Re: North-South Rail Link
The program's focus was on engineering challenges. Examples included an incredible maneuver dubbed, "the Eye of the Needle", where a massive boring machine passed 33 inches above an existing subway tube and 14 inches below another structure (reading those words I say to myself, "was it really inches?"). One employee's job was positioned in the subway tube below to monitor the ceiling for possible falling objects. As with the Big Dig, mitigation costs were expensive. Crossrails is being constructed in heavily congested areas and buildings had devices attached to monitor subsidence in real-time. Several large open concrete wells were constructed in anticipation of subsidence so that workers down in the wells could inject a liquid substance (don't remember what was used) into the ground to shore up any subsidence.
Oxford Street, if I remember correctly, was shown to be choked with traffic and Crossrails is passing nearby. Twenty-three billion dollars, when you need to spend the money, you spend the money.
For me, it was a fabulous hour of television viewing.
Click upper left thumbnail at this link, "Your questions answered: Crossrail".
https://www.theengineer.co.uk/issues/october-digi-issue-2/your-questions-answered-crossrail
What was the technical process used in ‘threading the eye of the needle’ — tunnelling between the very tight clearances between the escalators and the Northern line at Tottenham Court Road?
Andy Alder, Crossrail project manager, western tunnels: Crossrail used an earth pressure balance (EPB) tunnel-boring machine (TBM) to mine the western-running tunnels between Royal Oak and Farringdon. The EPB TBM mined the section of tunnel directly above the operational Northern line platform tunnels at Tottenham Court Road, directly below the London Underground station structures, with less than 800mm clearance to each.
The EPB TBM controls ground movement during the tunnel mining by maintaining pressure on the clay being excavated within the cutting face. The cutterhead rotates as the TBM advances, cutting the clay with tungsten carbide cutting tools. There is a steel bulkhead that creates the cutterhead chamber, separating the cutting face from the inside of the TBM. A screw conveyor removes the excavated clay from the cutterhead chamber, and the speed of rotation of the screw conveyor controls the clay pressure at the cutterhead. In this way the ground movement is controlled and minimised. Foam is mixed with the clay to create the right consistency of spoil to use the screw conveyor to maintain pressure. Pressure in the TBM is monitored in real time to allow it to be controlled.
Behind the TBM, precast concrete segments are erected to create the tunnel lining. Cement grout is injected around the segments under pressure to lock the segments in the ground, and to ensure that all voids are filled.
A laser guidance system is used on the TBM, to ensure that its position is accurately known. Variable pressure on the TBM shove rams around the circumference of the TBM is used to control the TBM position to within ±50mm.
Automatic monitoring was undertaken in the Northern line platform tunnels. This used an automatic theodolite to record the position of a number of prisms fixed to the existing tunnel lining and record any movement. This data was analysed in real time to monitor the effect of the mining on London Underground’s assets. Recorded movements were less than 3mm, well within predictions, and a remarkable feat given that the 1,000t TBM passed only 800mm above.
