结合消阻护筒的试验桩桩侧摩阻力和桩端阻力试验研究

石家庄地铁人民广场站试桩采用静载试验方案加载测试,设计要求除进行承载力测试外,还需确定桩侧各土层的分层极限侧摩阻力和桩端土的端阻力,以及桩侧摩阻力和桩端阻力占单桩极限承载力和承载力特征值的比例。利用消阻双护筒消除无效土层的侧摩阻力,通过桩身应力观测,利用弹性力学公式推算桩身轴力、桩侧摩阻力及端阻力的分布及变化规律,为设计提供依据。结果表明:1)双护筒消阻装置可直接消除无效土层段的侧摩阻力,使试验桩真实反映工程桩的实际承载力、侧摩阻力、端阻力及沉降值;2)达到极限承载力时,桩侧总阻力占比65%~66%,桩端总阻力占比34%~35%;达到承载力特征值时,桩侧总阻力占比76%~80%,桩端总阻力占比20%~24%;试桩承载力类型均为端承摩擦桩;3)局部范围内土层桩侧摩阻力表现为应力和位移的软化特征;4)桩端持力层主要为卵石层,对承载力的贡献平均占比约30%。

Experimental Study of Side Friction Resistance and  End Resistance of Test Pile with Double Sleeves

The static loading test is used to test the pile of Renminguangchang Station of Shijiazhuang Metro. The test of bearing capacity of the pile, determination of the ultimate side friction resistances of every soil layer and pile end resistance and the ratios of pile side friction resistance and pile end resistance to the ultimate bearing capacity and characteristic value of bearing capacity of single pile are required in design. The double sleeve is used to eliminate the pile side friction resistance of the invalid soil layer; the distribution and variation of the axial force, side friction resistance and end resistance of pile body are calculated by the elasto mechanical formula, which provides reference for project design. The results show that: 1) The double sleeve can directly eliminate the friction resistance of the invalid soil layer, which can actually reflect the bearing capacity, friction resistance, end resistance and settlement value of the test pile. 2) When the ultimate bearing capacity is reached, the total side resistance of the pile is about 65%-66%, and the total end resistance of the pile is about 34%-35%; when the characteristic value of bearing capacity is reached, the total side resistance of the pile is about 76%-80%, and the total end resistance of the pile is about 20%-24%; the bearing capacity of the test pile presents end bearing friction. 3) The side friction resistance of the pile in local range presents softening characteristic of stress and displacement. 4) The bearing layer of pile end is mainly composed of cobble layer, which accounts for about 30% of the total bearing capacity.