Today, there are more ways of creating tunnels than ever. As infrastructure in the Middle East becomes increasingly sophisticated with metros and large-scale utility requirements, clients are finding that modern methods, some of which are new to the region, are becoming increasingly attractive.

“Historically, the Middle East hasn’t had a lot of tunnelling work other than a few major highway cut-and-cover tunnels and small-diameter pipe jacking,” says Paul Groves, the UAE-based head of UK consultant Atkins’ tunnelling network group. “But in the last six years, starting with the Dubai Metro and followed by Abu Dhabi’s deep sewerage tunnels, this has changed and is set to drastically increase in the future.”

Simple as the cut-and-cover method may be, it is still the most appropriate choice in many situations. “From within a temporary deep excavation, you build up the concrete structure from the bottom, build the roof slab and cover it over again,” says Groves. “That has been the common method in the Middle East for 20-30 years and it is used globally all the time.”

Top-down approach

Although new methods are becoming available, not many are appropriate for very large cross-sectional areas at shallow depth. So, despite being well established, cut-and-cover methods continue to evolve.

On the Dubai Metro project, for example, a top-down approach to building the cut-and-cover tunnels was used for sections with depths of up to 27 metres below ground level, which connected the underground stations to the elevated viaducts above ground.

Permanent diaphragm walls were installed from ground level and excavation then commenced down to tunnel roof slab level using a temporary strut restraint to stop the face and walls from falling in. After this, the roof slab was cast before the lower part of the excavation was dug out beneath it to base slab level.

“The advantage is that in some circumstances, the top-down approach can offer simpler construction sequencing operations and the early slab casting gives stiff lateral restraint, which can be preferable to using temporary struts or anchors,” says Groves. “This is a great benefit when faced with looking after sensitive buildings and structures.”

Boring the main tunnels for the Dubai Metro, however, was undertaken using mechanised tunnel boring machines (TBMs), which resemble underground trains and were 9.56 metres in diameter. The machines, often referred to as ‘moles’, have a cutter head at the face and host a range of supporting infrastructure along the body, which can be as long as 150 metres.

Using shields

Huge shields protect the face of the excavation from collapse as the tunnel lining is constructed inside the bore. This principle is not new, however. When work started on the original Thames Tunnel in London in 1825, the late engineer Marc Isambard Brunel pioneered the use of a huge shield to protect workers from the risk of collapse at the tunnel’s face.

The methods used nowadays have evolved much beyond Brunel’s iron support mechanism. Today’s machines both support the face and build the tunnel lining behind it, usually by placing segments of concrete lining sequentially until a ring is created.

“The important thing here is that the TBM’s shield enables you to build the permanent lining in free air and dry conditions, without the ground and groundwater applying any load to it until the ring is complete and is expelled from the shield as the machine advances,” says Groves. “The current range of available machine types, despite being variable, all do the same basic tasks: they cut the ground and remove spoil, allow safe building of the lining rings and manage ground movements and groundwater control. All this is done in a tightly controlled process and at a potentially rapid rate.”

TBMs now average about 30 metres a day. During construction of the Abu Dhabi deep tunnel sewer, one machine managed to bore through 46.2 metres of ground in a day.

Flexible tunnelling machines

TBMs are capable of boring through most conditions, from soft, waterlogged ground to hard rock. In the Gulf’s coastal cities, machines suited to soft ground are most common. These include slurry (or mixed shield) machines and earth pressure balance (EPB) machines. These contain an Archimedes screw behind the face, and the rate of revolution of that screw determines the rate of advance of the cutter head and governs the back pressure in the chamber behind it. It is this back pressure that gives the tunnel its face support.

EPB machines were used in the construction of the Dubai Metro and are currently being used on Abu Dhabi’s 41-kilometre deep-sewer tunnel. One of the better known TBM manufacturers is Germany’s Herrenknecht, which manufactures the machines at its facility in Schwanau. Hubert Lang, an adviser to the company’s chairman, says the firm is currently suppling machines to more than 150 traffic-related projects and more than 850 utility projects.

Five of its EPB machines are currently in use in Abu Dhabi’s sewage interceptor tunnel scheme. “[EPB machines] are typically used in soft ground with cohesive or even incohesive soils, where there is high clay content and low water permeability,” says Lang. “Where shield excavation is carried out in unstable soils, a loss of stability of the tunnel face is prevented by creating a support pressure, known as the earth pressure balance shield.”

Slurry machines are also suitable for construction in soft ground and are often used where groundwater penetration is expected to be an issue. “Slurry machines are typical soft-ground TBMs used for tunnelling in highly permeable, unstable ground or beneath buildings sensitive to ground disturbances,” says Lang. The face pressure is created by mixing the excavated spoil with a solution of bentonite clay and pumping this around the tunnel face to provide resistance against any potential collapse.

Contractors may opt for slurry machines in Doha, which sits on karstic limestone where water has passed through the ground layers wearing away rock and creating cavities.

A TBM has an extensive array of support equipment on its body known as the back-up system. “A TBM is a highly complex, integrated operation unit,” says Lang. “What you find behind the cutting wheel is the segment feeder system, stone crushers, flushing lines, conveyors and belts to transport spoil back to the collection point, cylinders providing thrust, tanks of water, bentonite, grease, electricity transformers, personnel containers and even toilets. It is a whole factory.”

Although mechanised tunnelling is fast and effective, sometimes a manual method is most appropriate in soft ground. Experts tell MEED that conventional mining methods – known as sequential excavation, the sprayed concrete method, the new Austrian tunnelling method (NATM) and other names – have a place in the future of tunnelling works in Gulf cities.

Conventional tunnelling methods

“Mining methods can be used in competent soils [such as clay] and rock, which can be excavated by hand, mechanical breakers or back-hoes,” says Groves. Typically, the excavated face is sprayed with fast-hardening concrete, known as ‘shotcrete’, and is then supported by lattice girders or other elements before the permanent lining is put in place.

According to Groves, management of ground movements and groundwater are the main risks in NATM. But the method can be put to many uses. “In many respects, this is the most flexible tunnelling tool, being able to create tunnels of many shapes and sizes,” he says. “Yet it is difficult engineering work and prone to great risk if attempted by the inexperienced or if poorly planned. There are few cases of its use in the Gulf, such as the Dubai Metro [entrances].”

Some techniques have not yet found their way to the Middle East. Immersed tube tunnels, for example, have not featured in the region. But experts say that the shallow water and soft ground found in locations, such as Qatar and Abu Dhabi, make an ideal bed for this technique to make underwater road and rail crossings.

“The tunnel is formed by pre-casting lengths of the tunnel cross-section in a dock or casting basin; these tunnel elements are typically 100-200 metres long,” says Jonathan Baber, project director in the metro and civil division at UK consultant Mott MacDonald. “You then seal off the ends with bulkheads to create a large, hollow box. The dock is flooded and the section floated out to the site. It is lowered into a pre-dug trench using an immersion rig.” Once the required length of tunnel is in place, the internal bulkheads can be removed ready for internal fit-out.

As the Middle East continues to pursue work underground, new techniques will inevitably start to be used and existing techniques further refined to suit the region’s geology. Meanwhile, local expertise is growing as the number of such projects increases and clients are offered more choices and gain more confidence in the supply chain.