Elastoplastic Analytical Solutions for Borehole Contraction of Bored Piles by Boring Unloading Based on Unified Strength Theory

To quantitatively predict the process of unloading for bored piles, combining the unified strength theory (UST) with the non-associated flow rule, the elastoplastic analytical solutions of large strain and small strain for borehole contraction of bored piles were derived. And the influence of intermediate principal stress, dilatancy angle and parameters of piles on analytical values in the borehole contraction process were analyzed. The result indicated that the larger the coefficients of intermediate principal stress b and the internal friction angle of soil φ were, the smaller the dilatancy angle ψ of soil was, and the better the stability of borehole walls of bored piles was. The solutions based on the Mohr-Coulomb yield criterion and the small strain theory in a plastic state of borehole wall soil were both conservative. The larger the coefficient of intermediate principal stress b was, the larger the radial stress and the maximum circumferential stress at the same position of cylindrical borehole wall were, and the smaller the radial displacement at the same position was. With the increase in the coefficients of intermediate principal stress b, the maximum circumferential stress gradually approached the borehole wall, simultaneously, the increment of the maximum circumferential stress decreased with the increase in the coefficient of intermediate principal stress b. Both solutions for soil stress and displacement fields around the cylindrical borehole in the process of borehole contraction due to unloading by large and small strain theories all meet the requirements. The findings could be applied to the quantitative analysis for the stability of borehole wall soil during boring and the bearing characteristics of bored piles.