# Analysis of bearing mechanism of large-diameter under-reamed piles based on model tests

## TL;DR

This study uses model tests to understand how large-diameter under-reamed piles bear loads and fail, showing that the under-reaming angle significantly affects their performance.

## Contribution

The study provides new insights into the bearing mechanism and optimal design of under-reamed piles through systematic model testing.

## Key findings

- An under-reaming angle of 15°–20° optimizes bearing capacity, with a 204.99% improvement over uniform-section piles.
- The under-reamed structure shifts load transfer from shaft to end resistance, with a maximum end resistance ratio of 65.09% at 20°.
- Failure modes evolve from penetrative shear to fan-shaped compaction and then to bulging and collapse with increasing under-reaming angle.

## Abstract

Given the insufficient understanding of the bearing mechanism and failure modes of large-diameter under-reamed piles in complex strata, this study conducted scaled laboratory model tests based on similarity theory. A visualized “semi-model pile static loading-reaction frame” system was established to systematically investigate the influence of under-reaming angle (0° ~ 25°) and pile embedment depth (60 ~ 80 cm) on the bearing characteristics and failure mechanisms of the pile foundation. The results show that: 1) The under-reaming angle is the dominant factor controlling bearing performance. A reasonable increase in this angle can significantly enhance the ultimate bearing capacity, with a 204.99% improvement observed at 20° compared to the uniform-section pile. However, the enhancement effect weakens with increasing embedment depth. Comprehensive analysis suggests that 15° ~ 20° is the optimal under-reaming angle range; an excessive angle induces stress concentration at the “shoulder” of the pile, leading to interfacial detachment between pile and soil, thus limiting further improvement in bearing capacity. 2) The under-reamed structure effectively optimizes the load transfer path along the pile shaft. The end bearing resistance ratio increases first and then decreases with the under-reaming angle, reaching a maximum of 65.09% at 20°, indicating a transition of the load transfer mechanism from shaft-resistance-dominated to end-resistance-dominated behavior. 3) The failure morphology of the pile toe rock evolves from “penetrative shear failure” in the uniform pile (failure zone ≈ 1.25D) to “fan-shaped compaction” at the optimal under-reaming angle (≈ 4D), and further enlarging the angle results in unstable “bulging and collapse” failure. This study systematically reveals the full-process mechanism from load bearing to failure in large-diameter under-reamed piles, providing a theoretical basis for optimizing design parameters and predicting failure behavior. The findings offer valuable references for engineering design and improvement of design codes.