Management of the M4 Elevated Section substructures

The M4 Elevated Section in west London is a 1·9 km concrete viaduct structure providing a major arterial route into London. An intervention model bringing together structural assessment, corrosion and crack monitoring data plus current and future predicted condition has been developed in a three-dimensional (3D) visual platform to enable structural rehabilitation of the substructure cross-head beams to be prioritised and planned on the network over the 30 years of the M25 design build finance and operate contract for Connect Plus. An initial assessment of the cross-heads undertaken to identify residual strength discovered a deficiency in capacity compared with current standards. Further assessment, including 3D strut and tie, non-linear finite-element analysis and plastic analysis, was undertaken to confirm public safety with continued trafficking of the structure and to determine strengthening requirements. Additionally, extensive monitoring of the cross-heads, including crack surveys and remote monitoring techniques, has been implemented to provide early warning of deterioration to prioritise interventions. The long-term maintenance strategy includes strengthening, cathodic protection concrete preservation and concrete repair. This paper discusses the development of the 3D model, the prioritisation process, including deterioration modelling, and the advanced structural analysis used to formulate the strengthening and maintenance programme for these substructures.