With a project as large, as complex and as challenging as the CrossRail tunnel through central London, it’s hardly surprising that load cells have played a central role in its construction.
Crossrail will link Maidenhead and Heathrow in the west to Shenfield and Abbey Wood in the east. Running for a total of 118kms, the high frequency new rail service will pass through 21km of twin-bore tunnels which run right under central London.
Holding back the water at Canary Wharf
As part of the construction of the new Canary Wharf station for Crossrail, a cofferdam was created to hold back water from the construction site. According to the contractors Geosense:
“The primary structural support for the coffer dam wall consists of 160 temporary anchor piles offset in the dock with tie rods connecting the anchor piles to the Giken piled wall.”
Load cells played an important part in monitoring the wall during the construction period.
“21 load cells have been installed at key locations where the tie rods connect to the piled wall. The load cells, via an automated system, measure the force in the tie rods caused by the water pressure at the back of the wall while the dock is drained. This system will also be essential during the excavation of the station box to full depth.”
Beam testing at Whitechapel
Load cells have also been used to test temporary beams at the Durward Street shaft which will be used to lower down parts for the new escalators for Whitechapel station. The beams were tested using “A trolley, 5t chain block, 5.3t collar chain, and load cell via shackles” , and a wired 10t capacity load cell with a handheld reader.
Tunnelling under tunnels
Crossrail’s new tunnels often run under existing tunnels, many of them constructed over 100 years ago out of grey cast iron segments. Research was undertaken by Imperial College London to assess how these tunnels would react to deformation and what might cause them to fail, as grey cast iron is very brittle.
As part of a three-pronged approach, large-scale rings of cast iron were tested in the lab, and results used to create accurate predictions. The aim of these full-ring tests were to:
“Deform the ring to similar shapes observed in situ both prior to and after nearby underground excavation.”
A custom-made testing rig was devised, with load cells ensuring even loading as follows:
“The ring was loaded by 18 actuators positioned at equal distances around a reaction ring that surrounded the test ring… This allowed much greater control of displacements and loads than could be achieved with soil. The ring was loaded while in a horizontal plane on a structural floor. Each actuator bore onto a spreader pad via a load cell to help distribute the load.”
Load cells were also used in triaxial tests on rotary core samples of Thanet Sand Formation, one of the many different geological deposits Crossrail has to pass through in its journey under London.
And finally … where does all that excavated waste go?
If you’re wondering where all that excavated soil will end up, around 4.5m tonnes of it will be used to create a new RSPB nature reserve in Wallasea Island, Essex.