<p>Based on the treatment project of deep miscellaneous fill foundations in highway construction, several field tests involving different pile forming processes and spacings were carried out. Utilizing super heavy dynamic penetration tests and foundation bearing capacity tests, the improvement effects of down-hole dynamic compaction on the soil between piles in miscellaneous fill foundation soils were investigated. The results indicate that down-hole dynamic compaction significantly improves deep slag miscellaneous fill. Post-treatment, the compactness of undisturbed slag miscellaneous fill transitions from loose to medium dense or dense, with a substantial increase in soil compression modulus. Specifically, for punching piles, the compressive modulus increases by 45.3 to 226.6% when the pile spacing/diameter ratio is 2.8 to 3.9. Whereas for rotary digging piles, even with a smaller ratio, the increase is only 8.0 to 114.6% for pile spacings/diameter ratios of 2.1 to 2.9. Thus, small-diameter punching piles demonstrate superior improvement in bearing and deformation capacity compared to large-diameter rotary digging piles. Additionally, deep dynamic compaction in the hole can disturb shallow soil; hence, in engineering applications, the surface soil should be compacted after pile completion to ensure the overall treatment effect of the foundation.</p>