扩底抗拔桩桩端阻力的群桩效应研究

通过颗粒流数值模拟，从桩端阻力随上拔位移的发展与桩端周围土体颗粒位移表现等角度，研究了扩底抗拔桩端阻力的群桩效应问题。研究比较了单桩（墩）与群桩（墩）的抗拔性状以及不同墩距下中心墩与边墩阻力随上拔位移发展的情况。研究表明，在归一化上拔量 0.1时，单桩（墩）与群桩（墩）的上拔特性无明显差别；此后，随着上拔位移的发展，单桩（墩）的上拔端阻力要大于群桩中的桩（墩）的端阻力，桩（墩）周围土体颗粒的相互影响开始显现。在归一化上拔量 0.5的情况下，群桩（墩）中中心桩（墩）的端阻力要略大于边桩（墩）。在归一化上拔量 0.5的情况下，群桩中边桩的桩端阻力较中心桩的要大，而群墩中边墩的墩端阻力较中心墩要小，体现了桩身侧限对抗拔桩群中端阻力发挥的影响。随着墩距的增大，在较大的位移量以后群墩才与单墩的受力有显著差别。

Group effects of pile tip resistance of under-reamed piles on uplift loading

Using numerical simulation by PFC2D(particle flow code in two dimensions),the group piles effect of pile tip resistance of under-reamed piles on uplift loading is studied.The increase of tip resistance with uplift displacement and the variations of soil displacement around pile tip are observed in the simulation.The differences between single pile(pier) and group piles(piers) in uplift loading are shown and the development of tip resistance with uplift displacement for group piers in different spaces is compared.It is shown that the uplift characteristics are similar for single pile(pier) and group piles(piers) when the normalized uplift displacement s/D is smaller than 0.1.With the development of uplift displacement,the tip resistance of single pile(pier) is larger than that in group piles(piers) and the interaction of soil particles around tip emerges.When the normalized uplift displacement s/D is smaller than 0.5,the tip resistance of central pile(pier) is larger than that of side pile(pier) in the group piles(piers).When the normalized uplift displacement s/D is larger than 0.5,the tip resistance of side pile is larger than that of center pile in the group piles and that is reverse in the group piers,which embodied the effects of side restriction of pile shaft on the mobilization of tip resistance.The characteristics of group piers had obvious difference with single pier only in larger displacement with the increase of pier distance.