## 4 Conclusions  
1. With a reduction in the center distance of the pile, the dry density of the soil between the piles increased dramatically, but the self-weight collapsing coefficient and collapsing coefficient decreased significantly. When the center distances of the pile were 1.0, 1.25, and 1.5 m, the average dry soil densities in the depth range raised by 23.8%, 18.5%, and 3.1%, respectively, compared to a natural foundation. When the center distance of the pile was smaller than or equal to 2.5D, there was no possibility of soil collapse between piles. When bored piles are used to treat a considerable thickness of the collapsing loess foundation, a center distance of a pile of 2.5D is recommended to eliminate collapse.
2. Based on the hole expansion theory, a technique is presented for estimating the effective squeezing range of a single pile, which can offer theoretical support for selecting a suitable center distance of the pile when bored piles are used with a collapsible loess foundation.
3. Bored piles utilize the extrusion and expansion effects of the drill bit and the positive compaction effect during pile formation, substantially increasing the pile-soil interface stress and side friction. The compaction impact and side friction increased with decreasing pile separation. The average ultimate friction resistance of the bored piles improved by 63.5% and 48.5% when the center distance of the pile was 2.0D and 2.5D, respectively, compared to that of a non-extruded pouring pile.
4. With the extension of immersion time, the depth of water penetration and collapse deformation increased, and the negative friction of the pile side went through three stages: a gradual increase, a fast increase, and a steady fluctuation. In addition, the negative friction of the pile side exhibited two peaks during the steady phase of immersion and consolidation of the soil around the pile following immersion. When the pile diameters were 500 and 800 mm, the average negative friction of bored piles was 27.9 and 26.43 kPa, respectively. Compared with a non-extruded pouring pile, the average negative friction of bored piles with a pile diameter of 800 mm was 14% higher. Therefore, to maximize the technical benefits of bored piles in a loess foundation with large thicknesses and high collapsibility, the collapsibility of the loess foundation must be completely or partially eradicated beforehand.  
**Author Contributions:** Conceptualization, [PERSON] ([PERSON]) and [PERSON] ([PERSON]); methodology, [PERSON] ([PERSON]); software, [PERSON] ([PERSON]); validation, D.L., Y.Z. and D.J.; formal analysis, [PERSON] ([PERSON]); investigation, [PERSON]; resources, [PERSON] ([PERSON]); data curation, [PERSON]; writing--original draft preparation, [PERSON] ([PERSON]); writing--review and editing, [PERSON] ([PERSON]) and [PERSON]; supervision, [PERSON] ([PERSON]); project administration, [PERSON] ([PERSON]); funding acquisition, [PERSON] All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Natural Science Foundation of China (grant numbers 4236020077 and 52268058), a project supported by the Young Scholars Science Foundation of Lanzhou Jiaotong University (grant number 2022054), Gansu Province Youth Science and Technology Fund Program (grant number 22 JRSRA369), and Gansu Province science and technology project support (grant number 21 JR7 RA306).  
**Institutional Review Board Statement:** This study did not involve humans or animals.  
**Informed Consent Statement:** This study did not involve humans.  
**Data Availability Statement:** The data presented in this study are available in article.  
**Conflicts of Interest:** The authors declare no conflict of interest.