超厚粉细砂地层组合压浆桩压浆效果试验

为了研究超厚粉细砂地层中桩端、桩侧组合压浆效果，基于石首长江公路大桥工程中6根超长大直径钻孔灌注桩原位静载荷试验，通过对比分析组合压浆前、后钻孔灌注桩的试验结果，研究组合压浆对超长大直径钻孔灌注桩承载力性状、桩端阻力及桩侧摩阻力的影响。通过钻孔取芯、标准贯入试验分别对水泥浆液影响范围和桩基组合压浆的影响效果进行综合分析，得到该工程主桥试桩压浆前、后粉细砂土层侧阻力经验系数，建立压浆后侧摩阻力与压浆前标贯击数N的关系式。研究结果表明：与组合压浆前相比，组合压浆后的桩端阻力与桩侧摩阻力均有大幅度增加，且灌注桩极限承载力提高幅度为94.25%~151.51%，由此可见组合压浆的效果非常显著；组合压浆桩的承载性能明显优于桩端压浆桩，其对桩基的荷载传递特性产生了明显影响；钻孔取芯试验明确了水泥浆液在桩周和桩端以下一定范围的分布情况，证实了组合压浆的有效性；标准贯入试验结果表明组合压浆后桩侧土的标贯击数N明显提高，研究成果可直接运用于该大桥桩基设计，并可为类似超厚粉细砂地层中桥梁桩基工程提供参考。

Experiment on Grouting Effects of Combined Grouting Piles Under Extra-thick Fine Silty Sand Layer

To investigate grouting effects of combined grouting side piles and the tip of piles in extra-thick silty fine sand layer, in-situ static load tests on six cast-in-place piles with large diameter were conducted on the basis of the engineering of Shishou Yangtze River Highway Bridge. The influence of the combined grouting on the bearing capacity properties, the tip resistance and side resistance of cast-in-place piles with large diameter was explored by comparing the test results of cast-in-place piles before and after the combined grouting.Besides,the distribution of the cement slurry and the effect of the combined grouting on the pile foundation were comprehensively analyzed by the coring tests and standard penetration tests, respectively. The empirical coefficients of side resistance in the silty fine sand layer before and after the grouting were obtained based on the statistical analysis, and the relational expression between the side resistance after the grouting and the blow counts N before the grouting was established. The results show that compared with the numerical values before the combined grouting, both side resistance and tip resistance after the combined grouting dramatically increase, and the ultimate bearing capacity of grouted piles increases by 94.25%-151.51%, which shows that the effect of combined grouting is significantly obvious. The bearing behaviors of the combined grouting pile are obviously superior to bored pile with post-grouting merely at pile tip, and significantly affect the load transfer characteristics of pile foundation. Moreover, the distribution of the cement slurry of the side pile and the tip of the pile was clarified by the coring test. The effectiveness of combined grouting was further proved. Additionally, the results of the standard penetration test show that the blow counts after the combined grouting is significantly higher than that before the combined grouting. The results can be applicable to the design of the bridge pile foundation, and can provide references for the similar bridge pile foundation projects in extra-thick silty fine sand layer.