Besides the interaction of the multiple stress superposition from piles shaft friction

pile bottom resistance and contact stress between low caps and soil

the clamping action makes the cap-pile-soil interaction more complicated in pile foundation with large pile diameter and small spacing. 3D finite difference program is employed to evaluate the pile-soil-cap interaction with the change of vertical load

while the pile spacing and pile diameter remain constant in the analysis procedure. Analysis is carried out to explain the interaction of cap-pile-soil from the following aspects such as the lateral friction of soil in each layer

load on the top of characteristic piles

load-settlement relationship

vertical displacement of soil between piles

etc. The results show that: the clamping action on the upper soil among piles will decrease

and the pile shaft friction in the upper part will increase when the load exceeds 117.8 MN which is slightly larger than Pu/2 (Pu means ultimate bearing capacity of piles group); the lower part of pile shaft friction shows a trend of decrease due to the influence of the multiple stress superposition

and the decrease trend in characteristic piles has different influence scope; the load

which makes settlement of piles group achieve 5% of pile diameter

can be regarded as the vertical ultimate bearing capacity of piles group

and it is reasonable; when the ratio of piles group’s settlement to pile diameter exceeds 1%

the proportion of the load carried by piles decreases because of the cap contribution.