The Middle East is probably the most active market in the world for microtunnelling at present, according to Chris White, managing consultant at Bahrain-based Target Trenchless.

Last month, the firm completed work on the country’s 100,000 cubic-metre-a-day Muharraq sewage treatment plant, placing 5.2 kilometres of gravity sewer pipes using microtunnelling. 

Microtunnelling tends to be used on smaller diameter tunnels than the kind of mechanised tunnelling with tunnel boring machines (TBMs) used for roads, bridges and other major infrastructure schemes.

Trenchless methods

“It was in a very built-up area between houses, commercial areas, buildings and offices,” says White of the connection pipes, explaining that this meant contractors could not simply open trenches and drop in the lines. Trenchless methods had to be used. “We had four machines working 24 hours a day,” he says.

Although there are many similarities between microtunnelling and using TBMs, there are some important differences too. “The main difference between microtunnelling and using a TBM is that with microtunnelling, you are pushing a pipeline into place behind the microtunnelling machine, but with a TBM, you are building the lining in place behind it,” says John Wheeler, general manager at Al-Naboodah Specialist Services, part of the UAE’s Al-Naboodah Construction Group. 

Other characteristics of microtunnelling are that the face of the excavation is supported during the dig and the machines are usually remotely operated. As the pipes are jacked into place behind the head, the method is also sometimes referred to as pipe jacking.

Microtunnelling is highly accurate, with laser guidance systems enabling the cutterhead to reamin on track

 Wheeler says there are two microtunnelling concepts used in the region: slurry and earth pressure balance (EPB). The majority tend to be slurry-based machines, where water is pumped to the cutter head of the microtunnelling machine. This mixes with the excavated material and is then pumped back through the formed tunnel to a separation plant at the surface. “Slurry machines can cut through all types of ground, but may need a different type of cutting head,” says Wheeler. “You have three main types of head: a soft ground head, a mixed ground head and a hard rock head.”

These heads each host different cutting tools designed to deal with a variety of ground conditions. For example, disc cutters are used in hard rock, while smaller carbide teeth are used to break up the face in soft ground.

Meanwhile, EPB machines employ a range of cutting tools depending on the ground, but they can also be used in slurry mode when operating in saturated ground conditions. EPB machines exert pressure on the face of the excavation equal to the face pressure being exerted on the machine. This pressure prevents any inflow of material into the excavation and minimises the risk of settlement affecting the ground above.

Ground conditions in the region mean both machines are used, but, as Wheeler points out, slurry machines are most common, often using mixed ground cutterheads that can deal with soft ground, such as clay, and cope with cobbles, boulders and soft rock.

There is more soft ground in Dubai and Abu Dhabi, say experts, with rock prevalent in Saudi Arabia, Qatar, Bahrain and the northern emirates of the UAE. “When you look from the outside, people assume there is a lot of sand, but about 70 per cent of our work is in rock,” says White. “You get a lot of rock as you go deeper.” Target Trenchless has been undertaking microtunnelling for about 7 years. Prior to this, the firm had been focused on another trenchless method called horizontal directional drilling (HDD), which it continues to offer. Here, a rig is placed on the surface that pushes a pilot drill bit down into the ground beneath. This drill string is pushed along the line of the planned crossing to create a pilot bore. A reamer is then attached to the opposite end to the rig and pulled back through the ground, which widens the bore path and drags a pipe or casing behind it. 

“HDD has great capacity for long, large drives – we have done as much as 2,000 metres as a single tunnel in Saudi Arabia – but the accuracy is not suited for gravity work,” says White. Microtunnelling, meanwhile, is highly accurate, with laser guidance systems and live monitoring enabling the cutterhead to remain on the path of the designed bore. This is particularly crucial for gravity sewers, where the gradient of the pipe is vital to ensure the hydraulic profile of the wastewater system.

Saturated ground

Another advantage of microtunnelling is that it can be carried out in saturated ground where the water table is high, as is the case in many of the Gulf’s coastal cities. “Shafts have to remain dry but, once in the ground we can deal with the given water table within the ground formation,” says Wheeler. “At extreme depths, you may need a special machine that can withstand the imposed pressures. We are installing at 40-50-metre depths at the moment.”

However, microtunnelling at this depth is unusual, with more of the work in the 4-15-metre depth range and dominated by sewage, drainage and water network projects. In terms of the machines themselves, the market is currently dominated by Germany’s Herrenknecht, with other manufacturers such as Japan’s Iseki, Australia’s Terratec and Germany’s MTS also widely used. “All of our microtunnelling equipment is Herrenknecht,” says Wheeler, explaining that his firm buys most of its equipment new. “Some companies will hire; some buy for a contract and then sell them on,” he adds.

Someone who knows all about the second-hand market for equipment is Jim McNally, who started his company, TBM Trading Middle East, in late 2011. A former managing director at Herrenkecht, McNally now buys and sells microtunnelling machines.

“Demand has gone through the roof,” he says. “If you have to do more than 4-5km then you buy a new machine, but for up to 2km, a fully refurbished machine with a six-month warranty is better.”

Jobs are priced on the cost per linear metre and have to include the cost of the machines, labour, diesel and equipment. McNally says $1,100-$1,300 a metre is typical, but overheads and profits add another 25 per cent to the price. Ground conditions make a huge difference. “Cutters for sand are e100 ($125) each, but for rock they are e2,200,” he says. “So if you have to change the cutters every 50-100 metres, you have to calculate that in the price.”

The pipes pushed into place behind the machines are mainly concrete, fitted with a liner to protect from corrosion and infiltration. But in recent years, there has been some growth in the use of alternative products including clay jacking pipes and glassfibre reinforced plastic (GRP) jacking pipes. Hedley Industrial Group in Abu Dhabi manufactures both reinforced concrete and GRP pipes, and reports high demand for its composite products, which are concrete lined with GRP.

In composite pipes, the concrete itself is regarded as sacrificial, with the aggressive ground conditions expected to corrode it over time. So some clients are opting to use standalone GRP pipes despite their high cost.

“The pipe is more expensive but the savings are in the reduction of risk, better quality for the authority and the reduction in total cost of construction,” says Leon Bauer, Middle East sales manager for Austrian pipe manufacturer Hobas. Pipes with the same internal diameter as concrete have thinner walls, making them much lighter, and the smoother surface reduces friction. As a result, the force needed to push them in is lower and smaller microtunnelling machines can be used.

Plenty of work

Firms will need all the installation advantages available to them. “Schemes are becoming more complicated, with bigger pipes on longer drives,” says White. At the same time, the market is awash with new projects such as Qatar’s $400m package 1 of the Inner Doha Resewerage Implementation Strategy, which is set to be awarded imminently. Pipes will vary between 400-2,400mm, with more than 70km of lines in total being installed at depths of 10-36 metres.

Not surprisingly, the microtunnelling sector is positive about future prospects. “We have work booked in for the next five years,” says White.