The story


The Thames Tideway Tunnel is the biggest infrastructure project ever undertaken by the UK water industry.

It will be 25 kilometres long, wider than Big Ben’s clock face and deeper (in places) than Nelson’s Column is tall.

This is a serious piece of engineering, and we’re working with some world-class contractors to build this key infrastructure in the most sustainable and cost-effective way possible for one of the world’s greatest cities.


Tunnel Boring Machines (TBMs)

Tideway Tunnel Boring Machines on the move

To build the Thames Tideway Tunnel (and its two smaller connection tunnels), we’re using six giant TBMs.

These machines will excavate the ground using a rotating disc-shaped cutterhead, simultaneously creating the tunnel walls with pre-made concrete segments and removing the thousands of tonnes of spoil via a conveyor.

Spoil will then be transported up the deep shafts to ground level where it will be removed by barge to keep thousands of lorries off London’s busy streets.

Our machines will work around the clock, excavating up to eight metres per day.


Secondary lining

Tonnes moved by river

HGV movements avoided

How do you build a tunnel under the Thames?


We’re building more than a dozen shafts (deep, vertical holes) as part of our work, and some of these have been excavated to a depth exceeding 60 metres – taller than a 20-storey building.

To create the walls of some of these shafts, we’ve used another gigantic piece of equipment called a ‘hydrofraise’.

The hydrofraise (or hydromill) is fitted with a suspended rotating cylindrical cutterhead that drops through the ground, churning and excavating until we’ve reached our depth.

A special material is pumped into the void to stop the walls falling in before we begin pumping concrete, from bottom to top, in one long continuous operation until the shaft wall is complete.

Only when the entire circle of the shaft is complete (some requiring dozens of panels) can we start excavating the ground within the walls.


Once the Thames Tideway Tunnel is complete, London will be left with new pieces of public space, reclaimed from the River Thames.

But creating this new land is challenging work, requiring the construction of a ‘cofferdam’. First, we install ‘piles’ in the river. These are thin, interlocking, metal sheets that we drive into the riverbed, one by one until the perimeter is watertight. 

Then we drain the water from the new enclosed area, and fill sections of the cofferdam with material until it reaches the height of the river bank.

These new pieces of land are required to connect existing sewers (which spill directly into the river) to our new tunnel.



London's Super Sewer: What is a cofferdam?