Services

Inspections

Hydraulic Asphalt Inspections

Hesselberg Hydro's advice to clients on essential maintenance depends on regular inspections of previous projects to monitor performance.

While we will inspect any hydraulic structure, our work is most commonly on asphaltic concrete dam linings.

The linings are protected by a seal coat of either mastic or bituminous emulsion which generally needs to be re-applied at 15 to 20 year intervals. Before covering the asphalt, it is prudent to have the lining inspected and maintained.

Our inspections, which carry on well past any contractual obligation, also provide the knowledge of long-term behaviour of materials and structures that continually inform improvements to technology.

The construction of asphaltic dam linings is an important example of how technique has been modified in the light of experience:

• Older dam linings have two layers of Dense Asphaltic Concrete (DAC). During installation, heat caused adjacent layers to adhere leaving the interface indistinguishable. However, any moisture trapped between the layers during construction can lead to de-bonding and blistering. After initial de-bonding, progressive damage will follow , once blisters start to crack and allow more water in.
• The problem is avoided in modern installations by a single, thicker impermeable layer.
• Finding and repairing de-bonding damage is one of many aspects of the inspection work that has taken Hesselberg Hydro's team around the UK, and to the Faroe Islands and Norway.

Project

Winscar Reservoir, 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.

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