The stiffened deep cement mixing (SDCM) pile is a composite pile composed of the deep cement mixing (DCM) pile and an inner precast core pile. The excellent bearing performance of the SDCM pile that has been successfully witnessed in engineering practice is attributed to the double-layer load transfer mechanism, which effectively transfer the load from the stiffened core to the cemented soil and further to the adjacent soil. The mechanical properties of SDCM piles with stiffened cores that using large-size prestressed high-strength concrete (PHC) piles are rarely studied. This study aims to explore the bearing performance and failure behavior of the SDCM pile with a large-size PHC pile as stiffened core. The relationship between load and settlement as well as the distribution and development of axial force and lateral resistance was studied through field full-scale tests. The effects of the volume ratio, size, and concrete stiffness of the core pile, and the strength of cemented soil on the axial bearing capacity of SDCM piles were explored through the verified three-dimensional numerical model. The load transfer and failure modes at the internal and external interfaces of SDCM piles with different pile lengths were analyzed. Results show that the length of the core pile (Lcore) is a key factor for the bearing capacity of the SDCM pile. The bearing capacity of SDCM pile increases by 57.90% and 46.67% with Lcore increasing by 45% when cemented soil strength (qu, DCM) is 150 MPa and 300 MPa, respectively. The influence of qu, DCM and concrete stiffness on the bearing capacity of the SDCM pile is gradually significant with the increase of Lcore. The ultimate bearing capacity increases by 4.3% for every 100% increase in cemented soil strength at the optimal pile length. With the increase of Lcore, the investigated pile exhibits three failure modes, including the failure of pile end soil and cemented soil, the failure of pile top soil and core pile end soil, and the failure of pile top soil. The results of this study provide reference for the application of SDCM piles with large-size PHC piles as stiffened cores in the engineering field.

Precast prestressed high-strength concrete (PHC) pipe pile with cement-improved soil is a novel pile foundation technique that has been extensively utilized in contemporary years due to the enhanced lateral load-bearing capacity in soft grounds. However, though several studies have shown the damage mechanism of single piles with cement-improved soil, the group behavior of such pile foundations is still largely unexplored. Hence, this article aims to illustrate the lateral capacity and tension-induced failure characteristics of single and 1 x 2 group PHC piles reinforced by cement-improved soil. Extensive 3D nonlinear finite element analyses have been performed and the precision of the numerical models is confirmed by a previous experimental-numerical study. The effects of pile spacing, embedment length, and cement-improved soil thickness were examined in terms of various lateral responses and damages. Results have revealed that the addition and thickening of CIS around core piles enhances the overall pile performance and protects the core pile from excessive tensile damage. Declines in head displacement and bending moment were found to be up to 45 % and 25 % for single piles, and up to 47 % and 24 % for group piles, respectively. Moreover, the presence of CIS makes the stress distribution mechanism between the trailing and leading piles in group arrangement more uniform. Results of this study are expected to provide valuable insights for a better understanding of the damage behavior of group precast piles reinforced with cement-improved soil.

The PHC (pre-stressed high-strength concrete) pile foundation, serving as an innovative supporting structure for solar power stations, is subjected to complex loading conditions in engineering scenarios. In this study, field tests of the full-scale PHC Pile foundation were conducted in sand layer, loess layer, and double-layer sites to investigate its operational behavior under different load conditions. The study assessed the inclination of the column top, ground displacement, and torsion to analyze the stress and deformation characteristics of PHC pile foundations. The deformation of PHC short pile foundations exhibited distinct phases. Torsional load reduced the column crack load by 30%. The pile cap effectively controlled plastic deformation, minimizing foundation deformation, while torsional load increased lateral deformation. Under cyclic load, the PHC pile behaved with an approximate elasticity characteristic within the test load range. The deformation increased by approximately 10%. Furthermore, three-dimensional numerical simulations analyzed the effects of foundation dimension, bending-moment-to-lateral-load ratio, torque-to-lateral-load ratio, and pile cap size on internal forces and deformation. Simulations indicated that increasing the pile cap length was more advantageous for reducing deformation and internal forces. The bending-moment-to-lateral-load ratio was significant in design, while the torque-to-lateral-load ratio had a negligible impact. A comprehensive design program is proposed based on field tests and numerical simulations, considering deformation and bearing capacity. The study confirms the reliability of the PHC pile foundation as a support structure for heliostats, aiming to offer valuable insights for practical applications.

The soil arching effect is the key mechanism for load transfer in pile-supported reinforced road (runway) foundations. In order to investigated the formation and evolution process of soil arching effect in the whole process of embankment filling and soft soil foundation consolidation, a three-dimensional hydro-mechanical coupled numerical model of PHC pile reinforced soft soil runway foundation was established based on the foundation treatment project in Pudong Airport. The variation laws of soil settlement, pore water pressure, and pile soil stress were analyzed, and the influence of pile spacing was considered. These data from both numerical simulation and field test indicate the soil arching effect in the foundation reinforced by PHC piles and preliminary reveal the evolution of soil arching in the process of embankment filling and soft soil foundation consolidation. The preliminary results encourage the authors to continue this research to investigate the evolution of soil arching under aircraft dynamic loads through adding a more suitable constitutive model or subroutine in this numerical model.