Abstract：It is easy that the pile develops necking which can limit pile shaft resistance and cause the reliability index of bearing capacity for piles to reduce resulting in safety hazards to the project. Transparent soil technology is incorporated to load vertically on one intact pile with cap and seven necking piles with caps and the reduction coefficient of bearing capacity is investigated. The total probability formula is employed to investigate the influences of necking parameters(length，location，and diameter) on the reliability index of pile capacity. Subsequently，the speckle field of soil around piles is treated assisted with MatPIV software to obtain the soil displacement around piles，then the reduction reasons of reliability index caused by the necking to limit piles shaft resistance is explained. The study shows that the load capacity reduction coefficient and the reliability index change in the same trend，with the increase of the neck length，the closer the neck location to the top of the pile，the smaller the load capacity reduction coefficient and the reliability index，the more severe the loss of bearing capacity of the pile.
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