Many grouting projects are reactive to ground movement problems and residents of Glasgow are very familiar with these works as a result of widespread shallow coal mines across the Scottish city.
However, the latest fleet of drilling rigs and grout mixers are working to mitigate against the problem ahead of a major investment in a wastewater management tunnel to protect the city from flooding.
Accuracy is essential as the grouted mine workings will soon be in the direct path of a Herrenknecht slurry tunnel boring machine (TBM) that will be constructing Scottish Water’s £100M Shieldhall Tunnel. Main contracting joint venture Vinci Construction Grands Projets and Costain are currently preparing for the arrival of the TBM but the grouting work has been sublet to specialist contractor Soil Engineering Geosciences (SEG).
Shieldhall Tunnel route
The Shieldhall Tunnel is 5.2km tunnel to link the Shieldhall Treatment works with another part of the network in Queens Park, which is currently the main focus of the grouting. When completed in 2017, the tunnel will provide flood relief for southern Glasgow and reduce storm water overflow into the River Clyde.
“The scheme is part of a wider programme of work in Glasgow that is focused on large scale water quality improvements and removing properties from flooding register by 2021,” says Scottish Water senior project manager Dominic Flanagan. “It is the largest investment undertaken by Scottish Water and is aimed at making the network in Glasgow fit for purpose for the next 100 years.”
The project is based on 10 years of hydraulic modelling and is part of integrated investment planning that Scottish Water has undertaken with Glasgow City Council, the Scottish Environmental Protection Agency and Scotland Canals. This approach is now being rolled out to other Scottish cities.
“It was identified during the early stages of the project that there would be a requirement for grouting on the project,” says SEG business unit manager Tristan Llewellyn. “SEG has previously undertaken grouting work to stabilise collapsed coal and ironstone mines around Glasgow over the last 30 years, so we had a good idea of what to expect.
“The mining was mostly done in the 18th and 19th century but is largely unmapped. The mines are very discreet in nature, which makes it hard to predict where will or won’t be affected, but where the mines do exist most were driven using the pillar and stall approach.”
According to Llewellyn, the grouting work for the Shieldhall Tunnel is probably the most complex of all the grouting projects the company has undertaken in the city due to the size of the sites and varying locations. During the course of the scheme, SEG has worked in a residential street at Jura Street (see box), in park environments at Bellahousten Park (see box) and Queens Park, and on railway land and within live traffic at the Titwood Road site.
At Queens Park SEG has taken over an 800m long east-west swathe of Glasgow’s largest park, putting the work fully within the public spotlight.
SEG had specific experience of working in Queens Park after undertaking previous grouting work to shore up the bandstand for Glasgow City Council around a decade ago. Nonetheless, that work was not on the scale of the current project that will call for 2,000 grout holes to be drilled that are set to cover a linear length of 42km.
“The route of tunnel has been planned to follow Glasgow’s greenspaces and roads to minimise the impact of any potential subsidence on residential or commercial properties,” explains Llewellyn.
“With the TBM alignment set to bore directly through the grouted horizons, there is no margin for error with our work and minimising the risk of collapse, groundwater and ground gas is vital.”
SEG moved onto site in Queens Park in January this year and expects to complete the work in May 2016.
”With the TBM alignment set to bore directly through the grouted horizons, there is no margin for error”
Tristan Llewellyn, business unit manager, SEG
The grouting covers a 20m wide strip of the park with a spacing of 3.5m between treatment holes.
“The tunnel will pass under the park at 15m to 20m below ground level and we are treating the ground up to 12m below the base of the tunnel,” says Llewellyn.
There are five coals seams within the depth being treated by SEG but there is only evidence of workings within two – the Virgin and the Flint. There is some faulting and folding within the seams in the park so careful observation of the drilling arisings is vital. SEG is treating the seams from the top down and, so far with 25% of the drilling completed, voids have been found by most of the grout holes.
The grouting is sequenced to create a cut off wall to the north and south of the tunnel alignment and once this is completed the holes between are used to undertake bulk infill.
Costain and Vinci will also construct two shafts within the park and SEG will deliver more technical grouting to enable these to be excavated safely. “We will create a grout curtain, create a base plug using grout and undertake fissure grouting for the shafts,” adds Llewellyn.
Like Queens Park, SEG started work on grouting at Titwood Road in January. While the Queens Park work is being carried out during daylight only, the set up and railway elements of the work here will call for overnight working.
It is hard to miss that the grouting is in full swing – two cement silos tower above a supermarket car park that SEG has acquired for its site offices – and half the street – over a 200m section – is hidden by hoardings with just the drilling rig masts peeking over the top. Temporary traffic lights control traffic flows through what is thought to be one of Glasgow’s busiest junctions, giving road users plenty of time to view the work.
Work is focused on completing the grouting on one half of the road before the traffic management is switched over to allow SEG to access the other half.
The tunnel does not follow the road alignment so extensive use of inclined boreholes is being applied. However, this is not straightforward as short masted rigs have had to be specially brought in to prevent obstructing the traffic and part of the road is on embankment so the drill orientations need careful planning to prevent work daylighting.
The coal seams are more folded and faulted here than at Queens Park, so with just half of the site width available for treatment and the variability of the ground conditions, it has been challenging for the site team to build up an accurate ground model.
The next phase of work at Titwood Road will call for grouting below a railway line close to Crossmyloof Station, which will have to be undertaken during overnight weekend track possessions.
The other challenge for the Titwood Road grouting team is the location of a mine shaft in a road junction just off Titwood Road.
“Geophysics has been undertaken in the road to locate the mine shaft and guide the grouting design,” says Llewellyn.
While the scale of work at Titwood Road is smaller, it is certainly still challenging.
SEG had only completed six weeks on site when GE visited the work but so far everything has gone well.
“Our experience of grouting in Glasgow has certainly helped get us to this point but there are still some elements of the work that we need to manage carefully to ensure smooth running of the whole contract,” says Llewellyn.
“A major concern that still needs to be overcome is a decision on how to complete drilling for grouting work on the Titwood Street site which is close to a care home where residents are particularly sensitive to noise.”
“Our experience of grouting in Glasgow has certainly helped get us to this point”
Tristan Llewellyn, business unit manager, SEG
SEG has undertaken trials of a Soilmec SM8G rotary drilling rig at the Queens Park site to try and work with Costain, Vinci and Scottish Water to try and find a solution that will minimise the impact of the work on residents without affecting the programme for the grouting.
Another issue that will be a concern until the drilling of the last hole is completed is the need to ensure no steel is left in the ground. “The no steel drop is critical because the TBM will pass through the grouted areas and any steel left in the ground could seriously damage it,” says Llewellyn. “Just in case it does happen, we have developed some special equipment that would allow us to overdrill the hole and remove the steel before grouting is carried out.”
While it is clear that SEG has carefully planned the work and taken precautions against various pitfalls, the proof of the grouting will only come when the TBM safely passes through the site. With the TBM not set to reach Queens Park until spring next year, it will be some time before Llewellyn’s team can completely relax and class the work as a success.
Preparing for launch
In terms of tunnelling techniques, Shieldhall has it all with piled shafts, top down constructed shafts, sprayed concrete linings, pipe jacking, grouting, cut and cover and bored solutions.
Jura Street piling
At the other end of the new tunnel from the current Queens Park grouting works, Bachy Soletanche has already completed a large amount of piling, along with more grouting by SEG, to prepare for the launch of the TBM.
At the Jura Street site the ground conditions of clays and glacial tills overlying mudstones and sandstone combined with the shallow depth of the tunnel at Shieldhall have dictated the use of cut and cover techniques.
This led to Bachy undertaking 131, 900mm diameter cased secant piled (CSP) wall for the 15m diameter and 15.5m deep shaft one and a service chamber, which is just off Jura Street. The walls for the 240m long cut and cover tunnel have been formed by 750mm diameter CFA piles with the length changing from 15m close to the service chamber to 12m as the cut and cover tunnel progresses towards the TBM launch chamber.
The TBM launch chamber has been formed from 900mm diameter CFA piles installed to 13.8m depth with glass fibre reinforced cages used in the piles at the launch end of the chamber to create the soft eye for the TBM.
“The main challenge was the logistics due to the residential nature of the site,” says Bachy Soletanche site engineer Jake Young. “The site was literally the width of the street, so the sequencing of the work is critical.”
Jura Street grouting
Once the TBM is launched later this year, the tunnel will be bored south eastwards under Bellahouston Park and Pollock Country Park to Queens Park.
Despite value engineering, the bored tunnel work is not straightforward. The route has been selected to minimise the number of residential areas it passes, this takes the tunnel through a number of disused mine workings and complex faulting.
“Four areas were identified from the outset – Jura Street, Bellahousten Park, Titwood Road near Crossmyloof Station and Upper Queens Park,” says Walker. SEG has already completed grouting at Jura Street and Bellahousten Park.
The tunnel will also encounter faulted ground where it passes under the M77, the Paisley Canal and a railway line. “This area presents a settlement risk as we will only have one tunnel diameter of cover in this area,” says Walker. “We may undertake more compensating grouting in this area but this is still under discussion.”