Device for Load-Redistribution between Existing Piles and Underpinning Micropiles

A micropile preloading method using bolt-fastening was studied to improve the constructability and economic feasibility of underpinning micropiles for the vertical expansion of a building with existing pile foundation. Due to the close distance between the existing piles and new micropiles, and the difference in their stiffness, precise control of preloading is required for the micropiles. A new preloading device was developed for this purpose. The laboratory test showed that for precise control of preloading, direct bolt-fastening (like the bolt connection for steel structures) was better than the typical hydraulic jacking method. The results of laboratory and onsite tests showed that the proposed preloading device adequately redistributed the bearing load between the existing piles and underpinning micropiles. Further load-redistribution was also possible by applying post-loading to the micropiles with continuous long-term measurement of bearing force whenever it was needed, even after the construction of the building expansion.