The drilling process of a bored pile can cause radial unloading-induced borehole contraction. In this study

the stress-displacement solution for a borehole under radial unloading in cohesiveless soil was derived based on the SMP yield criterion and the non-associated flow rule. By combining with earth pressure formulas derived from cavity wall formula and Berezantsev’s formula

the variation curves of borehole wall displacement with borehole depth were obtained

which were compared the results of the case with elastic deformation ignored in plastic zone. Parameters study shows that the unloading factor n

borehole radius a0

and internal friction angle  had obvious influences on borehole wall displacement

and the effects of n and  had a critical value where the borehole wall displacement was significantly increased when n and  were less than the value. However

the influence of dilatancy angle ψ on borehole wall displacement was limited and could be ignored. In addition

n was a function only related to 

while rp/a was a constant value only related to n and  and had no relation with depth

which suggests that the self-supported depth of vertical drilling in cohesiveless soil was zero. Moreover

it can be found that the wall displacement was smaller and the plastic zone radius was larger when elastic deformation was ignored in plastic zone

which indicates that the solution of ignoring elastic deformation in plastic zone is not conservative in engineering applications. Lastly

the value of borehole wall displacement with different earth pressures along depth was quite different

which shows that the selection of earth pressure in vertical drilling should be combined with real field measurement.