Experimental investigation of vertical bearing capacity and group pile effect for combined long and short pile foundations in Loess

To simultaneously meet both bearing capacity and settlement requirements in loess areas, the introduction of novel pile foundation types is crucial. A large-scale indoor model experimental was designed and conducted. The experiment tested four variables: pile spacing, pile type, number of piles, and pile length. A comprehensive analysis was conducted to determine the vertical bearing capacity of combined long and short pile foundations, including the influence of the group pile effect on combined long and short pile foundation performance. The experimental results reveal that the lateral frictional resistance of both long and short piles in the combined long and short pile foundations system acted in an alternating manner. Furthermore, the axial force and lateral frictional resistance of the corner piles were slightly greater than those of the edge and center piles. Increasing the pile spacing, the group pile effect coefficient increases from 0.83 to 0.95. This results in a 16.7% enhancement in the bearing capacity of the group pile foundation. Modifying the pile type increases the group pile effect coefficient from 0.825 to 0.94. This leads to a roughly 16% improvement in bearing capacity compared to the ring-shaped pile configuration. Reducing the number of long piles, the group effect coefficient increased from 0.825 to 0.80. The overall ultimate bearing capacity decreases by approximately 20%. Increasing the pile length results in a group pile effect coefficient of 0.87, representing a 0.45 increase compared to the base pile configuration. Consequently, the bearing capacity of the group pile foundation improves by 18.1%.