人工挖孔嵌岩灌注桩承载特性现场试验与机理分析

以青岛市某大型工程为依托,对在泥质粉砂岩地基中的5根人工挖孔嵌岩灌注桩分别进行竖向静载荷试验与桩身内力测试。根据大直径嵌岩桩实测数据探讨大直径人工挖孔嵌岩灌注桩的荷载传递机理与竖向承载特性。试验结果表明:试桩荷载沉降(Q-s)曲线为缓变形,桩顶沉降量均小于11mm,卸载回弹率大,幅度为51%~75%,承载力较高,5根试桩均满足设计要求;在最大荷载下,5根嵌岩桩桩端阻力所占桩顶荷载比值均在10%~20%之间,随桩长、嵌岩深度(中风化)增大而减小,表现出端承摩擦桩的特性;桩身荷载自上而下逐步发挥,上覆土层先达到侧摩阻力极限值,在嵌岩段中部侧摩阻力达到峰值;桩入岩越深,安全储备量越大,在泥质粉砂岩中风化段,实测侧摩阻力约为规范推荐值的2.5倍,说明5根桩有较大的承载潜力;随着荷载的增大,嵌岩段分担的总阻力由39%上升至45%,嵌岩段侧摩阻力占主要比重,但桩端阻力分担荷载的比例上升速率较快;根据行业标准与静载试验数据,重新认识该地层人工挖孔嵌岩灌注桩的竖向承载特性,充分发挥其承载潜力,对工程桩桩身尺寸进行优化,达到节约材料和提高施工功效的目的,具有较好的经济效益。

Field test and mechanism analysis of bearing capacity characteristics of manual digging rock-socketed filling piles

Based on an important project at Qingdao, vertical static load test and the pile shaft stress test were conducted on five manual hole digging and pilling piles installed into argillaceous siltstone. The load transfer mechanism and vertical load bearing capacity of the large-diameter rock-socketed piles were discussed through the measured data. The test results showed that the load-displacement of five test piles was slow type, with the pile sedimentation less than 11 mm and larger unloading resilience ranging from 51%~75%, and all the piles had high bearing capacity which could meet the design requirements. Under the ultimate load, the proportion of head load transmitted and supported by the shaft base was between 10%~20% and decreased along with the increase of the shaft length and socked length (socketed into medium weather part), which showed the characteristic of end-support friction pile. The load of the shaft gradually worked from the top to the toe, and the soil side friction reached its ultimate value at first, while the peak side friction located at the medium of the socked length. The deeper of the socked length, the more safety stock of the pile, and the measured side friction was 2.5 times of the recommendation at the medium weathering rock. The percentage of the socked part supporting the total loading increased from 39% to 45% along with the increasing applied load, and the side friction of the socked part played an important role while the end resistance possessed a quick growth ratio. Based on the standard and data of static load test, vertical bearing capacity of the manual hole digging pile was refreshed, and the pile size was optimized in order to save the materials and improve the effectiveness of the construction, leading to the great economic benefits.