Prop Installation – Silvertown Tunnel TBM Launch Chamber

The installation and removal of propping with tight tolerances and site constraints for the Silvertown TBM launch chamber.

ContractorCognition Land and Water Limited

Primary contractorRiverlinx

Scope of Work

On the face of it the scope of work to install and remove props seems relatively straightforward. The tight tolerances for installing the props and site constraints made this a more challenging scheme.

The selection of the props had been made by Ayesa the lead permanent works designers, our role was to design the process for installation and removal.

The key constraints were:

  • The distance between the props had to be precise to fit the waler beams and embedded plates accurately;
  • The props had been specified such that even a 1° deviation in line would cause problems;
  • There were permanent concrete props above the temporary props making installation and removal impossible with cranes;
  • The steel walers were significant fabricaitons that had to be installed very accurately 

Installation Design Solution

A gallows bracket was designed to support the walers. The bracket had adjustability for line, level and rotation to ensure that the walers could be installed in the correct location. 

The design solution was for the props to be dropped into cradles and then slid into place with an excavator. The cradles were precisely positioned to allow the props to be placed in the correct position, and the gap between the props and the permanent concrete structure was minimized. The cradles were designed for ease of use, with a movement and jacking system designed in. 

The props were then grouted into position and adjusted with a jack to the correct level and plumb. 

Prop Removal

The prop installation was completed successfully, with the props fitting into place precisely and the excavation work able to proceed underneath.

The challenge for the removal was the lack of access for a crane due to a concrete block wall.

We designed a lifting beam to be suspended from the permanent concrete props above.

To allow the beams to be installed we added a counterweight to the beam, moving the COG location such that the beam could be lifted in to place. 

The lifting beams were then used to lift the props out one by one.


The props were installed and removed successfully and the TBM was able to start operation. A lot of thought and engineering was required to complete this project.

The team from Cognition Land & Water team delivered against very tight deadlines onsite. .