To counter chronic congestion in the Blackwall Tunnel, London, which dates back to Victorian times, the Silvertown Tunnel is being built in the east of the city, the first newly constructed underground road link between the two banks of the Thames in over 30 years.
Over a total length of 1.4km, two lanes run through each side of the tunnel. The diameter of the tunnel boring machine (TBM) of the EPB Shield type (Earth Pressure Balance Shield) designed and supplied by Herrenknecht is correspondingly large.
The TBM, christened "Jill" by the miners, is the largest ever used in the UK: the shield diameter is almost 12m. Including the gantries, the machine weighs 1800t and is 82m long. By the time work was completed on July 23, 2023, "Jill" had installed 1120 segmental rings, each weighing around 70t, to line the tunnel.
The contracting joint venture Riverlinx CJV thus completed the tunnelling with the highest precision and within less than one year. Breakthrough of the southbound tunnel took place in mid-February 2023.
180-degree turn with a 1450t machine
When building twin-tube tunnels, usually either two TBM are used or a single TBM is dismantled and returned to the original launch site for the second drive. However, due to the highly complex underlying conditions in terms of construction technology, time and finances, the project managers opted for a different, innovative approach for the Silvertown Tunnel. After the completion of the first tunnel, the entire machine was rotated by 180 degrees in order to subsequently bore the northbound tunnel. To this end, the tunnellers specially excavated an 18m deep and 40m long shaft that was open at the top.
In fact, turning such a huge machine required a special procedure developed by Herrenknecht. The shield machine and the cutting wheel are placed on a kind of sled. This, in turn, slides on cushions filled with compressed nitrogen so that with the help of pneumatic chain hoists, the machine can be turned precisely and brought into the new position. The method had already been successfully used in Paris and on the new Stuttgart-Ulm railway line. The 180-degree turn of the 1450t shield machine in London was accomplished by the Herrenknecht specialists in a single day.
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