Tideway TBMs well into excavations 26 Sep 2019

More than 8.5km of the total 30.7km of the Thames Tideway CSO main interceptor and its connecting tunnels has been excavated, with four of the six TBMs in operation and the fifth currently at the North Greenwich assembly site, ready to start work in early 2020. The first TBM, an 8.8m diameter NFM EPBM excavating the Kirtling Street to Carnwath Road reach of the Central Contract of the project, is expected to complete its drive in coming months.

Fig 1. Tideway route under the River Thames

The second NFM EPBM for the FLO JV of Ferrovial and Laing O’Rourke on the Central Contract is 1.5km into its 7.5km alignment from the Kirtling Street launch site to the reception shaft at Chambers Wharf. Completion of that drive is expected in late 2020, before which the two TBMs on the West Contract of the project will also have finished.

On the main West Contract alignment, a Herrenknecht EPBM for the BAM Nuttall-Morgan Sindall-Balfour Beatty (BMB) JV, is 782m into its 6.5km drive from the Carnwath Road launch shaft and is expected to breakthrough at the reception shaft at Acton Storm Tanks in Summer 2020. Meanwhile, excavation of the 1.1km long Frogmore connector by a refurbished Lovat EPBM, from King George’s Park to the main shaft at Carnwath Road, has an expected completion date of Spring 2020.

Two Herrenknecht slurry TBMs are procured by the Costain-Vinci-Bachy Soletanche (CVB) JV to excavate the route of the East Contract. Currently at North Greenwich, the first of these will excavate the 4.6km Greenwich long connection tunnel. The second, yet to be delivered from the Herrenknecht factory in Germany, will drive the 5.5km run between Chambers Wharf and the reception site at Abbey Mills Pumping Station. From Abbey Mills, the Tideway main alignment will join the Lee Tunnel, which was completed in 2015 and carries CSO flow to the Beckton Sewage Treatment Works.

Fig 2. Elevation of the gravity-fed sewer, the geological profile and boundaries of the three construction contracts

The main alignment runs 25km between Acton Storm Tanks and Abbey Mills. The conveyance system is gravity operated, running at a gradient of 1m:790m from the starting at a depth of 33m at Acton and finishing at 66m at Abbey Mills.

“Ground conditions vary along the alignment, from clay in the West section, mixed sand and gravel in the Central section, to chalk for the East section contract” a spokesperson for Thames Tideway told TunnelTalk. The geology in the East section is the most challenging due to the inclusion of hard flint within the chalk and under high water table. High compressed air pressures will be required for necessary man interventions for maintenance and cutterhead tool changes. "Each contractor was invited to look at the core samples before submitting their final bids to understand the requirements of each area,” explained the Tideway spokesperson.

Long connection alignments

In addition to the main alignment, the two long connector tunnels pose additional challenges.

“The entire alignment of the Greenwich connector runs through the Seaford Chalk Formation with an external water pressure of up to 5 bar and at a depth of 40m-50m below ground,” the spokesperson told TunnelTalk.

TBM arriving by river to the launch site

Including the main launch shaft, there are a total of three interceptor shafts to be connected to the Greenwich connector.” The other two, at Earl Pumping Station and Deptford Church Street, are online shafts, meaning the TBM will pass through the 17m diameter shafts, constructed using diaphragm wall support,” TunnelTalk was told. “To facilitate the crossing of the TBM at these locations, the shafts will be fully excavated and the base slab installed. The shafts will then be partially back filled with a cement-bound granular material to the top of the tunnel eyes. The shafts will then be filled with water to match the external water pressure of 5 bar.”

The Frogmore connector, at about 25m-35m deep within London Clay, is being excavated in two drives by the refurbished Lovat TBM. Starting at the launch shaft at Dormay Street, the first drive will progress 500m south to the transfer shaft at King George's Park, and the second 600m drive will be excavated after recovery and relaunch of the machine at Dormay Street, north to Carnwath Road.

“The Lovat EPBM was built in 1994 and has successfully completed five projects with a total length of 16.6km between 1995 and 2007,” continued the Tideway spokesperson. “It was overhauled for the Frogmore alignment and adapted to the requirements of the Tideway project. The machine has a diameter of 3.3m and the segmental lining comprises six segments per ring for an inside diameter of 2.85m, which is then supplemented with a 125mm thick in-situ concrete secondary lining.”

TBM lowered at Carnwath Road launch shaft

Shafts

The project also includes nine short connection tunnels and five main shafts. Combined sewer overflows - of which there are thirty along the Tideway route - will be connect mostly via culverts into interception chambers, then into shafts through short connectors to the main alignment. “In the East section, however, where the geology is predominantly chalk, the shafts will drop straight into the main sewer, instead of into a connection tunnel,” TunnelTalk was told. “This has led to the need for on-line vortex drop shafts to dissipate the energy of the falling water and prevent damage at the bottom on the shaft.”

“Shaft construction has varied across the sites, due to various site constraints, and have included diaphragm walls, sprayed concrete linings, secant piles and segmental linings. At the main Kirtling Street and Chambers Wharf shafts, a hydrofraise was used to construct the diaphragm wall shafts.”

Hydrofraise used for diaphragm wall installations

The hydrofraise is fitted with a suspended rotating cylindrical cutterhead that cuts through the ground until reaching depth. Bentonite slurry is pumped into the void to stop the walls collapsing, before concrete is pumped in from the bottom in one long continuous operation until the diaphragm wall panel is complete. Once the diaphragm wall complete the shaft is excavated. The diaphragm walls at Chambers Wharf are 1.5m thick and just over 72m deep and were completed in March 2018. At Kirtling Street, the diaphragm wall panels are 1.2m thick x 86m deep.

“The shafts encounter all geology, from made ground all the way to the underlying chalk, with varying depths as defined by the adjacent main Tideway alignment. The main shafts required to support the construction of the main alignment have all been excavated to formation level, with the secondary lining, internal structures, and roof coverings due to be completed post-tunnelling work.”

A supersewer in the making

The last of the excavation work for the project (in the East Contract), is scheduled for completion in 2021. By that time, the TBMs will have driven under 75 bridges, past 45 tunnels, and under 1,300 buildings. The project will cost about £3.8 billion, plus an additional £1.1 billion spent by Thames Water on preparatory works. Final completion of the project is expected in 2024.