Cairo is now one of the world’s largest cities, with an estimated population of 18 million and over one million vehicles on the streets. One of the city’s most urgent tasks is to address growth-related environmental degradation. In this respect, the development of underground infrastructure is seen as a new frontier.
Egypt’s National Authority for Tunnels, a state entity that has acted as owner of the Cairo Metro since the early 1980s, expressed an interest in building an urban road tunnel below the city centre. The tunnel would have the multiple purpose of decongesting vehicular traffic in the city centre, preserving Islamic Cairo from further urban degradation, providing faster cross-city access for cars and buses (lorry access being excluded) and reducing traffic pollution and noise locally.
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By capitalising on experience from the construction of lines 1 and 2 of the Metro, Vinci Construction Grands Projets (then Campenon Bernard SGE) put together a Joint Venture (JV) of contractors (Arab Contractors OAO & Co, Bouygues, Eiffage, Intetectra BTP, Soletanche Bachy France and Spie Batignolles BTP) to design and construct a twin-bore tunnel running from Opera Square to Salem Saleh Street (the main highway leading to the airport). A contract with the National Authority for Tunnels was let in mid-1998.
Developing a feasible design
The conceptual brief for the tunnel proposed an alignment below the most prominent road arteries on the surface, in order to reduce surface settlements from ground loss. One bore followed El Mosky Street and the other was under El Azhar Street.
Owing to the lack of appropriate data in the project area, traffic studies and topographical and building surveys had to be conducted along the proposed route in order to establish a workable design baseline. Many options had to be examined, evaluated and returned to the drawing board for revision.
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By GlobalDataAs data impacting the project came to light, it quickly became apparent that the flyovers that were to be demolished once the tunnel was in service were founded on piles. This meant the tunnel alignment had to be re-examined. Similarly, the subsequent discovery of the depth of piled foundations for the recently built El Azhar university and hospital, and recently built sewers, drove the central section of the tunnel alignment away from the conceptual routing below the main roads.
Although the extremities of the tunnel route could be maintained as designed, within the central area the tubes were pushed closer together and away from the road, below buildings in the Bazaar area (figure 1). The central ventilation stations thus became units serving both of the tunnels rather than each one individually.
Saleh Salem Street and Attaba Square are 50m and 19m above sea level, respectively. The tunnels were now passing below more buildings and fragile structures had to be protected with soil treatment, such as jet grouting and fine cement permeation grouting.
Fitting the road tunnel into the city sub-spaces was also conditional upon satisfactorily passing a 17m deep, 5m diameter main collector sewer, which was near Port Said Street ventilation station. Putting the sewer out of service (whether for protective bracing, diversion or post-passage repairs) was not an option. Consequently, the tunnel was designed to pass 4m below the sewer in order to limit tunnel-induced settlement to 10mm. However, this caused the profile of the tunnel to be lowered, increasing the tunnel gradient and the depth of the Port Said ventilation station to 37m.
Diaphragm walls
The ventilation stations were deeper than the Metro stations, particularly so at the Port Said ventilation station. Further investigation during mobilisation phase showed that deep layers of clay prevailed, which could form a naturally impermeable horizon. A different optimised design and construction method was quickly chosen for the ventilation stations. This involved constructing diaphragm walls through to the watertight clay layers. To do this, 1.5m thick, 87m deep diaphragm walls were sunk at Port Said, which was believed to be a new record in an urban environment. Important challenges to deep foundation construction were: avoiding structural damage to, and settlements of, surrounding buildings; avoiding damage to public utilities; working with limited headroom around and below the flyovers at Attaba; and working with the three diaphragm wall rigs, two cranes for handling, two 450m³ de-sanding plants and a Puntel drilling rig in a confined 2,000m² site at Port Said.
TBMs define traffic gauge
The fast-track project was based on using one of the 9.35m diameter Herrenknecht Hydroshield TBMs used for construction of the Cairo Metro. Although a wealth of knowledge of sub-soil conditions had become available during excavation of the Metro, around 30 boreholes were sunk along the alignment, with several serving as piezometers. These showed, at the tunnel alignment, Quaternary deposits of fluviatile origin deposited in a sinuous river.
The TBM, using break-out/break-in ground treatment at the entrance and exit of each station, had proved its worth on the Metro and the diaphragm walling was also a tried and tested success. The only real problem encountered during the drives was a localised pocket of flint that had been grouted in the break-out leading into Port Said ventilation station. This caused some delay and unexpected wear on cutting tools during the drive.
Keeping the air fresh
The ventilation design involved careful evaluation of air quality, air pollution and fire-life safety seeking to: prevent the dangerous accumulation of vehicle-emitted pollutants; help maintain visibility within the tunnel by preventing the accumulation of those pollutants; control the movement of smoke and hot gases during a tunnel fire; and be maintenance-friendly.
The recommendations of the Permanent International Association of Road Congresses (PIARC) provided guidelines for system design. Data gathered for Egypt’s environmental legislation, together with the traffic authority’s surveys and traffic counts, were reviewed to determine the fresh air needs in the road tunnels. After studying traffic densities, the tunnel profile, gauge and clearance, and the desire to minimise facilities at the surface, a longitudinal ventilation system with intermediary Saccardo nozzles and jet fans at the tunnel extremities was decided upon. The system operates on the principle that a high-velocity air-jet, injected into the tunnel, can induce high volume air flow, as well as renewing a portion of the air in the tunnel.
Fans in fire
Several fire scenarios have been considered at various points of the tunnel leading to different ventilation settings. In addition, an innovative strategy has been developed consisting of the following two phases. The first seeks to reduce air velocity to approximately 1m/s in order to reduce the spread of smoke and thus protect users during an evacuation. This phase is based on predefined settings and on an algorithm that modifies these settings according to real-time measurements.
The second phase aims to give emergency services a smoke-free route to the fire and protect the tunnel structure by achieving a higher air velocity (3m/s–4m/s) that reduces the maximum air temperature along the tunnel. The tunnel was equipped with a wet-riser fire-fighting system, served by a 200mm diameter supply with hydrants located at 100m centres.
However, fire protection design for the project was developed against the back-drop of a series of road tunnel fires in the Alps and led to a re-evaluation of safeguards for users, the tunnel itself and its equipment. The accidents sadly confirmed that prevalent safety concepts were no longer compatible with increased traffic flows. It was clear that bi-directional road tunnels presented a risk to users and that 400m spaced emergency exits could not guarantee safety.
Although heat is the biggest threat to the tunnel and its equipment, experience has proven that toxic gases and particles in undiluted smoke are more life threatening. Thus, people have a greater chance of escaping harm when the lower portions of the traffic space are free of smoke, and provide enough air and visibility to enable people to evacuate.
Avoiding spalling
The events in the Alps caused tunnel designers to refocus on standards for the El Azhar tunnel. What they came up with was a protective fire-coating for the tunnel and an ingenious emergency escape chute scheme. Although the tunnel does not allow the transit of lorries, the engineering team decided to protect the tunnel against the heat generated by a 100MW fire. In preserving the tunnel, it was necessary to safeguard against the ‘spalling effect’; when concrete is subjected to elevated temperatures, water contained inside expands as it evaporates and disintegrates the concrete in exiting the structure. This recurring process starts at approximately 250°C.
Considering that the water table is located just below the ground surface, spalling damage of the segments would have catastrophic consequences. Such a phenomenon could, within minutes, lead to the loss of the structure and cause huge surface settlement, which would, in turn, inevitably lead to building collapses and casualties. In this context, protection of the segments from heat was of prime importance and it was thus decided to protect the concrete surface from temperatures in excess of 200°C.
As the standards and requirements for protective coatings vary internationally, it was decided that testing would be performed in accordance with the most rigorous standards, namely those established by the Dutch Tunnel Authorities (RijksWaterStaat). The heat-testing curve defined by the RijksWaterStaat is currently the most stringent in Europe and requires test samples to be exposed to 1,350°C during a minimum period of two hours. It is note-worthy that the heat-testing curve is not directly related to the magnitude of the fire designed for (100MW in this case). Indeed, 100MW corresponds to an amount of burning material, whereas the resulting temperature is affected by ventilation, the volume of the tunnel and other external parameters.
The chosen fire-protective coating was selected on the following main considerations:
After comparing all products available on the market on the above criteria, Firebarrier 135, produced by Thermal Ceramics, was judged to be the optimum product.
It was decided not to rely solely on the lining bonding directly onto the segments, due to the smoothness of the latter. Plastic-coated wire mesh was fixed to the segments at the mid-thickness of the lining with stainless steel bolts. Bolt diameters were selected to be able to support cable trays and light equipment, such as cameras, in addition to the mesh. Connectors were homogeneously placed at 450mm centres and special traction tests were carried out to check the efficiency of the mesh and its connectors.
The nominal coating thickness was estimated at 47mm by theoretical calculations supported by electronic simulations, and then confirmed by fire tests carried out to RijksWaterStaat requirements in the SINTEF Fire Department in Norway. The nominal coating thickness was sprayed on to concrete plates, which were made of the same concrete mix, and were of the same thickness and reinforcement as in the El Azhar tunnels. The concrete plates were placed on top of a box-type oven and the required testing temperature was induced from below. The installation was equipped with several thermocouples to survey the temperature within the coating and at the concrete interface. The various test results all showed that the temperature at the interface between the concrete and the coating never even reached the 200°C limit. In addition, the wire mesh and connectors prevented coating collapse. These tests confirmed that the product, its calculated thickness, and the wire mesh with its fixing devices, were more than appropriate to protect the concrete lining from extremely high temperatures.
Escape chutes
The original El Azhar road tunnel design satisfied the most stringent European safety codes and provided emergency exits every 400m. These led into cross-passages, allowing users to escape from one bore to the other. After construction had already started, some European countries started to revise their tunnel safety regulations, so the exit spacing had to be redesigned to 200m centres. Cross-passage construction at a depth of 30m below the water table, in Cairo’s congested environment is a difficult operation, with the potential for significant construction risks. The contractor therefore proposed that the 3m wide, 2.07m high, technical gallery, located below the road slab, be transformed into an escape corridor (figure 2). Escaping users enter by short slides and then walk to the ventilation stations where there are stairs to the surface.
The slide scheme represented a considerable safety enhancement for the project, since emergency escape points from the tunnel were now set at 100m intervals. It did require, however, the gallery to be lightly pressurised to prevent smoke from entering, and well lit at all times. The chute entrances were equipped with warning lights at the carriageway level and lighting in the chutes themselves. Accordingly, extensive adaptations had to be carried out to the carriageway and gallery in the shortest of times in order to deliver the project on time.
Design and built in 40 months
The project was taken from an entry on the Government’s infrastructure planning list to operational reality in less than four years. This constitutes an impressive achievement under any circumstances. Design and construction performance may be summarised by the following principal milestones:
The El Azhar road tunnel is already bringing some relief to the chronic traffic congestion that has long been a characteristic of the city. The El Azhar road tunnel is now relieving the historic centre of more than 50,000 vehicle movements a day.
Related Files
Fig 2 – Cross section of the tunnel at one of the emergency exits, showing the escape chute down to the technical gallery beneath the road slab
Fig 1 – The final route design for the El Azhar tunnel avoided old flyover poles and resulted in fewer, larger ventilation stations
Fig 3 – Longitudinal profile was governed by the need to pass at least 4m below an existing main sewer
