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Rock stabilisation: Service life extender

2017 works drilling

Planning around the tourist season may not seem an obvious issue when working hundreds of metres underground but that was one of the requirements on a recent rock bolting project in North Wales. 

For almost 35 years Europe’s largest pumped storage power station has been helping to ensure the UK electricity network has capacity to meet peak demand at the touch of a button. Recent work to replace rock bolts in the power station’s 16km network of tunnels will help ensure that the site can continue to meet this demand for many more decades.

Bam Ritchies has just completed its fourth phase of work for Dinorwig’s operator First Hydro Company, which is owned by Engie, and knowledge gained through this long-standing relationship has helped the operation to run smoothly. Even a change in location resulting in the need to install three times as many rock bolts did not cause a delay to the start of the work in late September.

“Tours of Dinorwig are a popular tourist attraction so our rock bolt replacement work has to be scheduled around quieter periods,” said Ricthies geotechnical manager Stuart Jackman. “The visitors to the site were partly behind the change in location for the work this autumn. Originally we were going to be working on Tunnel 1 and installing 220 bolts but access was needed in this area so our work moved to Tunnel 104 with 616 bolts needed.”

Tunnel 104 is one of the main access tunnels to the bottom of the cavern and the drill and blasted route is 7m high and wide and D-shaped in profile. Rock bolts in this area were deemed to be nearing the end of their service life but Ricthies is supplementing the rock bolting by installing new bolts in a pre-planned array in between the existing ones, rather than removing the old bolts.

2016 works drilling & containment

Work has to be planned around tours of the power station

“The biggest challenge of working within Dinorwig is fitting around the day to day operations of the power station and the logistics of getting all the materials into the work area,” says Jackman.

According to Jackman, one of the reasons that Ritchies has won repeat work at Dinorwig is that the client has said “they don’t know we’re there” because of the good housekeeping practices the company has developed.

This long term relationship also means that Ricthies has built up detailed knowledge of the ground conditions, which is formed of Cambrian slate with near vertical dolerite dykes. “The rock we were working in this time was as we expected,” says Jackman. “We encountered water seepage at a few locations but nothing significant.”

For Tunnel 104, the rock bolts were installed in an array of five to a pattern designed by Mott MacDonald.

The new rock bolts are stainless steel so should have a longer service life than the ones they are replacing. The lengths of the 25mm diameter rock bolts vary from 3m for 400 of them to 3.7m for the remaining 216 locations. Each bolt is installed in a 32mm diameter hole and resin bonded to give a free length of around 1.5m.

Installation is undertaken using a JCB JS145 rubber tracked excavator fitted with a Ripamonte drill mast with a hydraulic drill head. “The use of the hydraulic drill head helped us to minimise the noise of our work,” says Jackman. “The hydraulic unit was also more reliable and fitting it on a rubber tracked excavator rather than a wheeled excavator meant that we could use our own operators which improved productivity.”

Water flush was used for drilling and Jackman says that grout bunds and submersible pumps were used to manage the flush and keep it contained within the work site.

Selected rock bolts have been tested to 165kN but they are designed to have a 110kN working capacity. Jackman says that the tests have been carried out under supervision from Mott MacDonald and the bolts have performed well.

Jackman says that the team was able to install up to 25 bolts per day with shifts operating between 8am and 6pm. “As well as a better understanding of the ground conditions, we learnt a lot about sequencing from our previous work on projects at Dinorwig,” he adds.

Work on installing and testing the final bolts was expected to be completed in mid-December.

Ritchies hopes to be back next autumn to complete the bolting on Tunnel 1 once the 2018 tourist season has quietened down.

 

Dinorwig fact file

dinorwig

The Dinorwig site was selected due to its past industrial heritage as a major slate mining area

Plans to build the Dinowig pumped storage power station started in 1970 and work on the civil engineering contract started in January 1976. It was the largest civil contract in Europe at the time. The power station first generated electricity in 1982 and was fully commissioned in 1984.

Dinorwig has the potential to deliver a maximum of 1,728MW into the UK network within 16 seconds and the engineering behind it set new records that still stand today. It is still the largest scheme of its type in Europe and its six generating units are sited within Europe’s largest man-made cavern at 1M.m3.

The power station’s 30m diameter, 65m deep surge shaft delivers water from the surge pond on Marchlyn Mawr to the power station located 750m within the mountain. First Hydro Company can use this water power to meet national surges in demand for electricity. The power station uses off-peak energy to reverse the pumps and transport the water back up from the lower reservoir to Marchlyn Mawr ready to go again when needed.

Not only is Dinowig an essential part of the UK electricity supply, it is also a major tourist attraction and over 225,000 people visit the site each year and many take a tour underground.

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