灌注桩浅部缺陷的形成原因及预防措施研究

Research on the Causes and Preventive Measures of Shallow Defects of Cast-in-place Pile

研究目的:按《铁路桥梁钻孔桩施工技术规程》(Q/CR 9212—2015)第7.3.3条的规定,机械成孔水下混凝土灌注桩,桩顶设计标高处超灌混凝土高度要0.5~1.0 m,以增加压力确保桩顶设计标高处桩的完整。可现场检测多处桩顶设计标高处桩浅部有缺陷,并做钻深0.5 m的芯样抗压强度试验,未满足设计要求,是何原因引起,又应怎样预防,均须进行研究。研究结论:(1)桩顶设计标高处超灌混凝土高度要2~3 m;(2)混凝土坍落度控制在180~220 mm;(3)裁桩时桩顶设计标高处外缘一圈要环切割,桩顶设计标高出露处要见有最大粒径粗骨料,并抽样钻深0.5 m的芯样强度满足设计要求;(4)混凝土导管埋深不小于3 m,灌注混凝土时导管要居中,采用短笼疏孔后要进行二次清孔,端承桩底沉渣厚度不大于50 mm;(5)本研究成果可为铁路、公路等领域提高桥梁钻孔桩施工质量提供参考。

Research purposes: According to 7.3.3 of Q/CR 9212-2015 in Technical Specification for Bored Pile Works of Railway Bridge Construction, the height of over-pouring concrete at the design elevation of pile top should be 0.5~1.0 m, in order to increase the pressure to ensure the integrity of the pile at the design elevation at the top of the pile. The defects in the shallow part of the pile at the design elevation at the top of the pile can be detected on the spot, and the compressive strength of the core sample with a drilling depth of 0.5 m is not satisfied with the design requirements. The cause and how to prevent the defects should be studied.Research conclusions:(1) The height of over-pouring concrete at the design elevation of pile top shall be 2 ~ 3 m.(2) The slump of concrete shall be controlled within 180 ~ 220 mm.(3) During pile cutting, a circle of the outer edge at the design elevation of the pile top shall be cut in a ring, the exposed part of the design elevation of the pile top shall be provided with coarse aggregate with the maximum particle size, and the strength of the core sample with a drilling depth of 0.5 m shall meet the design requirements.(4) The buried depth of concrete conduit shall not be less than 3 m, and the conduit shall be centered when pouring concrete. After the short cage is used for hole dredging, the hole shall be cleaned again, and the sediment thickness at the bottom of the end bearing pile shall not be greater than 50 mm.(5) The research results can provide reference for improving the construction quality of bridge bored pile in railway, highway and other fields.