Cairo's third metro line beats challenges

29 April 2012

Despite political unrest and technical challenges, contractors have minimised delays to Line Three and are now working on underground sections for the second phase

Residents have quickly made Cairo’s third metro line part of their daily commute, since it began operating in February. The line was inaugurated less than six months behind the original planned opening date and in time for a revised target, after facing significant tunnelling problems in 2009, as well as the political upheaval that began in 2011.

Technical challenges for contractors include waterlogged ground and cramped conditions for above-ground work

The popular uprising in Egypt have had a limited impact on the works, which continued almost uninterupted throughout the occupation of Tahrir Square by anti-government protesters and other unrest within Cairo and beyond. The French lead contractor on the metro project, Vinci Construction Grands Projets, confirms that work stopped for just a few weeks in February 2011 before restarting with additional site security in early March.

Short section of underground line

The political instability is expected to have more of an effect on the financing of ongoing work and, particularly, the later phases of the line. These are currently being redesigned to save costs by using more above-ground running sections. 

The short section of the underground Line Three cost $712m and has only five stations so far, including Attaba, which is close to the Nile. Not only does this 31.5-metre-deep underground station beneath the El-Azbakeya Garden lie in the heart of the central business district, it also connects the new section to the existing Line Two, which opened in the mid-1990s and was completed in 2001.

The two original lines run north-south, more or less parallel with the Nile, and already make up the 15th busiest metro system in the world, with more than 700 million passengers a year. Cairo is one of the fastest-growing cities and pressure on its public transport system is set to increase as the influx of people from the countryside expands the 11-million-strong urban population.

The new section of metro will not remain as it is, with a short spur ending at the Abbasia station. Line Three will eventually be about 45km long. The 7.1km phase 2 is currently being constructed by the same Vinci Construction Grands Projets-led consortium of four firms including France’s Bouygues and the local Arab Contractors and Orascom Construction Industries.

Underground tunnels will be added to the line from Abbasia, turning east towards the airport with five more stations, and ending at Al-Ahram Street in Heliopolis. Another extension will take the line to the airport, although this will not happen until the final phase.

Meanwhile, the third phase of construction will take the line west by another 4km underneath the Nile, linking to the large Zamalek island district above. It will then pass onwards underneath a smaller channel of the river to El-Kit Kat. Here the line will split into two branches, one heading southwards and one north into suburban and residential areas.

Travelling a further 6.6km, the northern branch, with seven stations, will terminate at the Rod el-Farag corridor, where there will be a an interchange connecting to other modes of transport, and a heavy maintenance workshop for the line. The southern link will be 7.1km long and will have six stations, the last one at Cairo University where the line finishes. Both these branches will be tunnelled only for about 50 per cent of their length.

According to Atta Abd Raboh el-Sherbiny, chairman of the National Authority for Tunnels (NAT), which is part of the Ministry of Transportation, the lines will have both at-grade and viaduct sections. After the underground El-Sudan station, the northern branch will rise on to a viaduct and run within the existing rail corridor on its way to Imbaba station, says NAT planning director Tarek Abouel Wafa. This sharing of Egyptian National Railways (ENR) land will avoid the problem of finding additional space in the crowded city. The line will stay on a viaduct until it has crossed the city ring road using an elevated, suspended bridge. After this, it will drop to grade level.

The southern branch will remain underground until Sudan Street, when it will cross below the Upper Egypt Railway before rising to run parallel along the El-Zomer Canal until the Cairo University station. This is a change from the original concept, which called for fully underground lines.

Phase 3 Cairo’s third metro line

France’s Systra is currently working on the designs for phase 3. These are being prepared in time for tendering construction contracts later this year. Mahmoud Dow, design project manager for Systra in Cairo, says this could happen in early autumn, but is more likely to be later. “In parallel, we are working on feasibility and outline designs for the phase 4 section,” he says. “This may be extended to 14km from the current 11km, with an additional two stations on top of the eight already in this phase.” Work began on the revised phase 4 in January.

The technical challenges faced by contractors working on Line Three included waterlogged ground, particularly in phase 1, and cramped conditions for above-ground work in the tight city streets. All phases of the project use a single-tunnel configuration for underground sections, as did Line Two. An 8.35-metre internal diameter tunnel carries a double-track line, rather than two bores running in each direction. In most places, the line lies 20-30 metres below ground, which means tunnelling under high pressure.

Stations are also deep, excavated within diaphragm walls forming watertight boxes. Not surprisingly for a route alongside the Nile, the ground is saturated. Many years of inundations have left deep deposits of sand and gravel and a water table close to the surface.

Slurry machines

These conditions necessitate a tunnel boring machine (TBM) with a pressurised face, known as a slurry machine. It works by using clay-based bentonite slurry to keep the excavation face under pressure in the head chamber, with the same slurry then extracting the spoil as it is pumped round in a circuit.

The 9.4-metre diameter machine used for the first part of the works was the same one that drove the original Line Two, from German manufacturer Herrenknecht. It reinforces the tunnel with 400-millimetre-thick, precast rings behind the shield as it pushes forwards. Each tunnel ring is made of seven segments.

The TBM drive went well for most of the route and the challenges mainly concerned the above-ground operations. Finding space for the spoil plant, where the piped slurry from the machine is cleaned to remove excavated soil before being pumped back down to the machine, was difficult. Further restrictions meant that a full day’s spoil had to be stored at the plant as trucks could only move at night to dispose of it in the desert beyond the city.

In September 2009, there was a failure in the tunnel lining due to a misplaced segment. The pressurised groundwater broke into the tunnel, carrying the ground above with it. A sinkhole appeared at street level, but fortunately the route of the tunnel followed the centre of the streets, which minimised damage to buildings.

To stabilise the tunnel and the ground above it, the machine and a section of the tunnel had to be flooded. Using diaphragm walls and ground freezing, the machine was dismantled and removed. After being reconditioned, the rescued machine began work on the second phase of Line Three in mid-March.

To complete the remaining 1,200 metres of tunnel on phase 1, a new Herrenknecht TBM was brought in, which would later be re-used on phase 2 works. If this machine had not arrived early, the delay would have added 11 months to a tight 48-month schedule.

The phase 2 works, which began in early 2011, have used slurry machines and an earth pressure balance (EPB) TBM, manufactured by France’s NFM. The second slurry TBM used to complete the phase 1 tunnelling has been used since early 2011 on the first 2km of phase 2, where the ground is made up of the same saturated gravels and sands as it was on phase 1. Away from the river, the ground changes to drier clay sand and sandstones, though water is still present. For this type of ground, the EPB machine is more appropriate.

Wafa says phase 2 is due to be completed in January 2014. By then, phase 3 should have commenced too. Meanwhile, he says, the study of Line Four is beginning and feasibility studies will begin for Lines Five and Six in 2014.

Recovering the lost machine

To retrieve the tunnel boring machine (TBM) flooded in September 2009, engineers constructed diaphragm walls by digging trenches 90 metres deep to reach a layer of impervious clay.

The trenches were then filled with concrete to create a watertight box around the machine head that could be pumped dry and the ground safely dug.

Behind the TBM, the ground was frozen by drilling and installing pipes to circulate a saline solution. This could be cooled below zero Celsius to chill the surrounding ground and carry away heat.

The groundwater surrounding the pipes therefore froze and further sealed the location of the machine so it would not flood further as it was being dismantled and removed.

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