Two CSM Bessac TBMs at Work in AlgiersIn November 2001, the poor districts of Bab El Oued, Frais Vallon and Beau Fraisier on the western side of Algiers were struck by devastating floods and mudslides. For 36 hours torrential rain washed fragile shacks from hillsides and flooded low-lying tenement neighbourhoods. At least 712 people were killed, 350 were injured and 116 are missing. More than 350 vehicles with passengers were buried under the mud. The M'kacel drainage tunnel was severely damaged. Waters surged in this sewer and dug a hole on the side of it. This was followed by a collapse of the ground. Then waters surged through the hole, which created a siphon similar to a whirlpool movement as when one empties the washbasin. A 10 m-diameter, completely circular crater was formed. At the bottom of the crater, the M'kacel tunnel was completely silted up.Dualling of the Oued M'kacel storm water drainage tunnelThe dualling of the Oued M'kacel storm water tunnel in Algiers consists of a 4,760 m tunnel, mainly bored with a TBM (see red one the map), on a route starting at Baranès roundabout up to Lamine Debaghine University Hospital in Bab El Oued. The last 230 metres of the sewer (blue one the map), which will transport rainwater to the sea near Ferhani Stadium, will be built in open cut due to the difference of height. Connecting structures are envisaged to link the existing sewer system and the subwatersheds of Oued Baranès, Oued Sidi Medjbar, Oued Frais Vallon, Oued Scotto Nadal and Oued Jaubert to the sewer under construction. The sewer will transport at sea more than 50 cubic metres of water/second and protect the area from a disaster. Construction cost is around DZD5.4 billion. It is a design-build project. The design studies led to the definition of a deep route, between 30 and 55 metres on average, with several drop shafts (stairway-like longitudinal profile) to limit the gradient of the three sections. The average gradient of the natural ground is 5%, with difference of height of 230 metres between the upstream end of the project and the sea. The owner is the Directorate for hydraulic resources and water economy of the Wilaya of Algiers, under the Ministry of water resources. The sewer and associated structures are constructed by a JV of CSM Bessac and its Algerian partner Hydro-Technique. Work supervision has been commissioned to Cabinet Merlin, France and CTH, Algeria. Visit www.csmbessac.com, www.hydrotechnique-dz.com and www.cabinet-merlin.com The TBM-driven tunnel stretches 4,530 metres. The excavated cross-section is 19 sq m for an inner diameter of 4,000 mm. The TBM dubbed 'Yasmine' arrived in Algiers and was lowered into the shaft on 28th March, 2007. It is a mixed earth and air pressure balance machine, built by CSM Bessac who not only designs and builds TBMs but is also civil engineering contractor. The machinery weighs 180 tonnes and is followed by 50 m trailing support mechanisms and a muck train. To date, 25 metres have been bored and assembly of the back-up is still under way.The geology comprises of rocks (schists and micaschists, crystalline limestone, gneiss), marls and backfill. The machine installs 4,000-4,600 mm universal precast reinforced concrete segments as it progresses. The segments are produced by the joint venture in a plant in the Algiers suburbs. Trelleborg supplied the sealing gaskets. A diesel loco pulls four 9 cu m muck wagons supplied by CSM Bessac to evacuate the material. Visit www.trelleborg.comThe project includes around fifteen working and/or connecting shafts to the existing sewers and oueds as well as a 230 m-long rectangular sewer in the downstream part built in cut and cover and a discharge structure at sea. The working and connecting shafts have variable depths and sizes, with diameters ranging from 10 to 15 metres and depths from 30 to 55 metres. The connecting shafts will be complex civil engineering structures (vortex shafts, energy-dissipating pools, etc.). The cut-and-cover sewer is shallow (average depth of four metres) and features a 2 x 4.75 m cross-section.The circular shafts are built in rock above the water table using traditional methods (steel arches and shotcrete) while the cut-and-cover sewer is built using temporary retaining walls, diaphragm walls and sheet piling. View pictures here. Pointe Pescade sewerTo protect the environment and improve the cleanup of the Algiers bay where most of the waste water is spilled, Algiers also launched an important sanitation programme by constructing a coastal sewer. The Pointe Pescade sewer is one of the key element within the general sanitation scheme of metropolitan Algiers. This 12 km sewer follows the coast and is intended to intercept all secondary systems that discharge at sea and transport waste water to a treatment station east of the city.The project is divided in four phases. The first phase is completed, it is a small diameter open cut surface sewer. The second and third stages have not been awarded yet (see green one the map). The fourth stage, between Algiers city centre and the harbour in the downhill part of the project, mainly consists of a 3,500 m-long 2,460 mm-internal diameter tunnel. The tunnel is lined with 17 cm-thick reinforced concrete segments. The tunnel route is perpendicular to the route of the M'kacel sewer and runs under highly trafficked urban arteries. The tunnel comprises two sections bored from an intermediate shaft built in diaphragm walls. The first 1,930 m section was completed on 27th March, 2007. It was driven through marls. The second, 930 m-long segment starts in marls, then crosses alluviums and finishes in backfill. The tunnel is built at some 10 metres below sea level in immersed ground. To date, 15 rings have been installed and assembly of the back-up train is also under way.CSM Bessac and Hydro-Technique are suppliers of an air pressure balance roadheader TBM, trailing gear, a muck train, the equipment to produce the compressed air to stabilise the face and the high pressure foam, as well as the segment moulds and bolts. The moulds are supplied by CBE. The segment production line is different from that of the M'kacel drainage tunnel but is housed in the same plant. Visit www.cbe-tunnels.com.The advantage of the TBM supplied by CSM Bessac is that the operator sees completely the face through a window and the excavation chamber can be accessed under confinement. This has allowed to demolish manually 1.2 m-diameter concrete piles while such obstacles could have been impassable for a cutterhead TBM. These piles belong to an access road to an expressway before the project was modified. 20/07.