On 23 May 2011 the construction contract for the biggest microtunnelling project in Morocco, a new connecting sewer in Casablanca, was awarded to the joint venture of Denys and Capep. The sewer will link a future water treatment plant, Sidi Bernoussi, to the sewer network. The plant, to be built in east Casablanca, will treat sewage that is currently just flowing into the Atlantic Ocean.
The project called for the construction of a tunnel with internal diameter 2.5m by microtunnelling over a total length of 2,570m. It runs parallel to the ocean, to the east of Casablanca’s harbour arm.
The underground option was chosen by water firm Lyonnaise Des Eaux de Casablanca (Lydec) and consultant Safege- C3E due to the highly water-permeable sandstone, the depth of the sewer under the groundwater table and the need to minimise the perturbation of the important economic activity on the surface.
Shafts and connectors
Next to the main sewer, two big connections have to be constructed as an overflow to the Atlantic Ocean. Also two launch shafts, two reception shafts and four intermediate shafts at 10 to 18m deep.
To realise the shafts, a lot of utilities had to be located and removed in this densely occupied urban area, thick with important harbour-related activities. For the first five to 8m the shafts were sunk by a system of vertical beams drilled in the ground, and horizontal sheets because of the presence of sand, clay and stones.
Afterwards excavation continued through the fractured and permeable siltstone. Iron nets were fixed against the side walls to prevent stones falling down, and a plastic membrane led the infiltrated water to the bottom of the shaft where it flows into a central pumping pit.
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By GlobalDataRock-anchors were necessary in a lot of places to consolidate the rock.
The first shaft was ready approximately five months after the starting-date of the project. At the beginning of December the first of the tunnelling equipment arrived on the jobsite after a long and difficult journey and the associated customs-related problems that had to be resolved.
Excavation and progress
Denys commissioned a microtunnelling machine from Herrenknecht. It is an AVN2500 equipped with a rock cutting head, a slurry-evacuation system and an integrated air-lock-system to enable cutting tool changes under compressed air.
Before the tender, probes were taken after vertical drilling to perform UCS, Lugeon and rock-quality designation (RQD) tests. These found 30 to 80MPa UCI rock. The geology that had to be crossed has been rather homogeneous, very permeable and fractured siltstone and sandstone. Compressive strengths range from 30 to 120MPa UCI. Water infiltration was manageable without injections.
The machine was lowered into the shaft in the middle of January, starting the first drive shortly after.
The total drive length of 2,570m is split into three separate drives with a length of 930m, 810m and 810m. The first drive of 930m, realised in a two-shift pattern, broke through on 16 May.
The drives are designed with a low vertical slope of approximately 1mm/m and with different horizontal curves with radii of 900 to 1,100m. The curves were to enable the location of launch and reception shafts at places where the surface impact would be minimal.
A different cutter-disc changing schedule had to be programmed during the first drive due to the abrasivity of the sandstone. At each intervention the full set of 18 of the 300mm-cutting-discs had to be replaced by new ones.
Guidance
Denys uses one of its SLS-RV-guidance-systems, bought from VMT. During the first drive, deviations after each control measurement in horizontal and vertical planes were minimal.
We are scheduled to finish both remaining drives by the end of 2012.
