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Llyn Brenig Hydro-Mechanical Upgrade




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To achieve recommendations set out by the Llyn Brenig Reservoir All Reservoir Panel Engineer (ARPE) following a Section10 report of the Reservoirs Act, Mott MacDonald Bentley (MMB) were faced with a multitude of complex temporary works scenarios in an extremely challenging, high-risk environment. 

Llyn Brenig Reservoir has a spill, draw off and emergency scour facility operated within a 50m-deep concrete tower, 150m from the reservoir shoreline. Following on from the Section10 report and feasibility study, it was deemed necessary to carry out remedial works to satisfy reservoir safety, some of which posed complex temporary works in very challenging conditions. Here are some of the activities that presented the biggest challenges:

  • Replacement of gantry crane support plinths;

  • Replacement of gantry crane; and

  • Replacement of scour gate whilst working behind a single-point of isolation (with 50m reservoir head)

Replacement of gantry crane support plinths

The Goliath Gantry Crane is primarily used to install the bulkhead gate for emergency or maintenance reasons, and to service remedial or maintenance works within the draw-off-tower.

The crane operates on two rails fixed to a steel structure and is supported by six concrete plinths. Following a structural assessment, four plinths were deemed necessary for replacement. This posed the team with the challenge of propping/supporting the 26t crane and support steelwork to allow safe replacement civils works of the plinths. We engaged with specialist contractor Freyssinet to design and implement temporary works which, whilst not compromising the existing structure, would jack the existing gantry crane and support steelwork up c.3mm to allow the removal and installation of the new plinths.

Gantry crane replacement

The next temporary works challenge was to replace the 26t, 40-year-old Goliath Gantry Crane. The main risks presented were use of a stabilised crane on water; protection from collision to the existing draw-off-tower/embankment dam; and supporting crane components during dismantling/assembly. 

We worked closely with Red7Marine to develop a temporary solution that overcame the issue of providing a mobile crane on a pontoon 150m off-shore without the option of completely anchoring to the reservoir bed (50m deep which included the earth embankment dam). Instruction from the ARPE was that the pontoon should be completely isolated from anchoring to the tower and embankment dam for the entire works. 

A temporary works design/methodology was developed between ourselves, Red7Marine and ARPE, which included a 100t telescopic crawler crane positioned on a 324m2 floating pontoon anchored back to the existing shoreline with c.250m long wire anchor ropes and 10t anchor points. Extensive stability checks considering wind loading and wave action were carried out, which contributed to the huge success of this activity.

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Replacement of scour gate (single isolation) 

At the base of the reservoir draw-off-tower some 50m deep sits the reservoir emergency draw-down and scour facility (actuated gates). Over time the primary scour gate deteriorated and presented the dam safety team with reliability concerns should an emergency event ever occur.

The project team were faced with the challenge of removing the concrete surrounding the existing primary scour gate whilst working behind single isolation, required to allow removal of the existing gate/frame to accommodate the new gate/frame. 

Following various engineering/temporary works meetings where the objective was to focus on the removal or reduction for operators to be working under single isolation, our team worked closely with Gnat Demolition-UK to develop a method to remove the concrete with engineering precision using remote/robotic control. The main risk was the potential to damage the existing structure, causing it to implode. This could have been fatal to anyone working within the confined space and also increased the risk of an uncontrolled release of the reservoir.  

The main and first indication of any potential damage to the structure or change to its condition was monitoring of the leakage rate past the bulkhead gate (first and only gate between the workforce and the entire reservoir). Over several weeks and months the team developed a robust aluminium screen which help to control a number of risks. The primary purpose of the screen was to monitor the existing leakage rate passing the bulkhead gate; this was completed by installing a throttled valve on each side of the aluminium screen and a float switch connection to a visual and audible alarm.  24hr CCTV monitoring was also in place to assess the condition of the work area prior to entry and throughout the working day by a confined-space-trained ‘top-man’. 


The second risk was to protect the bulkhead gate from any damage via flying debris which was achieved through a robust but manoeuvrable aluminium screen. The third risk/complication was managing the existing leakage water during concreting/grouting works, something overcome by installing two temporary valves within the aluminium screen to direct/pipe incoming waters around the working area. 

The project demonstrates a drive and commitment to achieving safety throughout the construction of permanent works by coordinating, designing, constructing, maintaining and removing complex temporary works in an extreme location with, at times, very extreme conditions.

Want to learn more? Speak to Jason Eddies @ MMB

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