Abstract: Radial unloading-induced borehole contraction is caused in the drilling process of bored pile. On the basis of the SMP yield criterion and the non-correlated flow rule, the solution for the stress and displacement fields in two stress-state conditions of plastic zone around the vertical borehole is deduced with the coefficient K₀ of lateral earth pressure considered. Parameter analysis shows that the K₀ and internal friction angle φ of soil have a great influence on the emergence and development of plastic zone as well as the radius r_e and r_p of plastic zone. The relative displacement and stress of borehole wall increase linearly along with the increase of drilling depth with different values of K₀ and mud's unit weight r_mud. The relative displacement of borehole wall intensifies with the growth of K₀, while declines with the rising of r_mud. However, the radial and circumferential stresses of borehole wall do not change with K₀. In addition, the range of plastic zone around the vertical borehole hardly changes along the depth in sandy soil, and the plastic radius r_p has a remarkable influence on the circumferential stress. The theoretical solution presented in this paper is of theoretical significance for the unloading problem of borehole in initial in-situ stress field of sandy soil.

Key words: sand, cast-in-place pile, coefficient of lateral earth pressure, vertical drilling, radial unloading, theoretical solution