On the Calculation of Ultimate Shaft Resistance of Bored Piles in Service by Considering the Effect of Additional Stress and Load-Adding Procedure

Ultimate shaft resistance had relation with the cohesive strength between the soil and pile surface. On the other hand, when shaft resistance was large enough and the stress combination in soil around a pile satisfied Coulomb’s law, the soil would be in shear failure state and could not bear more loads. This showed that shaft resistance played at such time would also be considered as ultimate shaft resistance. Hence, ultimate shaft resistance had relation with both cohesive strength and the stress state of the soil. Hence, a method for calculating ultimate shaft resistance was put forward when it was controlled by the stress state. The working stages for a bored pile in service were divided as construction, service and maintenance and the characters of additional stresses caused by subsurface friction in soils in each stage were analyzed, based on which, the calculation of compression module of soils during different working stages were set up. Thus, the influence of the additional stress mentioned above on the changing of compression module of soils and on the transformation of τ~s curve during different working stages were discussed together and it was deduced that how to decide the increase ratio of the bearing capacity of a pile in a bridge after the pier and superstructure of the bridge strengthened or rebuilt to its initial one in normal service stage after it was bored. The amend factors for side resistance of soils around a pile with stable settlement were put forward, which finally forms the evaluation method for subsurface friction of soils around a bridge pile in service on the basis of the additional stress in the soils.