Centrifuge model tests and settlement calculation of belled and multi-belled piles in loess area

Belled and multi-belled piles are cast-in-place piles with one or several bell-shaped enlargements. In this study, centrifuge model tests were conducted to explore the bearing mechanism of belled and multi-belled piles in loess area. It was found that both the bell shape forming and pile loading procedure lead to the compaction effect of soil adjacent to the pile which was shown intuitively as the mutation of axial force and side friction resistance at certain positions of pile shaft. The compaction effect due to the bell-shape enlargement transformed to the squeeze out behavior when the bell is oversize, with negative effects on the bearing capacity of piles. The compaction effect due to bell tip stress together with the bell tip resistance strengthens the attenuation rate of pile axial force. A stable relationship in the percentages of bell tip resistance, pile tip resistance and the upper load was also found at the later load stage of multi-belled piles. A settlement calculation formula for multi-belled piles was proposed based on Mindlin solution. Finally, the accuracy of the formula was verified by model test results. The method for predicting the ultimate bearing capacity of multi-belled piles was proposed, and it was verified by an in-situ worked-out example. •The bearing mechanism of belled and multi-belled piles in loess area was first explored.•The centrifuge data which includes ultimate capacities, pile/bell tip resistance, axial forces, side frictional resistance can provide significant reference for proper design of belled and multi-belled piles in loess area.•The compaction effect due to the bell-shape enlargement transformed to the squeeze out behavior when the bell is oversize, with negative effects on the bearing capacity of piles.•A method for predicting settlement of multi-belled piles in loess was established and verified by an in-situ worked-out case.