Using blinding as struts in cut and cover excavations

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The term blinding is used to describe the thin layer of unreinforced over-site concrete which is used to protect the base of excavations. Blinding is not generally seen or exploited as a beneficial structural element even though it clearly provides some temporary lateral support to the walls of cut and cover works until the base slab is constructed. This proposal stems from Powderham's pioneering work at Mott MacDonald, which shows that blinding struts can eliminate the need for temporary steel propping in cut and cover excavations. Mott MacDonald has used blinding struts in the construction of the Channel Tunnel cut and cover works, the Limehouse Link and the Heathrow Express cofferdam. The hallmark of these projects was the elimination of temporary steel propping from a series of deep excavations which paved the way for further time and cost saving schemes. It is important to note that in all these applications, the actual capacity of the blinding struts and the loads they were carrying were unknown. Therefore, a very conservative approach was adopted for the blinding struts which resulted in the struts being thicker than necessary with consequent cost and environmental implications. Research is required since many clients are unwilling to sanction the use of blinding struts because their performance has not been definitively established.The research will develop a numerical method for predicting the response of blinding struts in cut and cover excavations that is calibrated with data from laboratory tests and field data where possible. Predicting the failure loads of blinding struts is complex since the buckling load depends on factors including cracking, creep, shrinkage and the rates at which the lateral and transverse loadings are introduced into the slab relative to the development of concrete strength. This problem will be solved by carrying out a coupled analysis using both geotechnical and structural finite element programs. A simplified model for the structural response of the blinding strut will be used in the geotechnical analysis. This will enable strut loadings to be determined which will then be used in a refined structural analysis of the strut itself. The overall process will involve iteration, with the structural analysis refining the simplified strut model used in the geotechnical analysis, which in turn will provide improved estimates of the strut loading. This process, combined with the results from the laboratory testing, will lead to a proposed model for the non-linear behaviour of the blinding strut that can be adopted by others in their geotechnical analyses accounting for the specific site and construction conditions.The numerical analysis of the blinding strut will be carried out using the nonlinear structural analysis program ADAPTIC, developed by Izzuddin, which already provides all the necessary elements and material models for assessing the timedependent response of the blinding strut. The geotechnical analysis will be performed using the finite element program ICFEP developed by Potts. This program has been written specifically for geotechnical engineering and has been applied to a wide variety of soil-structure interaction problems. The geotechnical analysis will investigate the behaviour of an embedded cantilever wall with and without a single prop near its top. In both cases, a temporary blinding strut will be modelled at final excavation level. The construction sequence will be based on the procedures adopted at the Limehouse Link, Heathrow Express Cofferdam and Airside Road Tunnel constructions. Soil conditions will also be based on these sites with analyses conducted for excavations in predominantly London Clay and in the Lambeth Group. A series of 20 laboratory tests will be carried out on scale models of blinding slabs with variations in material properties, end conditions and lateral imperfections. The test results will be used to validate and refine the numerical analysis.