Field study on post-grouting effects of cast-in-place bored piles in extra-thick fine sand layers

The post-pressure grouting technique has proven to be an effective method to enhance axial resistance. In this paper, field tests were conducted to investigate the performances of large-diameter cast-in-place bored piles for six combined side-and-tip grouting piles and two side-grouting piles in extra-thick fine sand layers. The load-displacement response, shaft resistance, and mobilization of unit base resistance were discussed. The field results indicate that compared with the piles before post-grouting both the shaft resistance and base resistance for the piles after post-grouting are significantly improved. The piles after post-grouting have higher load-bearing capacity. Meanwhile, the load-bearing capacity of combined-grouting piles is also greater than that of side-grouting piles. Additionally, the strengthening effect of the surrounding soil improvement due to side grouting on the base resistance can enhance the unit base resistance, and the unit shaft resistance can also be increased by the soil improvement at the pile tip due to tip grouting. Moreover, the distribution of the injected pressurized grout at the pile tip and side is detected by an electromagnetic wave computerized tomography (EWCT) technique. The results reveal that EWCT can detect the distribution pattern of the pile, injected grout and geotechnical interface and evaluate the diffusion range of the grout of the soil at the pile tip and side, which can be used to assess the grouting effect of grouted piles. Finally, an in situ standard penetration test is employed to evaluate the grouting effect of grouted pile by variation of the SPT N value before and after post-grouting. The research results have reference value and guiding significance for the design and effect detection of post-grouting piles.