Micropiles are widely used for underpinning existing buildings that are being vertically extended. They can be preloaded after installation, whereby partial loads are transferred from existing piles to the micropiles to prevent the existing piles from being overloading during the vertical extension work. This paper evaluates the effect of preloading on the load transfer from the existing piles to a micropile and the load sharing mechanism among pile groups subjected to subsequent vertical loading. This study presents a series of centrifuge experiments conducted on a 2 × 2 pile foundation underpinned with a micropile. Before the foundation was subjected to vertical loading, a preload of 10% of the micropile's bearing capacity was applied to its head. The experimental results show that the preloading facilitates the transfer of partial loads from existing piles to the micropile, with the load-transfer efficiency being more than 90%. During subsequent loading tests, the application of preloading made the load-sharing ratio of the micropile about 20% higher than that in a comparable case without preloading.

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预加载微桩对离心实验基础支撑性能的影响

微型桩被广泛用于支撑垂直延伸的现有建筑物。它们可以在安装后进行预加载，从而将部分载荷从现有桩传递到微桩，以防止现有桩在垂直延伸作业中过载。本文评估了预压对从现有桩到微型桩的荷载传递的影响，以及在后续竖向荷载作用下桩组之间的荷载分担机制。这项研究提出了一系列在微型桩支撑下的2×2桩基础上进行的离心实验。在基础承受垂直载荷之前，将微型桩承载能力的10％的预载荷施加到其头部。实验结果表明，预加载有利于将局部荷载从已有桩向微桩的传递，荷载传递效率大于90％。在随后的加载测试中，预加载的应用使微桩的负载共享率比没有预加载的情况高出约20％。