Performance of the Underpinning Piles for Basement-Supplementing Retrofit of a Constructed Building

Abstract

Development of underground space in urban areas may be obstructed by the aboveground facilities that are in good condition. Instead of building new aboveground structures, an alternative solution is to add new basements under the existing structures. One of the key issues arising from this complicated technique is the behavior of the underpinning piles. This paper considers this issue in terms of a real case recently addressed in China. The retrofit construction procedure was carefully monitored. Observations included the axial-loading and excavation-unloading responses of the underpinning piles, the axial forces transmitted from the piles to the replacing pillars, and the variations in building settlement. The results indicated that the piles acted well as an underpinning role in transferring structure load into surrounding geomaterials. The mobilization degree of the pile-shaft friction increased as the shallow strata were excavated further. Abrupt increase in pillar force and building settlement were detected when the exposed portions of the piles were cut, and therefore the cutting operation should be careful for safety control. The performance-based analyses support the pile-pillar load transmission method and offer a fairly successful reference to future applications.