Carsington Dam—The Near-Miss Which Became a Bulls-Eye

The slide of a large part of the core and upstream shoulder of Carsington Dam in July, 1984, took place against an unusual background. One of the largest earth-fill dams in Britain, it was designed by a team that appeared to show limited geotechnical expertise. Far too few shear strength tests were undertaken, unrealistic slip surfaces were analysed, instrumentation data was not utilised; and the design factor of safety was not adequately specified. The dam incorporated an oddly-shaped core which maximised the length of potential slip surface through this comparatively weak element of the dam cross-section. Towards the end of the second earth-moving construction season, as the dam embankment rose towards 5m of its final height, movements in the fill and the Engineers' concern about a high pore pressure and no consequential change of design, caused the contractor to seek an assessment of the dam's stability from its specialist consultant, an experienced earthfill dam designer. In a report reviewing stability, undertaken by both Authors with assistance from a number of colleagues, the eventual failure mode was identified, and likelihood of failure, due to low factors of safety, was correctly predicted. Appropriate corrective measures were identified. This could have been a "near miss" case — if the original designers had discussed technical issues. In the event, strained relationships arising out of other issues prevented action being taken to rectify matters. Further earthfill was instructed to be placed in the third construction season, and the avoidable failure took place just as the embankment dam was being completed to full height. Post-failure investigations were carried out and published by others, including a Governmental Report and several technical papers. The dam has been rebuilt satisfactorily on the same site. There are several lessons to be learnt from Carsington, including that clear channels for the debate of technical issues must be provided, so that they can be debated free from commercial or personality related issues, without fear of litigation.