Collapse of a brick outlet culvert

During the routine 6 monthly testing of the penstock valve, on the upstream end of an overflow culvert at an upper pond a large amount of brick rubble was carried through in the discharge to the lower pond. The penstock valve was immediately closed. The overflow culvert runs through the centre of the earth embankment dam and is 32 m in length. The upstream 20 m comprises a circular 530 mm dia concrete pipe. The final 12 m length comprised a brick arch culvert, 1080 mm high, 610 mm wide. There were no visible signs of embankment movement or crest settlement. A CCTV survey was undertaken, entering the culvert from the downstream end only, which identified a large void in the brick culvert section 8 m from the downstream end of the culvert. This survey also identified that the invert of the brick culvert comprised timber sleepers. Following consultation with the last Inspecting Engineer, a precautionary and controlled drawdown commenced. A further CCTV survey was completed, entering the culvert from both upstream and downstream ends, to help ascertain further information for the most appropriate repair. This revealed the full scale of the void, approximately 3 m in length and extending approximately 1.5 m away from either side of the original line of the brick culvert. The crest path was closed to both pedestrian and vehicular traffic and temporary pumps were brought in to continue the pre-cautionary drawdown at a rate of between 300 and 400 mm per day and hold the water level at an agreed level whilst a design solution was developed and repairs to the overflow were undertaken.

The root cause of the incident was failure of the brick culvert overflow leading to potential instability of the homogeneous earth embankment dam.

The Undertaker promptly arranged for a CCTV survey to be undertaken to establish the condition of the overflow and extent of damage. The Undertaker and Supervising Engineer liaised closely with the most recent Inspecting Engineer, as the severity of the damage and the full composition (timber sleeper invert) of the culvert was established to close the embankment to the public and undertake a pre-cautionary drawdown. The Undertaker accepted the need for temporary pumps and secured their procurement promptly. The above team of personnel also promptly engaged with a couple local contractors to consider alternative repair options and established that an open-cut full length replacement of the overflow culvert was the preferred solution, driven by the presence of rotting timber sleepers forming the invert of the original brick culvert.

Not applicable

The path that runs across the crest of the embankment dam was closed to the public (walkers, horse riders etc).

A CCTV survey was undertaken on 28th May 2024, entering the culvert from the downstream end only, which identified a large void in the brick culvert section 8 m from the downstream end of the culvert. This survey also identified that the invert of the brick culvert comprised timber sleepers. A further CCTV survey was completed on 17th June, entering the culvert from both upstream and downstream ends, to help ascertain further information for the most appropriate repair. This revealed the full scale of the void and the extent of rotting/missing timber sleepers on the culvert invert. The Supervising Engineer engaged with two local contractors to consider potential repair options either re-lining and grouting or an open-cut replacement. A full length open-cut replacement of the overflow pipe was agreed, to provide the greatest longevity. The overflow culvert was replaced with a concrete pipe over the full length, including a concrete collar on the upstream end and a filter collar on the downstream end and the embankment re-instated with a cohesive Class 2A material between September 2024 and January 2025.