Theoretical Research on Bearing Capacity Calculation and Optimal Design of Pile Bottom Grouting Root-reinforced Cast-in-situ Piles

Post-grouting technology for cast-in-place piles involves injecting cement slurry into the soil layers at the pile tip and pile side after pile formation. This reinforces the sediment at the pile tip, the bearing stratum at the pile tip, and the mud cake around the pile, thereby improving the stress state of the pile tip soil and enhancing the strength of the pile-soil interface. This ultimately aims to increase the ultimate bearing capacity of cast-in-place piles and reduce the settlement of pile groups. However, traditional post-grouting cast-in-place piles, which adopt a "pile side first, pile bottom second" grouting process, still rely on empirical parameters for calculating the bearing capacity of individual piles. This leads to highly subjective calculation results and significant dispersion in test data, which in turn affects their economic efficiency and the accuracy of project cost estimation. This paper systematically reveals the manifestation and exertion coefficient of pile side resistance, summarizes and compares the methods for determining pile tip resistance in different industries and the existing problems; based on a comprehensive analysis of domestic and international pile foundation bearing capacity calculation models, it proposes a theoretical method for determining the design values of pile side resistance and pile tip resistance of cast-in-place piles with pile bottom grouting, based on in-situ test results. Specifically, for the pile side resistance of cast-in-place piles with pile bottom grouting, the return slurry section is consistent with the current industry standard "Technical Code for Building Pile Foundations" (JGJ94), while the non-return slurry section considers the effect of pile tip resistance on the increase of pile side resistance. For the pile tip resistance of cast-in-place piles with pile bottom grouting, it draws on the working behavior of prefabricated pile tips and introduces an exertion coefficient less than 1.0 for reduction. Furthermore, it proposes an optimized design theory for cast-in-place piles with pile bottom grouting based on construction technology and deformation control. The theory is validated through an example of a 800 mm diameter cast-in-place pile in the People's Insurance Company of China Tower project: the originally designed post-grouting compressive piles with a length of 36 m and non-post-grouting tensile piles with a length of 26 m were both optimized to 20 m long cast-in-place piles with pile bottom grouting. The experimental results confirm their significant technical and economic benefits, providing reference and guidance for the optimized design and theoretical research of similar cast-in-place piles.