The Protection of the 400kV Cables at Pudding Mill Lane


The ground movement assessment and associated mitigation measures for the National Grid Electric 400kV power cables at Pudding Mill Lane, has been the subject of extensive debate and analysis since the commencement of the Crossrail project. These power cables are very sensitive to ground movement and hence elaborate schemes were developed for asset protection. A refined ground movement assessment was undertaken to ascertain the risk to the cables. This assessment concludes that ground improvement works alone, would be sufficient to protect the cables. The settlement predictions where undertaken by standard Empirical assessment, modifications to the Empirical method (accounting for ground treatment stiffness) and Finite Element Modelling. The report details the good correlation of maximum settlement between the modified Gaussian method and the Finite Element Modelling, in the order of 25mm. The observed movement shows that the most critical period for the cables were during the ground treatment phase, where a narrow heaved profile was observed. Analysis was undertaken at this point to ascertain the strain on the cables during depressurisation and tunnelling. This analysis summarised that the tunnel excavation would provide a relaxation in strain that could have built up in the cables during ground treatment. The Eastbound TBM excavation was undertaken with minimal surface settlement (10mm), equating to a volume loss of 0.5%. The observed movement showed two distinct settlement profiles either side of the TBM, due to the extent of the ground treatment works.