El Azhar road tunnels
Construction of two 2.7km tunnels under old Fatimid city and Khan El Khalily working class district.
This was a quite exceptional job on several counts: - The preliminary feasibility study for this ambitious, complex project in the very heart of Cairo began in March 1998 with construction of the diaphragm walls scheduled to commence in August... of the same year!
- The whole job lies in an urban fabric which is severely congested both at street level (Khan El Khalily bazaar and old buildings in narrow crowded streets) and under ground (buried services, often inadequately mapped, even though passing near the tunnel and affecting the final route, horizontally and in depth).
- Despite this unsuitable site for a major construction project requiring heavy constructional plant, it was vital to meet the tight, 16 month, completion deadline, which included the time needed for the site survey and project planning studies.
The works included 1km of cut-and-cover tunnel with diaphragm walls 0.6m to 1.2m thick, TBM starting and arrival shafts, also with 1.2m diaphragm walls (one of which had to be sunk under a fly-over carrying traffic), and two ventilation shafts 30m and 37m deep (one of which needed a 1.5m wide Hydrofraise trench right against dilapidated buildings in a state of imminent collapse).
The special works needed for the starting and arrival shafts deep below the water table called for a variety of techniques combining bentonite cement and silica gel grouting, jet grouting and plastic concrete pavings.
Soil treatment was continued downwards below the bottom of the ventilation shaft diaphragm walls by jet and standard grouting or with a special tool capable of reaching the required depth directly, where there were sufficiently impervious geological horizons to enable the shafts to be dewatered under a groundwater head of 35m. The cut-and-cover trenches had watertight grouted bottoms.
A noteworthy feature was the jet-grouted arch where No. 2 underground railway line, built a few years earlier by the same consortium, passed over the road tunnel. The fly-over foundations and sewers near the diaphragm walls and tunnels were protected by appropriate grouting methods combining jet grouting or Microsol grouting, to suit project requirements and local geology.
The Port Said ventilation shaft chalked up two world records, for:
- Deepest (87m) retaining wall in an urban setting, 1.5m thick, built in separate panels.
- Greatest density of ground-level plant, i.e. one KS 3000, one Hydrofraise 4000 with 40m capability, one Hydrofraise 8000 with 90m capability, two general purpose cranes, two 450 m3 desanding units, and one Puntel drilling rig, all working in perfect harmony on an area of only 2000 m² closely surrounded by residential buildings, a mosque and working fly-over.
A diaphragm wall is a reinforced concrete wall that is made in situ. The trench is prevented from collapsing during excavation, reinforcing and casting by the use of supporting bentonite slurry. The slurry forms a thick deposit (the cake) on the walls of the trench which balances the inward hydraulic forces and prevents water flow into the trench. A slurry made of polymers can also be used.
Grouting involves the injection of a pumpable product (slurry), which will subsequently stiffen, into the soil or into man-made material (masonry), in order to consolidate the soil or structure or make it impermeable, through filling all the voids it contains. The slurry can fill the voids in the ground, the cracks within rock, solution cavities (it is then referred to as fissure and permeation grouting) and/or displace the surrounding soils through a bottom-up process or by fracturing (compaction grouting or solid injection - see the section on the subject - and strain injection). Grouting with soil displacement may be used to prevent potential damage to the structure brought about by excavations (galleries and tunnels, major urban excavations, etc.) and this is called compensation grouting (see the relevant section).
Jet grouting is a construction process that uses a high-pressure jet of fluid (generally 20 – 40 MPa) to break up and loosen the soil at depth in a borehole and to mix it with a self-hardening grout to form columns, panels and other structures in the ground. The parameters for the jet-grouting process and the desired final strength of the treated soil depend on a number of characteristics, such as the soil type, the technique used and the objective to be reached. In granular soils, the high-pressure jet breaks up the grains through erosion, while in a cohesive soil, such as clay, the jet breaks the mass up into small particles. High pressure is needed to produce the kinetic energy required for the jet through a small-diameter nozzle. Waste material from the process (a mix of soil, water and binder) is recovered at the surface before being taken away for disposal.
Soletanche Bachy has developed a number of techniques for use in road structures; cut and cover tunnels, viaduct etc.
In the tunnelling field, Soletanche Bachy’s subsidiaries Soletanche Bachy Tunnels and CSM Bessac specialise in management of tunnel sites and also, for the latter, the design and construction of tunnel boring machines.
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