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Hesselberg Hydro

Marchlyn Mawr Dam, Snodownia, Wales

Inspection & maintenance contract for asphaltic concrete lining


Marchlyn Mawr dam is 620m along the crest and 65m at its maximum height. It is owned by First Hydro Company who provide electricity for the National Grid.  There are various types of dams and most store rainfall from large water catchments.  The Marchlyn Mawr Dam is different in that it is the top reservoir of an enclosed system.  The water is circulated from dam to power station and back again, according to the levels of demand for electricity.

All dams need regular maintenance and, with a rapidly changing enclosed system, extra hydraulic pressures are exerted on the dam wall – in this case, 50 000 square metres of asphaltic concrete on the upstream face.

In 1996-1998 Hesselberg Hydro performed inspections and some remedial work on several dams with asphaltic waterproof linings, in the UK, Norway & the Faroe Islands.  In 1998 they were asked to inspect and maintain the Marchlyn Mawr Dam nestling high in Snowdonia. 

A thorough investigation was carried out and some minor surface defects were detected. These were closely monitored over the next seven years. It was discovered that the defects were generally vertical surface cracks along construction joints, with other minor defects apparent on the lining in various locations.

The defects were on a steep upstream face of the dam and with the potential for vicious winds and heavy rain, remedial work can become a challenge even for specialists.  On top of this the power station operation was not to be interrupted so the asphaltic membrane works could only occur between 1200 – 2400hrs and the lining had to be left impermeable at the end of each shift.


In 2005 therefore, Hesselberg Hydro turned to an innovative Shell Bitumen product for the solution.  The cracks in the dam wall were cut out with a hand-held chaser, penetrating up to 40mm deep and 20mm wide.  The void, once cleaned and primed, was filled with Shell Tixophalte Wet Seal And Fix, a special Thixotropic adhesive.

In the past, hot mastic would have been used. This can be problematic when applied in harsh weather conditions and presents serious safety risks when handled on steep slopes.  Also, hot mastic is not very viscous, and on an incline can run or weep if applied too heavily.  Contractors would be required to build up layers to fill cracks or holes, which is both time consuming and messy.  Shell Tixophalte can be used straight “out of the pack” without heat application and can be applied in more or less any climatic conditions, and even underwater.

The remedial work, lasting more than five weeks, forms part of Hesselberg Hydro’s term contract with First Hydro as maintenance engineers at the Marchlyn Mawr Dam.

During the 2005 works the surface of the lining was cleaned and it became apparent that the protective mastic seal coat had worn away in many areas so Hesselberg Hydro were asked to develop a method for replacing the seal coat.

In 2006 the majority of the seal coat was replaced using hot mastic placed by a purpose-build spreader suspended from the crest.  Prior to placing the mastic the existing surface had to be cleaned and primed.