Keywords: blinding, struts, upheaval buckling, soil-structure interaction.

The term "blinding" is used to describe the thin layer of un-reinforced over-site concrete which is used to protect the base of excavations from being churned up by construction traffic. The aim of the research is to exploit the blinding strut as a beneficial structural element, making use of the temporary lateral support that it provides to the walls of excavation works until the base slab is constructed.

The present research involves combined analytical, numerical and experimental work to obtain a better insight into the soil-structural interaction between the strut, the retaining walls and the underlying soil, with the aim of producing design guidelines that will aid current practice. This paper focuses on predicting the failure load of blinding struts in upheaval buckling. Comparisons are made between numerical and analytical solutions, highlighting the limitations of the latter in respect of more realistic numerical modelling. Some comparisons are also made with the results of laboratory experiments on blinding struts which form part of the overall research programme.

Parametric studies, using the structural finite element program ADAPTIC [1] are reported which highlight the most influential factors in the analysis. The parameters considered are i) the maximum imperfection of the strut profile, ii) the depth of the concrete strut, and iii) the length of the imperfection profile. The results from these numerical studies are compared with the predictions of the analytical model developed by Croll [2,3], and favourable comparisons are achieved within the limitations of the analytical model. The results also demonstrate the superiority of the numerical model, which can account for a variety of complex phenomena, including i) arbitrary imperfection shape, ii) rigid and flexible contact between blinding and the ground, iii) material nonlinearity, iv) time-dependent phenomena, and v) dynamic response.

The results from several laboratory-based experiments are finally used to validate the numerical models. These experiments have shown that the buckling response of the blinding strut is very sensitive to the position of the axial load within the cross-section, an observation that has been largely mirrored by the numerical models.

1

Izzuddin, B.A., "Nonlinear Dynamic Analysis of Framed Structures", PhD Thesis, Imperial College, University of London, 1991

2

Croll, J.G.A., "A Simplified Model of Upheaval Thermal Buckling of Subsea Pipelines", Thin-walled Structures, 29, 1-4, 59-78, 1997. doi:10.1016/S0263-8231(97)00036-0

3

Croll, J.G.A., "A Simplified Analysis of Imperfect Thermally Buckling Subsea Pipelines", International Journal of Offshore and Polar Engineering, 8, 4, 283-291, 1998.

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