Like many major cities, Algiers is investing to improve its wastewater and drainage network. The key, common strategy for the many projects underway internationally in coastal cities is to construct larger sewers that will intercept the myriad of smaller tunnels that often drain to the sea, and the Algierian capital is no different.

Within the next couple of months, contractor Denys is expected to commence pipejacking for a sewer project that will be the latest stage in construction of the large-scale wastewater interceptor being built in Algiers. Preparatory works, including final stages of shaft construction, are underway to enable contractor Denys to build this almost 3km long section of new sewer in the heart of the city, in the northwest section of the Bay of Algiers.

The microtunnelling project will jack concrete pipes below the built-up and industrialised coastline, passing below roads, railways and a number of buildings, though always above groundwater level. Denys will work from two launch and two reception shafts for the project. The excavation work along a north-south alignment is expected to take approximately one year to complete.

The almost EUR 17M (USD 22.7M) sewerage project is the third phase of a five-stage ‘Collecteur Pointe Pescade’ scheme being constructed to improve the wastewater collection and sewerage system performance. The entire scheme will be 25km long to drain flows to southwards from the west arm of the bay, and then southeast inland, to the existing treatment works in Bariki, beyond the most urbanised area.

Planning and procurement
In the west arm of the bay, the third stage of the sewerage project extends from coastal part of the Bab El Oued area, near the Rais Hamidou area, at its north end to pass the Casbah and then, at its southern end, terminates in the Tafourah area. With the built-up terrain sloping eastwards towards the bay, the alignment of the sewer runs closest to the coastline and below the main transport routes.

The client for the scheme is the department water authority for the city – Direction de l’Hydraulique et de l’economie de l’eau de la Wilaya d’Alger. The client undertook the preliminary design work for the third phase project over 2007-8. Bids were called in mid-2008 and contract award was provisionally made in December 2008 to Denys. The award of the contract was finalised on 1 September 2009, and marks the first contract for Denys in Africa.

Stage 3 is being undertaken as an EPC contract with a bill of quantities, and the contract formof the Algerian Conditions of Contract according to the law of public works.

Local subcontractor Teixco is responsible for site investigation and shaft construction, using secant piles, for the microtunnelling works to be carried out by Denys. Site investigation was carried out in the third quarter of 2009, in the closing months to the contract award and being accounted for as part of the contract.

The initial programme for the works suffered some setbacks due to delays getting the necessary authorisations and difficulties with re-routing utilities near the proposed shaft locations. The detailed design work is continuing as preparations are made to receive the tunnelling equipment, once finally cleared from customs at the port, and then commence the main underground works.

Project plan
The new collector will be formed of a 1800mm i.d. concrete pipeline that will pass through an area of the city that is geologically difficult, said Denys, as it has both gneiss and schists bedrock but also some areas that were, historically, backfilled with clay and sand, and reclaimed from the sea, during the French colonisation era. Parts of the alignment are in the backfill and other sections pass through bedrock.

Additional complications come from the alignment passing below important areas and buildings and, of course, from the works being undertaken in congested, busy and industrious parts of the city. Running north, the alignment of the third stage of the interceptor starts at a shaft nominated P1 for the microtunnelling, at Rue d’Angkor, at Tafourah, near a port entry and the administrative centre of the capital. The shaft is also the north end of the completed Stage 4 of the overall scheme.

The sewer is then routed near government buildings, then buildings from the French colonial period of around 1870, and then the terrain rises with the tunnel then running deeper to pass below one of the city’s oldest mosques (El Djeddid) and the Chamber of Commerce until an esplanade is reached. From there, the sewer enters its last leg, in the popular Bab El Oued district, to the existing pumping station on the Boulevard Emir Khaled. This northern part of the city was the most affected by the earthquake of 2003.

A tunnel inceptor was the only feasible solution investigated, and the main parameters of adjusting the layout related to the number of starting and arrival shafts to satisfy the needs of tunnel curves, and accounting for existing structures – buildings and roads – for the micro-tunnel drives.

Four drives totalling over 2.7km are to be executed by pipejacking to install the interceptor, which will have a total of 11 shafts, or manholes. In total, between Lot 4 at the southern end and the existing pumping station in the north the total length of the new section of sewer, however, is almost 3km. The microtunnelling works are to be executed with two starting, or launch, shafts and two arrival shafts.

The almost EUR 17M (USD 23.3M) project includes all tunnelling and construction works, including supply of the reinforced concrete pipes which will be manufactured by Denys’ sister company, Socea, at a rented industrial facility in Rouiba. Equipment is being transferred from Socea’s manufacturing facilities for the project.

The Denys group decided on this approach for its bid as there is limited experience in Algeria for production of concrete pipes suitable for pressures placed by pipejacking procedures. The plant is to manufacture four pipes per day.

Works on the project also include:
• Construction of two new circular shafts of 13m i.d. with secant piles for the TBM
launches and pipejacking. They are to be 9m and 17m deep, respectively.
• Construction of a new circular shaft of 6m i.d. and 8m depth, and modification of an
existing shaft from a previous stage (Stage 4) of the scheme, to receive the TBM after drives.
• Modification of the existing pumping station on Boulevard Emir Khaled, at thenorth end, to be the third of the three arrival shafts required for the works.
• Construction of visiting chambers at the two launch and the three arrival shafts.
• Construct connections to the live sewers.
• Connect possible, but as yet undetermined, sea outfall pipes.

Denys’ division manager for North Africa is Geert Heytens, the project manager on the contract is Kurt Mussche and Geert Goossens is technical project manager. The division manager for tunnelling is Dirk Derycke.

Tunnelling plan
The pipejacking is to be performed between five shafts constructed, or modified, for the microtunnelling works – shafts P1 – P5, which run from the southern end to the north in that order.

Shaft P1 will be a modification of the existing shaft that terminated Stage 4, where jet grout piling will be done. At the northern end of the sewer, shaft P5 will constructed by modifying the pumping station.

Near the middle of the sewer, shaft P3 will be a new construction of 6m i.d. and 8m deep. All three shafts will function as receptions for the TBM.

At present, as Stage 4 works have yet to be completed by state contractor Hydrotechnique and the modification work needed for shaft P1 will have to hold off. The client has yet to decide how shaft P5 is to be realised, and there is the possibility, notes, Denys, that an extra 500m of sewer may be built. Shaft P3 is almost complete.

The large, launch shafts for the TBM and the pipejacking work – P2 and P4 – will be constructed near the rail station and at the esplanade on Boulevard Amara Mohamed Rachid, respectively. Shaft P2 has been finished, and P4 should be completed in late February.

Denys has planned the excavation sequence to be focused in the southern half, initially, and to be:
1st Drive: P2 – P3; length approx 728m; depth of cover 6m-7m.
2nd Drive: P2 – P1; length approx 715m; depth of cover 6m-7m.
3rd Drive: P4 – P3; length approx 788m; depth of cover 15m-20m.
4th Drive: P4 – P5; length – still under study; depth of cover 12m-15m.

Detailed design has been completed and approvals received for the first drive, and work continues for the others withmost expected to be approved shortly. The construction work is being executed under the supervision, for the client, of the Hydraulic works agency – Organisme de controle technique de la construction Hydraulique.

Tunnelling will be done using a Herrenknecht AVN 1800 TBM and intermediate jacking stations at about 100m intervals. Warmann slurry pumps will be used for the drive, and a Bauer slurry separation unit.

The project will have four Westfalia jacks capable of up to 200 tonnes of force, to give a total of 800 tonnes of the 3.3m long concrete pipes being jacked. Lubrication will be by bentonite injection. Blower ventilation will be employed, and a VMT guidance system.

When micro-tunnelling gets underway, Denys hopes to achieve progress of 6m per shift.


Shaft P4 is under construction near the northwest corner of the Bay of Algiers A Denys group company, Socea, will manufacture concrete pipes for the pipejackin, which will start around March at shaft P2