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Work starts on Lee Tunnel

Site workers broke ground this week on the £635M Lee Tunnel, marking the start of the major construction phase on the tunnel access shafts.

Spades in the ground

Thames Water chief executive officer Martin Baggs cut the first sod at the Beckton shaft site last Thursday.

In total four shafts will be constructed for the Lee project. The first is at Abbey Mills to take flows from sewer overflows to the tunnel. A second will take the flows from the tunnel to a pump station shaft at Beckton sewage treatment works.

At Beckton, a third shaft will pump sewage to the treatment works itself, while a fourth shaft at Beckton is an overflow shaft.

The shafts will be built using diaphragm walls up to a maximum depth of 75m. Contracting joint venture Morgan Sindall, Vinci Construction Grands Projets and its geotechnical sister company Bachy Soletanche (MVB) believe these will be the UK’s deepest diaphragm walls.

Trench work

Construction of the walls will involve using Bachy’s Hydrofraise rigs, which have circular, rotating cutting heads.

These take bites out of the ground to excavate a trench which is supported using bentonite slurry before reinforcement is installed and concrete is pumped to the base.

The circular shafts will range in diameter from 20m to 38m and will be between 1.2m and 1.5m thick.

The [existing] sewers are simply not big enough to cope with a population which has trebled in size and continues to grow”

Martin Baggs

“The [existing] sewers are simply not big enough to cope with a population which has trebled in size and continues to grow, and a city which has paved over many green spaces preventing natural drainage,” said Baggs.

“We have a plan in place to tackle this. Abbey Mills is the largest sewer overflow in London so the Lee Tunnel will deal with this first so we can have the biggest impact, most quickly.”

CH2M Hill Lee Tunnel project manager Robert Hayden said the groundbreaking ceremony marked an exciting milestone on the project. “It’s really the start of the main works,” he said.

“Incredible” project

Thames Estuary Partnership executive director Jill Goddard said: “The Lee Tunnel ground breaking is great to see as the start of this incredible engineering project.”

The 6.4km tunnel will prevent 16M.t of sewage overflowing into the River Lee each year.Tunnelling work will start in 2012, and the project is due for completion in 2014.

The Lee Tunnel is the second phase in Thames Water’s London Tideway Improvements programme.The proposed 32km Thames Tunnel running from west to east London will be the final and most challenging phase. Thames Water last month confirmed revised costs for the Thames Tunnel, which could be between £3.6bn and £4.3bn.

  • For the full project report see the Thames Tunnel major project report published with this week’s NCE.

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