On 1 August 2019, the police evacuated the town of Whaley Bridge after the partial collapse of the northern section of the spillway of the Toddbrook Reservoir dam in the High Peak District of Derbyshire, UK.
RSK associate director and specialist geotechnical engineer Chris Eaton, a Reserve Army Lt Col in the Royal Engineers, Engineer and Logistic Staff Corps, was mobilised as part of a military assistance team to assist with the emergency response. The request for assistance was answered in less than six hours, and Chris and his Royal Engineer colleagues had gathered in the tactical command centre in Whaley Bridge.
“After a briefing by the incident command team and assessing the dam and surrounding area, we assisted in providing appropriate recommendations to the agencies and dam owners. This included correctly monitoring and assessing the dam’s stability and systems of warning affected people downstream,” commented Chris. “Temporary repairs at the eroded part of the dam with bagged granular soils – placed at about 30 tonnes per hour by RAF helicopters – were already underway. Amazingly, more than 500 one tonne bags were placed by the RAF by the end of Monday, 5 August.”
The dam stabilisation work developed by the dam owner and its engineering consulting team comprised an infilled buttress of granular materials, plus cement grouting followed up with expanded polymer injection. The Royal Engineer specialists had experience in these stabilisation methods and were able to provide additional comment to the consulting team on site. Active liaison and coordination assistance were also provided to the fire brigade, police, mountain rescue, Canals and Rivers Trust, Environment Agency and to the civilian consultants and contractors undertaking the remedial work.
“We worked closely with the police to fly micro-drones very close to the damaged parts of the dam to film the underside of the collapsed slabs. This allowed us to assess the condition of the clay core and the supporting soils and structure before sending in engineers,” continued Chris. The detail and accuracy of these surveys proved invaluable in planning later work. “The drones were also equipped for infrared imagery, so surveys were flown at dusk looking for cooler spots which might reveal early water seepages through the embankment dam. The drones were also flown far out of the line of sight to create a helicopter’s eye view of a further Chinook drop-zone in a gorge where river diversion work was needed. As a result of the real-time transmission of the images to the RAF planners, tree felling in the drop zone commenced immediately so that the Chinook delivery could be made before nightfall.”
The military assistance team also provided advice on the placement and selection of pumps, which were provided by the Environment Agency and fire and rescue services. The immediate aim was to lower water levels to around eight metres below the spillway height, which is close to or below the base of the failure. Geotechnical consideration was given to the upstream damage to the dam because of the rapid drawdown of the reservoir. Teams also enhanced the original Victorian diversion weir headworks and the creation of a 1 km bypass canal with a 100-person workforce comprising civilians, the police, fire and rescue services and the army.
“We demobilised over the 4th and 5th of August,” concluded Chris. “The operation showed how the cooperation, coordination of all emergency teams, and the general public, provided an excellent response to a serious incident. Homeowners were given the all-clear to move back into their homes on Wednesday, 7 August.”
Relevant authorities will continue to monitor dam pore water pressures, structure stability and pumping levels.