Tunnels make a major contribution to economic, environmental and cultural life around the world. In the UK, for instance, London Underground’s Jubilee line tunnel – and, by extension, the tunnel boring machines (TBMs) that helped create it – has brought redevelopment all along the line and boosted the city’s economy.
TBMs can also support the environment by digging tunnels that help alleviate traffic congestion, reduce the impact on landscapes and protect wildlife. For example, working with its joint venture partners Morgan Sindall and BAM Nuttall, Balfour Beatty has seen one of its two TBMs become the first to break through the ground to complete a section of tunnel for London’s new super sewer – the Tideway project (main image).
The 1.1 km tunnel will take sewage overflows from King George’s Park in Wandsworth, south London, into the main 25 km super sewer at Fulham, where it will be transferred to east London for treatment, instead of polluting the River Thames. The refurbished TBM working on this project (it previously operated on a water ring-main project in north London) is 3m wide and more than 70m long. So far, 10 km of the overall Thames Tideway Tunnel has been built, with four tunnelling machines in the ground. Once complete in 2024, the tunnel will help stop tens of millions of tonnes of raw sewage pouring into the river every year.
Another project designed to minimise environmental impact involves an 1,800 tonne TBM being used to construct Sirius Minerals’ 23-mile mineral transportation tunnel from seaside town Whitby in Yorkshire, to Teesside, a conurbation around Middlesbrough.
The tunnel is part of the company’s multi-billion-pound project to extract polyhalite (a natural fertilizer). The material will be transported underground on a high-capacity conveyor belt system (a mineral transport system) located in a 37 km long and 4.9m wide tunnel at an average depth of 250m below ground, to the materials handling facility on Teesside.
Tunnel contractor STRABAG plans to push a 225m tunnelling machine into the ground down a pre-excavated portal to bore the first of three separate tunnel drives, which will make up the tunnel. Two further machines will be launched in due course from Whitby and Lockwood Beck, near Guisborough, to complete the other 15 miles of the 6m diameter tunnel. Each TBM will operate 24 hours a day, seven days a week, lining the tunnel with 150,000 concrete segments to form rings that reinforce the tunnel walls.
TBM DESIGN & type
TBMs – also known as ‘moles’ – essentially comprise a large metal cylinder and trailing support mechanisms.
Typically, there is a rotating cutting wheel at the front end of the shield, behind which there is a chamber for the excavated soil to be either mixed with slurry (a mixture of solids with suspended in liquid) or left as it is, depending on the type of the TBM.
A set of hydraulic jacks behind this chamber, supported by the finished part of the tunnel, is used to push the TBM forwards. Once a metre or two has been excavated, a new tunnel ring is built using the erector – a rotating system that picks up precast concrete segments (example pictured, above right) and places them in the desired position. Support mechanisms behind the shield include dirt removal, slurry pipelines if applicable, control rooms and rails for transport of the precast segments.
Patricia Kossek, corporate communications officer at TBM supplier Herrenknecht, explains that each individual tunnel route has special geological and hydrogeological conditions: “The machine type appropriate in each individual case is chosen after all available project parameters have been considered. There are several different types of TBM.
“Soft ground (including cohesive soils such as clay, silt or loam with low water permeability and a smaller range of grain sizes) is a classical area of application for earth pressure balanced tunnelling methods. Heterogenous ground (which includes non-cohesive soils such as sand, gravel or stones with high water permeability, or mixed geologies of solid and loose rock) is more likely to use slurry-supported tunnelling methods, such as the Herrenknecht Mixshield (www.is.gd/hulegi).”
Meanwhile, drilling through rock, such as sandstone, limestone, basalt or granite with high compressive strengths, typically involves using hard rock machines. Hard rock TBMs excavate rock with disc cutters mounted in the cutter head. These cutters create compressive stress fractures in the rock, causing it to chip away from the tunnel face. Excavated rock is transferred through openings in the cutter head to a belt conveyor, where it runs through the machine to a system of conveyors or muck cars for removal from the tunnel.
The twin secrets of extending the life of a TBM are proper design and effective maintenance. The former, which will depend on ground conditions and tunnel length, is best carried out by the TBM supplier.
Maintenance needs to be scheduled at regular intervals with the tunnel contractor completing daily planned cutter inspections.
Other maintenance tasks include checking and topping up, if necessary, of hydraulic fluids and oil levels, with daily monitoring logs kept for all major systems on the TBM.
If the tunnel is particularly long, it may be necessary to include planned refurbishment of components, such as gearboxes, during the operation.
BOX OUT: A brief history of TBMs
Modern TBMs are engineering marvels, but tunnelling has its origins in far humbler beginnings. The first tunnel ever constructed successfully beneath a river was the Thames Tunnel in London. Started in 1825 and completed in 1843, the project used engineer Isambard Kingdom Brunel’s tunnel shielding technology – a temporary support structure designed to protect the labourers excavating the tunnel from unstable ground. The shield was moved forward as the excavation of each section was completed, being progressively replaced with pre-built sections of tunnel wall.
However, Brunel’s tunnel shield had a design deficiency – it was rectangular, which meant that it lacked strength. In 1840, a US engineer called Alfred Ely Beach suggested that a circular shield design would work better because round tunnels are more stable and sturdier than their rectangular counterparts. (The stresses around a circular opening are said to be more evenly distributed). The original Thames Tunnel shield design was improved still further by mechanical and civil engineer James Henry Greathead during construction of the Tower Subway under the River Thames in London in 1870.
The first boring machine reported to have been built was Henri-Joseph Maus's Mountain Slicer in 1846, to dig the 8.5 mile-long Fréjus Rail Tunnel between France and Italy through the Alps. It consisted of more than 100 percussion drills mounted in the front of a locomotive-sized machine, mechanically power-driven from the entrance of the tunnel.
Meanwhile, 11 TBMs were used to dig the Channel Tunnel; five dug from France and six from the UK. Ground conditions at the French end were wetter than at the British end. This meant the French machines moved more slowly and that British TBMs dug more of the tunnels. Engineers used two tunnel lining systems – cast iron segments bolted together and precast concrete rings. The TBMs excavated a huge amount of chalk. On the French side, this was crushed, mixed with water and pumped inland behind a specially built dam. On the British side, engineers used the chalk to build a landscaped platform at the foot of Shakespeare Cliffs near Dover. One of the TBMs from the British side remains buried under the Channel and another was reportedly sold on eBay in 2004.
Other project examples include Crossrail, which used eight of these tunnel borers to construct the rail tunnels under London. The colossal machines carefully weaved through the capital’s congested sub-terrain, snaking between the existing Tube network, sewers, utilities, and London’s hidden rivers at depths of up to 40m.