming to systematically reveal the mechanisms by which thorn length and the number of thorn layers affect the compressive bearing properties of thorn piles through indoor model tests, thereby providing a theoretical basis for establishing a performance-based optimized design method. [Methods] Using a self-developed loading device, a geometric similarity ratio of 1∶75 was adopted, and PVC hollow circular pipes and 304 stainless steel cylindrical pins were used as the model piles and pile thorns, respectively. Using the controlled variable method, a total of 21 test groups were designed, and indoor model tests with different thorn lengths and number of thorn layers were conducted to investigate the compressive bearing properties of thorn piles in sand. [Results] (1) compared with smooth piles, thorn piles exhibited superior compressive bearing and settlement resistance. Under sand conditions, the ultimate bearing capacity of the 17-layer Type-38 thorn pile increased by 278% compared with that of the smooth pile. (2) Both thorn length and the number of thorn layers showed positive correlations with the bearing capacity of the pile foundation; compared with thorn length, the number of thorn layers was more sensitive to the ultimate bearing capacity of a single pile and the pile shaft resistance. (3) From an engineering economic perspective, when the thorn spacing was approximately one-eighth of the pile length and the thorn length was about equal to the pile diameter, the thorn pile exhibited optimal compressive bearing properties. [Conclusion] (1) The presence of thorn structures increases the interlocking effect between thorns and soil as well as the effective stress of the surrounding soil, delays the softening of pile shaft resistance, expands the influence range of the surrounding soil, and effectively redistributes the applied load, thereby significantly enhancing the overall bearing capacity of the pile foundation. (2) The concept of mid-bearing degree is introduced to quantify the mid-bearing contribution of pile thorns, and a calculation method for the ultimate bearing capacity of a single thorn pile is proposed, laying a foundation for subsequent theoretical studies. (3) Future research may consider the effects of layered foundations, soil squeezing during thorn formation, and thorn geometry on the compressive bearing properties of thorn piles, so as to further improve the comprehensiveness of bearing mechanisms and the applicability of theoretical models. The findings of this study have important theoretical and practical value in promoting the transformation of modern pile foundation engineering toward green and intelligent development, and their application will profoundly influence future modes of underground space development.