In early 2015, the Glasgow Subway's operator, Strathclyde Partnership for Transport (SPT), engaged civil engineering company Freyssinet UK for a £16m programme to modernise 10 km of tunnels.
Appointed as principal contractor and principal designer, Freyssinet had 22 months from the April 2015 start date to complete the works, which posed significant challenges from the outset.
Completed in 1896, the Glasgow Subway is the world's third oldest underground railway, after London's and Budapest's and its Victorian design makes it difficult to maintain and repair the ageing infrastructure. Significantly, there was no mechanical means to move repair materials and equipment from street to track level.
Freyssinet regional safety manager Darren Collinswood CMIOSH set out the challenges in March when he addressed the UK Health and Safety Executive's (HSE) first musculoskeletal disorder (MSD) summit, where the company won the HSE and Chartered Institute of Ergonomics and Human Factors' joint risk reduction annual design award. Collinswood said traditional but slow manual handling solutions such as stair walkers were out of the question due to the narrow working window each night.
"We would have needed hundreds of people to carry out the manual handling that was needed to get all of the bags down into the system," he said.
Around 28,000 of the 46,000 injection holes were drilled 2 m above the tunnel track bed
SPT wanted to avoid a full-scale shutdown of the tunnels to keep the public transport system operating during normal hours. It put in place a short closure period in the early hours for the upgrade, leaving Freyssinet a 6.5-hour nightly shift to undertake the works.
In these hours, Freyssinet's team of 150 employees and subcontractors would need to transport 3,000 cu m of concrete grout (that is 900 25-kg bags each shift) and 9,000 litres of resin down the narrow stairways. Once in the tunnels, workers would then clean the tunnel lining, track bed, slip and drainage channel before further investigations and assessments to determine the condition of the lining.
One of the first jobs involved drilling 5,500 probe holes to measure the thickness of the tunnel lining and the presence and size of voids behind it. Once they knew the extent of the repairs, workers would drill 46,000 32 mm diameter injection holes in 8,019 linear metres of tunnelling. The holes were filled with grout to prevent damage to the tunnel lining from any of the voids.
Throughout this period, workers monitored the 3.2 m diameter tunnel at five points to ensure there was no unexpected movement of the lining. The concrete lining repair work involved manually applying concrete patches to the damaged tunnel walls and spraying up to 2,700 sq m of mechanically applied mortar.
Workers also drilled 18,000 injection holes and pumped in 68,000 litres of resin to leak-seal the brick and concrete linings of the tunnels. Weep pipes were installed to manage water ingress and track bed repairs were completed to seal cracks.
Handling risks
The drilling operations presented hand-arm vibration and manual handling risks. Around 28,000 of the 46,000 injection holes were drilled 2 m above the track bed, requiring a scaffold or access platform. The location also required workers to hold 7.5 kg Hilti TE60 drills above shoulder height. Using standard equipment, Freyssinet calculated that the drilling would create vibration levels of 7.5 sq m a second, with a calculated exposure action value (EAV) of 53 minutes.
To reduce the hand-arm vibration risk, Freyssinet developed a bespoke drilling rig with French manufacturer Sept-Tools. The custom-built rig had to meet several design objectives. It had to be compact, easy and safe to move to and from the working areas during each shift, easy to assemble and disassemble, and easy to operate. The rig had to serve as a handling device for the Hilti TE60 drill, so it had to be compatible with both the drill and its dust extraction system.
"There are robotic drills and other drilling frames out there, but we had two problems with them," Collinswood told the HSE summit. "First, we couldn't store anything in the subway. We had sub-surface fire regulations to contend with and space issues. Most of the kit out there was quite cumbersome and heavy. It would have taken several hours to set up."
The advantage of the bespoke rig was that it could sit on the track trolleys that moved equipment and materials through the tunnels. Because it was compatible with the dust extraction system, the rig helped cut the amount of respirable silica dust.
The rig comprised only a few parts, each weighing between 3 kg and 4 kg, making them easy to carry and assemble. The design also eliminated the need to hold the drill at head height.
The rigs, mounted on track trolleys, removed the need for specialist access equipment and the associated work-at-height risks. Collinswood said the kit enabled workers to drill around 70 holes into the tunnel wall each shift, tripling production and halving task hours.
Another problem was the significant manual handling challenge in moving materials down to Kelvinbridge and St George's Cross stations, where the main grouting work took place.
Designing out this risk took inspiration from the food production sector where overhead conveyors are used in abattoirs to transport heavy animal carcasses. Freyssinet worked with specialist manufacturer Amber Industries to design and install such a conveyor to move grout down the stations' fire escape stairs from street level. Workers transported 16 bags of grout at a time using custom-made 700 mm x 700 mm pallets, each with a safe working load of 500 kg, as well as a cradle made to carry track trolleys and other equipment.
Industry benefits
Collinswood told the summit that the technology offered wider lessons for the construction industry.
"We work mainly in refurbishment and repair where sometimes it's even harder than on new-build sites to implement measures to get rid of the causation factors," he said.
"Because of that it's very important that we get our designers and project managers on board. They need to understand MSDs and that they are just as important as mainstream risks. They understand manual handling but not fully. They think manual handling is picking heavy loads up. They don't realise it's supporting a drill above your head for four hours a night."
Collinswood also highlighted the benefits that engineered solutions can provide to improve the longevity of the industry's workforce.
"A lot of skills in the construction industry are waning, he says, "and we need to keep guys into their 50-plus, 60-plus years. We still want them to be operating for us.
"A big selling point that we need to get across in the industry to project managers -- the guys who hold the purse strings -- is that, if we can bring solutions in like this [to] reduce the MSDs, that will have a clear, direct effect on increasing production. It's money well spent all round, not just in reducing claims from injuries but also in making a lot of time up on projects and keeping them to plan."
