The network of Algiers Metro was extended by an additional 4km between July 2008 and January 2012. Construction work mainly included a 2.7km long tunnel, four new subway stations, an approx. 300m long bridge across the motorway and a river undercrossing.
The 2.7km long tunnel runs through clayey sands and sandy clays, marl, interstratified sandstone and unconsolidated rock. Due to the widely varying geological conditions and the relatively short tunnel sections, the tunnel for the Metro was built using the New Austrian Tunneling Method (NATM). As the excavation work was carried out using excavators, the team was able to react quickly and flexibly to changing ground and stability conditions by adapting support systems in accordance with the concept of NATM. The tunnel was stabilized using steel mesh and shotcrete immediately after excavation. This process lead to a high loading capacity in the ground with only small relaxation processes.
The dimensions of this project were impressive. With a full cross section of 88m², more than 1,200t of steel and approx. 17,000m³ of shotcrete were used during tunnel construction. For stabilizing tunnel advancement, DSI Austria supplied R32-280 DSI Anchors including the complete range of accessories such as drill bits, couplers, nuts and anchor plates. Especially in the sandy clays, the self-drilling DSI Anchor proved to be the best solution due to the fact that the injection anchor can be drilled into non-stable boreholes without a casing.
The length of the anchor rods consisting of special hollow bars with cold rolled outside threads could be modified as needed using couplers. Thanks to matching types of drill bits, the DSI Self-drilling Anchors were adapted to the different rock and soil conditions quickly and flexibly during tunnel advancement.
The easy operation principle, which remains the same even in different rock and soil conditions, and the time-saving installation procedure due to simultaneous drilling, installation and grouting also proved to be significant advantages during this project.
In addition, DSI supplied the AT-Pipe Umbrella System with an exterior pipe diameter of 88.9mm and a wall thickness of 6.3mm as a pre-support element in the partly soft and subsidence prone ground conditions. The use of the AT-Pipe Umbrella System offers the following principal advantages:
- reduction of subsidence due to ground improvement and load distribution
- increase of stability and safety by ground improvement ahead of the tunnel face
- ground support in the open span in the face region
The quick and easy installation of the self-drilling pipe umbrellas and the accurate alignment of the pipe umbrellas during drilling, which was carried out using a conventional drilling rig available on site, repeatedly proved of value during the use of the AT - Pipe Umbrella System in Algiers Metro.