# Study on Vertical Uplift Resistance Characteristics of Pre-Drilled Planted Piles in Boulder Strata
## 1 Introduction
The coupled FLAC3D-PFC3D model, which combines the advantages of continuous and discrete media, is suitable for multi-scale complex problems and can simultaneously simulate macroscopic deformation and micromechanical behavior. [PERSON] et al. [28] used FLAC3D-PFC3D coupling to analyze the stability during the contact process between piles and soil, studying the micro-deformation of the surrounding soil, stress distribution, and changes in the pile itself during pile penetration. [PERSON] et al. [29] employed a coupled continuous-discontinuous medium numerical model to simulate the deformation and failure process of a single gravel pile under load in an indoor model test. [PERSON] et al. [30] established a coupled finite element-discrete numerical model and combined it with experimental measurements to investigate the bearing capacity and deformation of a series of composite piles. Previous pile foundation studies have predominantly utilized field tests [31], laboratory soil tests [32], and numerical simulations [33], but have offered limited observations of internal soil deformation. Transparent soil technology enables the visualization of internal soil deformations, facilitating in-depth studies of the interactions between pile foundations and soil. [PERSON] et al. [34] conducted indoor model experiments using transparent soil to investigate the compaction-induced deformations of soil during the installation of rectangular piles. [PERSON] et al. [35] examined the bearing capacity of threaded and helical piles in soft soil layers through transparent soil model tests, focusing on the influence range on soil and the direction of soil movement. Existing studies have employed transparent soil technology to explore the impact of pile foundations on soil, demonstrating the practicality and reliability of this technique. To mitigate the effects of boulders on projects, a method involving pre-drilling followed by the insertion of high-strength concrete precast piles is employed, integrating the boulders into the load-bearing system. This method sets the drilling location in the core region of the boulder, allowing for the construction of piles that share the load with the boulder, resulting in a new type of pre-drilled composite pile. This method combines the advantages of embedded piles and enlarged base piles, making it suitable for boulder-rich strata. Previous studies, such as that by [PERSON] et al. [36], have investigated the load-bearing capacity and load transfer mechanisms of piles when the boulder is located at the pile tip, without considering the contributions of boulders positioned at different elevations along the pile shaft to the uplift capacity of the pile. This study aims to fill the research gap regarding new pre-drilled composite piles when boulders are located at various positions along the pile shaft. Based on transparent soil technology, destructive uplift tests were conducted on planted piles and piles containing boulders to analyze the load transfer mechanism of the piles, the interaction between the piles and the surrounding soil, and the development pattern of the sliding failure surface angle. Concurrently, a discrete-continuous coupled 3D numerical model was established to analyze in comparison with the uplift test results and investigate the effects of different boulder locations along the pile body on the load-bearing capacity of the piles and the sliding rupture surfaces. This study may provide valuable references for the handling of large boulders within boulder strata.