# Study on Vertical Uplift Resistance Characteristics of Pre-Drilled Planted Piles in Boulder Strata
## 7 Conclusions  
This study, based on actual engineering problems in the Xiamen region of China, employs transparent soil technology to conduct non-intrusive pull-out model tests. It compares two scenarios--with and without a boulder--to investigate the influence mechanism of the boulder on pile bearing capacity, the influence range of the pile on surrounding soil,  
Figure 14: Sliding failure surface diagrams of planted piles during uplift under different boulder burial depths: (**a**) boulder located at the pile bottom; (**b**) boulder located 50 mm above the pile bottom; (**c**) boulder located 100 mm above the pile bottom; (**d**) boulder located 150 mm above the pile bottom. and the development of sliding failure surfaces. Additionally, FLAC3D-PFC3D numerical simulations are used to analyze the relationship between the contribution of the boulder to pile bearing capacity and their position along the pile. The study concludes with the following findings:  
1. The presence of boulders significantly enhances the overall bearing capacity of the pile. Specifically, boulders increase the effective cross-sectional diameter of the pile, providing additional vertical support. Additionally, boulders enhance load transfer pathways to the surrounding soil and expand the contact area, thereby extending the influence range of the pile on the surrounding soil. 2. During the pulling process of the planted pile, the vertical impact depth on the soil primarily concentrates within the length range of the pile body. The presence of boulders modifies the morphology and horizontal influence range of the soil displacement field. Under extreme uplift loads, the displacement field of the surrounding soil for piles without boulders exhibits an \"inverted triangle\" distribution, with the maximum horizontal influence range on the surface soil being approximately four times the pile diameter (4D). However, when a boulder is positioned within the pile, the displacement field around the pile shows an overall distribution of two \"inverted trapezoids\" with the horizontal influence range on the soil expanding from 6D above a boulder to 8D at the surface. 3. The presence of the isolated boulder can amplify the impact on the surrounding soil during the uplift failure of a grouted pile. When the cross-sectional area of an isolated boulder is approximately four times that of the pile and it is in the middle section of the pile, the distance to adjacent piles should be greater than eight times the pile diameter in engineering practice. 4. The presence of a boulder alters the angle of the sliding failure surface of the pile, and as the burial depth of a boulder decreases, the angle of the sliding failure surface also gradually reduces. When the boulder is at distances of 0, 50, 100, and 150 mm from the end of the pile, the angles of the sliding failure surface of the pile are \(56^{\circ},53^{\circ},49^{\circ}\), and \(41^{\circ}\), respectively, whereas under the condition without a boulder, it is \(75^{\circ}\). 5. Boulders significantly enhance the ultimate uplift bearing capacity of the pile. When a boulder is at distances of 0, 50, 100, and 150 mm from the end of the pile, the uplift bearing capacity of the pile increases by 3, 2.5, 2, and 1.5 times, respectively, compared to the piles without boulder. This is attributed to the additional uplift resistance provided by the overburden from the boulder. The thicker the overburden, the greater the vertical resistance, thereby significantly improving the uplift bearing capacity of the planted pile. 6. When the boulder is embedded in the middle or lower part of the pile, their effect on enhancing the bearing capacity of the pile is more pronounced. Therefore, in practical engineering, when considering the inclusion of the boulder in the bearing system, priority should be given to the boulder located near the design depth of the pile, with a preference for the boulder positioned at the pile tip. Conceptualization, Q.C., W.L. and H.L. ; methodology, Q.C., W.L. and H.L. ; software, H.L. and W.L.