Strengthening and underpinning of a sinking two storey building in Lagos State Nigeria

Abstract

Many existing foundations, particularly those that are old and dilapidated, are no longer strong enough to support the pressures they bear or support new loads applied to them. In order to boost the load bearing capacity, a unique underpinning technique is required. Underpinning technique provides secure, efficient, and reliable solutions to foundation and geotechnical problems affecting the foundations of buildings. However, the practice of underpinning is not common in Nigeria. This paper presents a case study on the settlement failure of a two-storey residential building in Lagos, Nigeria. The residential building encountered excessive differential settlement due to the variation in the soil strata in in 06°26′12″N Long: 3°30′44″E area of Lagos state. Some portion of the building was laid on weak and loose clayey sand which showed signs of foundation distress and led to the eventual choice of an underpinning technique adopted to extend the foundation depth to a stable stratum, to salvage the failed areas of the foundation in order to strengthen the bearing capacity and to minimize settlement. Underpinning was done because the original raft foundation was inadequate for vertical and lateral loads. The procedure used for underpinning was summarized. The measured settlement points of the underpinned foundations after six-year service life were less than 0.333 mm. This result meant that every point (from points 1–10) measurements were below the settlement limit which guaranteed the bearing resistance of the building. The result showed that the underpinning technique salvaged the areas that initially settled and showed distress signs.