Lee Tunnel
 

Launch shaft assembly with a unique start-up situation

The task

The Thames ecosystem is at risk by London’s overstrained sewage system. A long-term answer is to be provided by expanding and building new efficient pump systems and sewage works. One measure for improving transport capacity involves a major tunnel construction to the east of the city from the Abbey Mills pump station to the sewage works in Beckton – the Lee Tunnel. A Herrenknecht Mixshield is to excavate the tubes extending for almost 7 kilometers. One of the core challenges presented by the project is site assembly in a confined launch shaft 70 meters below ground.

The Herrenknecht service solution

The urban planning context necessitates construction of the Lee Tunnel alignment at an extensive depth of 70 meters. The TBM is therefore designed for an operating pressure of up to 8 bar. The very confined launch shaft also means that assembly and the start-up situation need to be given particular consideration. Together with the customer, service engineers at Herrenknecht develop an integrated solution concept to meet this challenge.

First of all, the entire shield is assembled above ground before being lowered by a crane into a small underground start-up cavern. Owing to the very confined space underground, it was impossible to assemble the entire TBM there. The individual back-up segments built specially for the tight launch shaft are then successively lowered into the cavern by crane. The “incomplete” TBM is obliged to bore the length of a back-up ahead in order to create space for the subsequent gantry. Accordingly, the entire machine grows bit by bit until it reaches its entire length while supplies are ensured from above through pipelines in the shaft. The last assembly steps and final commissioning are then carried out underground, enabling the machine to commence tunnelling as soon as possible, and offering assembly personnel a high degree of safety thanks to comparably easy TBM assembly above ground.

Project advantages

• Project-specific machine design and site assembly as the right answer for the tight launch shaft 70 meters below ground

• Experienced service experts from Herrenknecht provide support during planning and implementation of the unusual service solution on site.

• Greater safety at work for the site crew thanks to assembly above ground (when compared to underground cavern assembly)