Upgrading the capacity of foundations by using a hybrid expanded footing–micropile system

Abstract

This paper introduces an innovative approach, where a hybrid expanded footing–micropile system is utilised to improve the load-bearing capacity of shallow footings on loose sand. Finite-element analyses were carried out with a validated numerical model to simulate a shallow foundation on loose sand supporting a concentric vertical load. The simulated foundation was then upgraded by expanding its area and underpinning the additional area with pressure-grouted micropiles. The micropile installation process was treated as a pressure-controlled cavity expansion problem in the numerical simulation to account for the associated increase in radial stresses in the adjacent soil. A comprehensive parametric study was conducted, focusing on the primary determining factors: number of micropiles, micropile diameter, micropile bond length, grouting pressure, width of the additional area around the perimeter of the footing and footing aspect ratio. The results showed that the micropiles acted as excellent settlement reducers once installed. If reducing the settlement is a high priority, when designing an expanded footing, the hybrid expanded footing–micropile system should be preferred rather than expanding the footing without the use of micropiles. The load capacity of the underpinned foundations was highly dependent on the grouting pressure applied during micropile construction.