Upheaval buckling behaviour of blinding struts

Abstract

The term ‘blinding’ is traditionally used to refer to the thin layer of unreinforced concrete laid at the bottom of excavations to prevent the surface of the excavation from being churned and eroded by construction traffic. Blinding is usually assumed to be non-structural. However, Powderham and others have shown that significant cost savings can be achieved by using blinding as a temporary strut in cut and cover excavations prior to casting the base slab. This article describes a research project being undertaken at Imperial College to investigate the key issues which need to be addressed in the design of blinding struts. The research has shown that blinding struts can fail in upheaval buckling. The main parameters influencing the buckling load are identified in the paper. Comparisons are also given between the measured and predicted axial responses of scale model blinding struts tested by the authors. The article also discusses the effect of creep of concrete on the failure load of the blinding struts. It is shown that struts can creep to failure under sustained loads greater than around 70% of the buckling load at first loading when a sinusoidal ground imperfection is assumed. The article concludes with a brief overview of the geotechnical numerical analysis undertaken to date which gives some insights into the nature of the soil-structure interaction.