Study On Bearing Properties Of Large-diameter And Super-long Bored Piles With Tip Post-grouting

The large-diameter and super-long bored piles were widely used in large construction projects such as high-rise buildings and bridge works. In order to minimize the negative effects of sediment and slurry to increase ultimate bearing capacity, reduce pile length and construction cost, the tip post-grouting technique was used intensively.On the basis of the former researches, the bearing properties of large-diameter and super-long bored piles with tip post-grouting was studied in the thesis by way of analyzing test data and numerical simulation.Firstly, by means of the large-diameter and super-long bored piles self-balancing test results of the Hangzhou Bay Bridge, the thesis studied the difference of vertical load transfer behavior before and after the tip post-grouting, the influence on tip load transfer behavior after tip post-grouting of the loading box's position, slurry and grouting flow. The results showed that tip post-grouting had great impact on tip resistance and skin friction of the soil layer adjacent to the pile end. The loading box's position and grouting flow also have influence on tip load transfer behavior. The slurry had marked impact on ultimate tip displacement, but litter to tip resistance.Secondly, a summary of computational methods for ultimate bearing capacity was carried out which are frequently used of bored piles with tip post-grouting. A computational method for ultimate bearing capacity which applied to large-diameter and super-long bored piles with tip post-grouting was proposed. On the basis of the piles test results, the enhancement coefficients of skin friction and tip resistance in the Hangzhou Bay Bridge area under tip post-grouting were presented. Afterwards, contrast of ultimate bearing capacity results which were gained respectively by this method and custom method.Thirdly, a load transfer function model before and after tip post-grouting was proposed according to the load transfer properties, meanwhile, Based on the characteristic of large-diameter and super-long bored piles and the shortcoming of traditional load transfer method, a improved load transfer method which considered the continuity of soil mass was put forward and validated by an example.Finally, by using ABAQUS finite element analysis software and supposing the grout body is a cylinder which its diameter equaled to pile's diameter, the influence of grouting flow and length-diameter ratio on ultimate bearing capacity was studied by numerical simulation on the condition that the soil used Mohr-Coulomb constitutive model, in the meantime pile and grout body used elasticity constitutive model. The results indicated that the influence of grouting flow was larger than grouting flow. The ultimate bearing capacity increased with the increase of grouting flow or decreases of length-diameter ratio, however, it existes optimal grouting flow and length-diameter ratio, when the grouting flow was larger than the optimal grouting flow or length-diameter ratio less than the optimal length-diameter ratio, the ultimate bearing capacity increased slowly.