微浸水对桩基负摩阻力影响的试验初探

Preliminary exploration of tests on effect of micro-immersion on negative skin friction of pile foundation

为使湿陷性黄土地区桩基负摩阻力问题得到实质性的解决,提出了一种在桩基施工过程中通过桩周土体浸水消除部分黄土湿陷以提高其承载能力的思想,并以此为出发点,阐述微浸水概念,设计现场试验,进而分别对不同地层条件下及不同微浸水程度后桩侧负摩阻力的分布特点、发展规律展开初步探索。研究结果表明:高压循环注浆成桩工艺既能实现桩周土体的微浸水,使其首先发生预湿陷,又能使漏浆层以下一定深度范围内土体浸水程度明显增强形成强浸水段,引发再湿陷,产生负摩阻力;土层中漏浆层的不连续分布,致使桩周土体在桩体受荷后分段湿陷,桩侧负摩阻力沿桩身呈现交错分布的形态;随着微浸水程度的逐步增加,桩侧正摩阻力逐渐受到削弱,单桩极限承载力逐渐减小,与此同时桩侧负摩阻力逐渐增大,但增幅不大、数值较小。最后,指出本次试验的不足之处,并对后续研究提出建议。

In order to solve the problem of negative skin friction of pile foundation in collapsible loess areas, a new idea is proposed to improve the bearing capacity of pile foundation by immersion of the soil around the pile to eliminate part of the loess collapsibility in the process of pile foundation construction. Taking it as a starting point, the concept of micro-immersion is introduced, and the field tests are designed. A preliminary exploration is made on the distribution characteristics and development laws of negative skin friction of pile side under different geological conditions and the degree of micro-immersion. The results show that the high technology of pressure circulation grouting forming pile can achieve micro-immersion of soil around the pile to make it pre-collapse at first, and it can enhance the moisture content of soil significantly within the scope of certain depth below leakage layer and form strong immersion sections, and then it may lead to re-collapsibility and cause negative friction. The discontinuous distribution of the leakage slurry layer in the soil layer results in discontinuous collapsibility of the soil around the pile after the pile is loaded, and the negative skin friction of the pile side shows a staggered distribution along its length. With the gradual increase of the degree of micro-immersion, the positive friction resistance of the pile side is gradually weakened, and the ultimate bearing capacity of the single pile decreases gradually. At the same time, the negative skin friction of the pile side increases gradually, but the increase is small and the value is small. Finally, the shortcomings of this experiment are pointed out and the future research is prospected.