Thames Tideway Tunnel progress continues

Mon, 14 August 2017

It’s been 18 months since ‘Maritime Journal’ visited the various sites of London’s new so called ‘Super Sewer’, so we took the opportunity in August 2017 to see how the marine civil engineering side of the project has been developing.

Red7Marine continues to be a key provider of marine plant for the (estimated) £4.2bn project, particularly at this stage which entails the construction of cofferdams at the three main ‘drive sites’ which will eventually contain the vertical shafts down which the tunnel boring machines will be lowered to begin their work.  The tunnel boring machine (TBM), of which there will be six employed on this project, excavates a circular tunnel using a rotating cutterhead, while simultaneously creating a tunnel wall using concrete segments. As it excavates it also transports material from behind the machine via a conveyor belt or pump, which is progressively extended as the machine moves. Once each TBM has done its job and cut the tunnel through to the next drive site it is disassembled and removed, and the tunnel shaft lined with concrete.

Of the three ‘drive sites’ Chambers Wharf, approximately a kilometre downstream of Tower Bridge is the most advanced we saw.  Red7 Marine deployed the 200t jack-ups Haven Seariser 1, Haven Seariser 3 along with Haven Seaconstructor to install this sheet piling cofferdam. The sheet piling is complete here and backfilling the cofferdam is well advanced. Once the backfilling is completed, a deep circular shaft will be excavated, which will have a large steel acoustic shed erected over it to reduce the tunnelling noise, as the boring machines will work 24 hours a day.

According to Red7Marine Engineering Consultant Scott Wiltshire, the long trading history of the River Thames makes for some challenges: “It’s quite common for a pile to hit something on its way down, usually a piece of old timber from a previous landing stage or similar, knocking it off course and slowing progress.”

When the piling is going well and obstructions are not encountered the soft London clay means progress can be quick. “The piling operation is conducted one 30m length of the jack-up at a time, called a ‘gate’, and often it is possible to move the jack-up onto a new gate every second day”.

The variable ground conditions mean that it is critical to preload the legs of the jack-ups prior to jacking up. According to Scott, this is done by placing the whole weight of the vessel in turn on both pairs of diagonally opposed legs. The clay, however can create problems with suction when it comes time to move the vessel onto its next location. There is also considerable tidal flow in the river, meaning vessel movements are planned to the second- usually around slack water.

Red7Marine has built up plenty of experience with deploying its assets to the tidal Thames as it has been involved in numerous other projects and was involved from the start of the Tideway tunnel project carrying out the site investigation works.

The bridges, of course, have presented a challenge when it comes to getting the jack-ups from site to site. A crane barge is used to lift the legs out and lay them on deck to transit a bridge, then refit them once the jack-up is in position and ready to work. The most demanding example of this sort of leg juggling was the positioning of the Haven Seaway for 14 months in the very tight gap between the Blackfriars road and railway bridges, in order to perform enabling works (the construction of a waterside lift system) as covered in more detail in MJ in March 2016.

Travelling up and down the river, visiting the various construction sites of this huge nine-year project, and talking to multiple partners and stakeholders it became clear that the project is both required and overdue. While on a dry summers day, the river is undoubtedly cleaner than it was 20 or 30 years ago, the problems only really come at times of heavy rain.

London’s antiquated Victorian sewers while a marvel of engineering at the time, are configured so that at times of heavy rain, the incidences of which are only forecast to increase, there are combined sewer overflows (CSOs) that carry the excess flow, including raw, untreated sewage into the river.

The Thames Tideway Tunnel solution is a 25km section of 7m bore pipe, that carries the sewage under the Thames at depths ranging from 30-66m for treatment downstream at Beckton. It will require 24 construction sites and the use of six TBMs (Tunnel Boring Machines) to complete the project.

Currently, around 10,000 tonnes of sewage related litter enters the tidal River Thames from CSOs every year. This will be reduced by 90% once the Tideway Tunnel is in operation in 2023.

Source: Jake Frith

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