This is the first time that work has been carried out in the tunnel in around 40 years. The infrastructure is life-expired. And there are other factors to consider, according to Reilly. Those renewals in the 1970s were carried out at weekends, meaning that the teams were ‘always chasing the clock’.
The tunnel was being dug out and concrete poured in, but the water wasn’t managed properly, he suggests. And with natural springs in the ground surrounding the tunnel, that factor is important.
“There are canals and streams and all sorts running over the top of it. So quite a lot of water finds its way into the tunnel.”
Reilly says the tunnel is in good condition and fairly secure, but nevertheless the slab track needs securing. NR must dig down at least 1.5 metres, depending on the rock type, to secure the track.
“There’s a hydrostatic pressure which could break it,” he explains. “So we’re putting some dowels down to hold the slab track onto the bedrock. And we’re putting in a deep six foot drain to remove all the water from there.”
Furthermore, the tunnel is on a 1-in-40 incline, which will affect what plant can and cannot be used. Runaways also need consideration.
The site will be split into platforms, the throat and the tunnels. Up to 100 people could be working per shift, and three shifts will work around the clock.
The conductor bar for electric trains will be installed along the length of the tunnel and will be directly fastened into the tunnel lining. There will be seven metre centres, so there’s no catenary system - the bar just spans between the supports. This was the system installed in Winchburgh Tunnel and also tunnels at Princess Gardens in Edinburgh.
Access must be tricky for the teams, however. Reilly explains: “We’ve got access from the north through to the tunnel. There are also two tunnel airshafts we’re going to use to supply concrete into the tunnel. We’ll set pumps on top of them to pump concrete into the tunnel. It’s not an easy job… it’s not a great location, but we can’t move it. One of the key interfaces is managing the site compound along with ScotRail passengers.”
He adds that when NR builds the station frontage area, that will be done while the facilities will remain in use: “The station will be used as it is today. We’ll knock the building down and build a new one.”
The Queen Street project has been ongoing (Reilly says he has been working on it for two years), with preparatory work for the slab-tracking carried out at Christmas.
Reilly explains: “The slab-track system is a three-part system. There’s a base slab we’re going to install. It’ll knock the slabs out and at these points we’ll put in a spigot or spindles - that makes the lines level of the slab track. When we’ve sufficiently put these in we’ll put a length of rail in for 100 metres or so. Then we’ll fine-tune the position of the slab track of the rails so the geometry is perfect. There’s a self-compacting concrete, which we pour into these gaps. We just pour the concrete in, which fills the sides in.”
Once the concrete ‘cures’ the system is ready to go: “Essentially it’s all ours, we can run a whole rail track over it. In three days you can run trains over it. Once you’ve got the old base slab out and the new base slab in, the job becomes fairly straightforward.”
A production line will then work its way through the tunnel. Reilly explains: “We start around the Up line, which is the Edinburgh line - the Glasgow end of it - and we break the existing slab out and head towards Edinburgh. Basically remove the slab and then once you’re so far ahead you start installing the new base slab. Once that’s up ahead you start putting in the slab track.”
Attention then turns to the Down Line, going in the same direction.
“It’s easier. If you try to pour the concrete on the top end it just runs down. So you need to start at the bottom end and work your way up. You clean the site behind you, so what you’re cleaning you can work on, as long as you have trains in that are bringing sufficient supply of the panels - and we’re going to have a stack of these in the station.
“Something’s going to have to stop work to allow a train coming past us. But the beauty of doing one line at a time is you’ve always got a working line. You’ve got a way in and out.
“Queen Street is pretty straightforward because it’s all a pretty consistent gradient. Winchburgh had a bit of a dip in it, so mining out water was more problematic.
“Going to our methodologies, we’re going to install a drain as we go. Then there’s always an outlet for water, which will go all the way along that base slab, so we’re never going to be swimming - we’ll have our outfall.”
Reilly is not expecting any surprises. “We are going to encounter spring water at certain times, but we’ve got a plan. One of our designs is we can capture that and channel out water. We manage that water at a one-off level, so it doesn’t impact into the build.