Abstract: When a load is applied on soil around the pile group foundation of the existing bridge pier, it can result in the settlement and lateral deformation of soil and negative skin friction, which would have a great influence on the deformation and bearing capacity of the piles. To investigate the stress difference among piles at different locations under one-side load and surrounding load, model tests of 3×3 pile group foundation were performed to determine the change rules and the differences of axial force, negative skin friction, neutral point position, and safety factors of bearing capacity of piles among different piles. The results show that under the surrounding load, the axial force and skin friction values of the piles at the corner of pile group reach the maximum, those of side piles are the second, and those of the pile in the centre reach the minimum. In addition, the neutral points of the piles in the corner locate at the deepest, those of the side piles locate slightly higher, and those of the pile in the centre are closest to the pile tops. Under one-side load, the axial force and skin friction of the piles in the row near the side load and in the middle row change similarly to those under the surrounding load, and piles in the row far away from the side load are less affected by the load, and there is no negative skin friction acting on the piles. The change rule of the neutral point position in each pile is also similar to that under the surrounding load. Under the side load, the axial force, negative skin friction of the piles at the same location are smaller, but the neutral point position is higher than those under the surrounding load. The safety factor FS of bearing capacity of each pile under the surrounding load condition is less and decreases faster than that under the one-side load condition. This study could provide references for design of pile-group foundation under different loading modes.

Key words: surrounding load, side load, pile group, negative skin friction, neutral point, model test