Kayley Edwards, Reporter

London is close to constructing its largest sewer project since Sir Joseph Bazalgette's revolutionary sewerage network built in the 1800s. The city's £2.5 billion Thames Tideway project comprises nearly 40km of deep level bored tunnels to intercept combined sewer overflows and convey stormwater to downstream treatment plants for eventual discharge of treated effluent.

Bazelgette's more than 150-year old open cut sewers built into the embankments of the central reaches of the Thames and in brick-lined tunnels for the lower gravity feed sections, are still seen as one of the greatest engineering achievements of history. Urban development and population growth over the decades has put the system under great pressure and in need of being adapted for today's environment. Up to seven TBMs, many working concurrently, will build the nearly 40km long modern day equivalent of Bazalgette's original concept.

The Thames Tideway tunnels run deep beneath the alignment of the river

The 'super sewer' will be the deepest ever wastewater tunnel in the capital and will prevent millions of cubic metres each year of untreated sewage and rainwater overflow into the Thames and Lee Rivers.

The first phase of the project to be undertaken is the estimated £400 million, 7km long Lee branch tunnel in East London. Tenders for the design-build contract are currently being reviewed and proposals from an original line-up of four major UK and international contracting consortia have been shortlisted to :

• Murphy/Hochtief JV

• Morgan Est/Vinci/Bachy JV

Proposals from FCC of Spain and the Laing O'Rourke/Impregilo JV rounded out the initial four-group competition.

Passing under Bazelgette's original Northern Outfall Sewer, the 7km long Lee Tunnel will be driven by TBM at up to 75m deep through the Upper Chalk and extens from an 80m deep shaft at Beckton sewage treatment works and a reception shaft at the Abbey Mills pumping station. The design-build proposals were based on a reference design completed for the project by Faber Maunsell. Support was also provided by the CH2M Hill-Halcrow partnership which is appointed by client Thames Water as Programme Manager for procurement of the main Thames Tunnel. With a scheduled start before the end of 2009, the Lee Tunnel is expected to be complete in 2014.

A further two shafts at the Beckton treatment works will connect to and provide pumping facilities for the main Thames Tunnel.

Main Thames Tunnel connection A further two shafts at the Beckton treatment works will connect to and provide prumping facilities for the main 32km long Thames Tunnel that follows the alignment of the river from Hammersmith in the west to Beckton in the east. It is currently programmed for construction between 2012 and 2020. To pass beneath existing tunnels crossing the river the new sewer tunnel will be 30-40m deep and will descend to 80m at Beckton.

Since both the Lee and Thames Tunnels will be located below major rivers in permeable chalk strata, high water pressures are anticipated, especially to the east of the project where pressures of up to 8 bar may be encountered. This will present particular challenges and will set a new record for closed faced pressurized TBM tunneling in the UK.

The types of TBMs used for the Thames Tunnel will depend on the ground conditions. For the chalk strata a slurry-type TBM may be more appropriate whereas in the overlying Lambeth Group and London Clay there is a proven track record of using EPBMs in both closed and open-faced mode. In addition to high water pressures, the principle challenges to be tackled in both the Lee Tunnel and the deep Thames Tunnel will include managing cutterhead wear due to the presence of hard flints embedded in the chalk; treatment and disposal of excavated material; the depth and size required for the construction of shafts; and forming connections from the dropshafts to the tunnels.

Both tunnels will be lined with a primary segmental concrete lining and finished with a secondary in-situ concrete lining to a final internal diameter of 7.2m.

Overlay of the project on satellite image of London

For the 32km long Thames Tunnel, main drive shafts are anticipated to be 25m in internal diameter, with depths ranging from 40m in West London to 80m at Beckton. Smaller reception shafts will be used to remove the TBM from the tunnel. Six main shafts are likely to be used on the Thames Tunnel, allowing up to five TBMs to operate concurrently.

The main large diameter shafts will be lined with concrete and could be constructed using diaphragm wall techniques. Caissons or underpinning may be preferred for the shallower, smaller diameter shafts. Flotation of the shafts will be a challenge for shaft designers given their depth and the high ground water levels.

Barge transportation is likely to be the preferred for haulage of the volumes of materials to be transported in and out of the main shaft sites. Rail transportation is also possible but transportation by road will likely be restricted in order to minimise disruption to surrounding communities.

In addition to the main drive shafts smaller diameter intermediate shafts will be required along the tunnel route to intercept up to 32 CSOs. Interception structures will be built to redirect flows from the combined sewer and transfer flows into the main tunnel while continuing to use the existing outlets when the tunnel is full. Where one CSO is to be intercepted a single deep drop shaft and connection will be built. Where two or more CSOs are intercepted, rider tunnels connecting the CSOs to the drop shaft may be used. Both arrangements will include an interception chamber and a connection adit.

Although similar CSO interceptor projects have been undertaken in other parts of the world, the Thames Tideway scheme is set to be one of the most challenging tunneling projects ever undertaken. Other cities around the world will view its progress with great interest and the venture may be a catalyst for the modernisation of their sewerage systems.

Thames Tideway Project