Winscar Dam, Yorkshire

Completed in 1975 with a rockfill embankment dam (53m high and 520m long), Winscar is one of several reservoirs in the UK with an asphaltic concrete, watertight element on the upstream face. The membrane is of the older, two-layered type. Very low water levels throughout the winter and summer of 1995/6 provided Hesselberg Hydro with the opportunity to inspect the lining for our client, Yorkshire Water Services.

• Various cracks and blisters were found in the upper DAC layer
• Defective areas were removed, and the DAC was replaced and compacted
• Vacuum and water penetration tests, showing the lower DAC was still watertight, confirmed that damage was restricted to the upper layer, and was due to de-bonding
• De-bonding appeared to extend beyond the repair sites
• Conductivity tests confirmed the presence of water between the layers, even where there were no surface defects
• Random cores identified the wide extent of de-bonding
• Tests on core samples showed a normal composition, with materials in good condition. We concluded that the potential for damage was probably introduced during construction, with rainfall entering the interface being the most likely explanation.

We recommended further inspections and repair whenever low reservoir levels allow..

We also calculated that, irrespective of damage to the upper DAC, the intact lower layer should withstand predicted wave attack over the next 30 years. Thereafter, fatigue and damage to the asphalt is a possibility.