With just the width of a residential street in which to deliver the start of a £100M tunnelling project in Glasgow, logistics and planning are proving to be critical.
The approach to Jura Street near Glasgow’s shipbuilding district of Govan does not indicate that you are about to arrive at the launch site of a £100M tunnelling scheme. But looks can be deceiving – the narrow residential streets on the 1950s housing estate soon give way to site offices and the first view of a piling rig.
The launch shafts and a section of cut and cover tunnel for Scottish Water’s Shieldhall Tunnel are very much up close and personal to houses on Jura Street. This residential environment – and the narrow site it creates – is where Bachy Soletanche is currently undertaking piling work for joint venture contractors Vinci Construction Grands Projets and Costain.
The work is the downstream end of a new 5.2km tunnel to link the nearby Shieldhall Treatment works with another part of the network in Queen’s Park. When completed in 2017, it 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 the 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.
The scale of investment and potential for the scheme to become a role model for planning in Scotland means that the scheme has a high profile. At Jura Street the work is even more sensitive as residents here are not at risk from flooding and will only benefit from a new road surface once the work is complete.
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 (see box for tunnel details).
Bachy has already completed the 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 company is now working on a CFA contiguous piled wall for the 240m long cut and cover tunnel and CSPs for the tunnel boring machine (TBM) launch chamber that runs along the alignment of Jura Street itself.
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.
A Soilmec SR70 is being used for the CFA piles for the contiguous piled wall. The CFA piles are 750mm in diameter and the length changes 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 piles will be 900mm in diameter and installed to 13.8m depth.
“The main challenge is the logistics due to the residential nature of the site”
Paul Doyle, Bachy Soletanche
“The main challenge is the logistics due to the residential nature of the site,” says Bachy Soletanche contract manager Paul Doyle. “The site is literally the width of the street, so the sequencing of the work is critical.
“The piling platform is being installed just ahead of the piling work. Getting the materials onto site is also a challenge, so we have daily planning meetings to coordinate the work.
“We have no option other than to use one rig for this section. We did consider using two and working from each end of the site but it was just too difficult logistically.”
Cages for the piles are being supplied by Romtech and with room to store no more than 16 on site, planning and coordination is key, especially as the cage design varies with the length of the piles.
“The first CFA piles encountered the sandstone at a shallower depth than expected, which meant different cages were needed to the ones that had been delivered to site for the work,” says Doyle.
Work so far has gone well despite the challenges presented by unexpected – and often disused – utilities in the road which have mostly been undisturbed since the 1950s. “In some places the original cobbled road was directly under the asphalt,” says Doyle.
The only other thing that still concerns Doyle is installation of glass fibre reinforced cages to create the soft eye for the TBM launch. “Normally glass fibre cages are installed in an open hole but as we are using CFA piles we will have to plunge the cages,” he says. “We have worked with Romtech to develop a steel tube inside the cage to create enough stiffness to allow us to lift and plunge the cage without damaging the glass fibre sections.”
Originally it was planned for the excavation work to follow behind the piling but the logistical challenges mean that the shaft will be excavated at the same time and work on the cut and cover section will start once the piling is complete.
The cut and cover will be propped at two levels to reach the formation level and the invert will be cast with fibre reinforced concrete to overcome uplift pressures, resist cracking and maximise durability. The crown will also be cast insitu ahead of the TBM launch next year.
Bachy expects its work to be completed in November but it will be another two years before the rest of the scheme is completed.
Once the Herrenknecht slurry TBM is launched next year, the tunnel will be bored south eastwards under Bellahouston Park and Pollock Country Park to Queen’s Park.
The upstream end of the tunnel – shaft four – will connect into an existing chamber at Queen’s Park using pipe jack techniques, while the Jura Street end will be connected to Shieldhall via a 2m diameter cut and cover tunnel.
“Shafts two and three were located at changes of direction in the tunnel bore but have been removed through a value engineering exercise,” says Costain Vinci surface works project manager Brian Walker.
Despite the 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, Bellehousten Park, Titwood Road near Crossmyloof Station and Upper Queen’s Park,” says Walker. “Soil Engineering is undertaking consolidation grouting of the mine workings themselves and permeation grouting around the launch and reception shaft.”
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. Further ground investigation is planned this autumn.”