3D FE modeling of a complex multi-stage foundation underpinning for a high-rise building

Abstract

ABSTRACT The piled foundation of an existing high-rise building in Cairo was assessed as inadequate. The building foundation consists of 370 piles arranged below the columns and walls. The building was constructed in 1977, and up to the start of this investigation in 2014, it was just a concrete skeleton without finishing. The existing foundation was subjected to underpinning in 1985 using micropiles to support new lateral load-resisting elements proposed by the project structural consultant. According to the new Egyptian codes of practice, underpinned foundations are still considered inadequate and it was proposed to add additional micropiles with a raft connecting the different pile groups. This paper presents the 3D FE modeling for this complex multi-stage foundation underpinning considering the construction sequence and the different interactions between the original piles, existing and proposed groups of micropiles, supporting soil and the capping raft. The load transferred by direct contact between the raft and the soil is also investigated. Available detailed geotechnical investigation and evaluation of the available micropiles loading tests were used to estimate the parameters of soil and pile−soil interface. The advanced 3D FE model is used to ensure that the proposed number of additional micropiles satisfies the required safety and stability. The outcome of the analysis proved to conservatively satisfy safety requirements. The raft transferred a considerable part of the total load directly to the soil, hence increasing the capacity of the combined piled raft foundation system.