French winemakers talk about the ‘terroir’ of a vineyard to describe all the vagaries of soil, topography and weather that affect the wine. In the notoriously variable and challenging ground conditions of Scotland’s largest city however, it probably hasn’t occurred to the engineers of Vinci Construction Grands Projets and Costain to think about the ‘spirit’ of the ground they are contending with. The local whisky probably makes a more fitting analogue.

Mixed face conditions under low cover and abandoned coal mines are just two of the challenges to be overcome on the project. Mercifully the tunnel is mostly under parks and roads – but there is the Paisley Canal Rail Line to pass under and also the M77 motorway.

The JV team is working to construct a new sewer in Glasgow, a bypass tunnel around the existing network to increase capacity by 100,000m3 and relieve pressure on the water treatment plant at Shieldhall. The tunnel runs through the southwest of the city from the Craigton Industrial Estate to Queen’s Park. The idea is primarily to alleviate flooding in the city, which can be severe, but should also lead to river quality improvement, habitat improvements and economic development.

THE TUNNEL AND LINING

The main portion of the works is a 5kmlong, 4.7m-diameter segmentally lined tunnel designed by Aecom. It is being excavated by a Herrenknecht slurry TBM that will erect around 3,250 rhomboidal rings during its drive, which are each 1.5m long, 250mm thick and reinforced with 35kg/m3 of 4D Dramix fibres supplied by Bekaert Maccaferri. Tunnel designers said that the 5D product was considered but they were satisfied with the strength of the 4D, which resulted in a compressive strength of around 80N/ mm2, easily exceeding the specified 60N/ mm2. The segments themselves are cast by FP McCann at its plant in Derbyshire in the English Midlands.

There will be no special inner lining to contend with the waste, just the concrete lining and VIP gaskets making the seal. The machine was launched from a 200m-long open-cut ‘false tunnel’. The ground conditions were clays and made ground with a low cover and not suitable for mechanised tunnelling. Bachy Soletanche was brought in to sink 650 secant and contiguous piles along the area to be cut. Heavy propping was needed, with two rows of props before the bottom was reached at 10m deep and the invert was constructed. This was the first civil work on the site and eventually the tunnel ring would be completed and properly backfilled and covered.

This 200m section also meant that the full 180m TBM backup could be deployed, saving time when excavation started, as opposed using a launch frame and dropping the backup down as the machine advanced.

GEOLOGY, METHANE AND AVOIDING MINE WORKS

The tunnel is relatively flat – a 5km length but only 3m difference in terms of level to satisfy the gravity requirements – but the geological differences are impressive. After the clays and made ground of the launch pit, the TBM hit bedrock made of sandstone and mudstone. This was also the first place where mine grouting works were undertaken.

Glasgow is famously riddled with coal mines that date back centuries. In fact, the mines were also for ironstone and building materials, but the coal measures are the famous target. Every few years a story relating to the mines will appear in the news. Sometimes this is to do with a structural risk of collapse, damaging houses, at other times discussion has ventured into the familiar realms of intelligent use of underground space. Speaking to Scottish paper the Herald in 2013, researchers from Glasgow Caledonian University suggested that 40 per cent of the city’s heat could come from geothermal sources, facilitated by the honeycomb of mine works.

For tunnellers on Shieldhall, the mines are a danger. At the very least if a slurry machine hit an abandoned mine the immediate result would be a sudden loss of slurry.

To avoid this, prior to construction Scottish Water engaged JWH Ross to get an accurate picture of the mine workings and their history. The company used a grid system as part of their investigations, which allowed the work to focus where it was most needed. So far nothing has been encountered by surprise. Any mine workings that were a potential risk to the tunnel were fully grouted (with PFA) and neutralised.

The more usual risk from coal is methane. While it had not been encountered as Tunnels and Tunnelling visited site, the project has been set up to deal with it. This takes the form of monitoring stations in the tunnel. Air is used to maintain pressure in the working area, which can be adjusted or even flushed if it becomes contaminated by passing through an area with methane. The slurry treatment plant supplied by M-S of France also has monitoring stations in order to detect methane. Back to the geology, the machine will then pass through glacial till, granular alluvium and in places a mixed face of sandstone in the invert, glacial till in the middle and clay at the top. This is particularly challenging for the slurry treatment plant, needing small adjustments to get the best balance for the varying conditions.

The geology in the area is challenging according to the contractor and although the machine has the capacity to probe ahead of the face, it is not seen as necessary to date. Although probing will begin if the TBM encounters anything unexpected.

CONNECTION TO EXISTING WORKS

The connections to the existing sewer are by 2.4m-diameter microtunnelled drives carried out by Joseph Gallagher. The actual link upstream is in the bifurcation shaft which forms a Y-junction to join the parallel networks. Once everything is complete, a new permanent structure will be built to handle the inflow.

MACHINE AND SITE PERFORMANCE

The Herrenknecht machine has performed well achieving a best advance rate of 26 rings per day in mudstone. This was a pleasant surprise to the contractor as the machine is designed for a mean production of 20. The contractor’s construction manager Guillaume Lefrere says he is reasonably satisfied with its ability to perform in differing ground conditions and to control settlement where there is limited cover. The former is fortunate as part of the value engineering on the project was to eliminate two of the four shafts (shafts two and three – tunnel maintenance shafts) and drive from start to finish without surface intervention, saving money and time, but requiring confidence in the equipment and personnel.

In terms of settlement, a key section was the crossing of the motorway M77 (CH1550-1600), the designed settlement predictions were about 16 to 19mm, while in reality less than 4mm was detected by the monitoring system.

Site personnel are picked out by Lefrere for special mention, “The people who run the machine make the difference. You can have the best machine, capable of going through all variations of ground along your alignment, but if you don’t put competent people with the machine, in this case people with experience of slurry technology in various ground conditions, you will not go anywhere.

“For example our tunnel manager Rafik Karaouzene, and also the varied experience of both companies in the JV are a huge boost to us. Karaouzene was the tunnel manager for the Lee Tunnel project and various others around the world utilising a slurry TBM, which is a huge plus.

“[Theoretically] putting together a team of people who spend most of their time in London Clay is not necessarily a recipe for success, we needed people with a variety and with Costain we got the right people together.”

Performance bottlenecks have depended on the situation, with hard or abrasive ground, testing the machine more, with stickier and softer materials proving a trial for the STP, although Lefrere says they have been problematic the team has worked hard to adjust the parameters and to keep the process as efficient as possible.

The lining has also stood up well, with a cracking percentage around 3 of the utilised segments, of which 10,000 had been placed at the time of T&T’s visit.

Cutters and shifts

Cutter wear has been higher than expected during interventions, with more wear in the sandstones in the area of the M77 motorway and beyond. The team compensated by having interventions prior to important crossings so there Interventions, which can be carried out under hyperbaric conditions up to 2.4 bar, are also undertaken when moving from one ground type to another and took the form of an inspection to appreciate wear, then swapping out the tools as needed. This time is also used to plan the time and location of the next two interventions. A typical intervention takes four to six shifts and the site operates a 24 hours per day, five days per week schedule of up to 12 hours per shift. Fridays are shorter and nontunnelling works are sometimes carried out over the weekend.

Design features

To avoid the need to purchase a second TBM and the associated costs, as well as the additional launching chamber, second STP, acoustic shed, cranes and all the rest, careful attention was paid to the design of the machine. There is a feature of the TBM for use in sticky ground that was provided specifically for the messy Glaswegian geology. The machine has the ability, if it is getting bogged down, to rapidly flush the chamber with fresh slurry, this allows the cutters to be showered off for better cutting of the ground. The engineers think it might be a first use of this technology.

A plastic membrane has been added to the tail skin and is an improvement on the sealing arrangement used for the Lee Tunnel TBM. This membrane reduces the grout ingress in the brushes when the drive direction changes. According to Karaouzene, it makes leaks appear for less than 30 seconds, sometimes less than 10, rather than for more than five minutes. This keeps flexibility in the brushes for longer, extending their working life. As Tunnels and Tunnelling goes to press, the membranes were operating fine after 2,300m. Also for this situation, foam gaskets are being installed at the gap between the EPDM gasket and the extrados in the radial segment joint. The measures are so far improving efficiency.

The team also insisted on a plan to change the main bearing if it became necessary. Lefrere comments: “Therefore to reduce the risks of stoppage the bearing has been designed with safety coefficients. If such a stoppage occurred, it would require the supplier to provide a new bearing with the associated lead time, and a month to change the main bearing in the tunnel.”

COMMUNITY ENGAGEMENT

One of the ongoing challenges in residential areas is traffic to site, muck away wagons and segment delivery and so on. At Shieldhall, located close to the M8 motorway but on the edge of a residential estate, something like 150 trucks per day are needed. This was solved early on by creating a temporary road at the back of the launch site that links to the motorway and bypasses the residential area.

The team have had few complaints as community engagement was carried out in advance by Scottish Water, the engineers on site remark that it’s their experience that locals are generally happy with work if they understand the need for it in advance and what sort of disruption to expect, mostly from the ancillary works and structures.

In June 2016 some 60,000 letters were dispatched to local residents and businesses. Overall 100,000 letters were posted. As is now usual, spoil is reused for other projects where possible – mostly as backfilling product, depending on the nature of the material.

FINAL THOUGHTS

Despite the challenging mixed face ground conditions, the engineers are confident of a successful finish which will provide a substantial improvement to the Scottish Water infrastructure, “As I said, if you have the right equipment and people, it is all manageable,” concludes Lefrere