TBM - a tunnel factory

TBM is short for tunnel boring machine, but a better name would be tunnel factory. The 120 m long device bores, seals with concrete segments and removes the waste material – all at a speed of approx. 18 m per day.

Two TBMs

The Citytunnel is made out of two parallel tunnels and therefore two TBMs are operating at the same time. One machine was assemled before the other and therefore had a slight head start.

Inside the shield

The first 10 m of the TBM is like a can. It is the steel shield which seals off the bored  tunnel. At the front of the shield, there is a rotating cutter head, with a diameter of almost 9 metres. There are two types of tools on the cutter head: discs and scrapers that grind down the limestone into smaller pieces. The spoil is collected in the excavation chamber and removed from there with a screw conveyor on to a conveyor belt. The belt will take the waste through the 110 m long back-up unit of the TBM and further out along the tunnel.

Sealed with lining

As the TBM works its way through the limestone, the tunnel will be lined. The lining is made of watertight concrete rings that create a tube in the bored tunnel. The lining is assembled in the back part of the shield and every ring is made out of eight prefabricated segments. The tunnel tube is lengthened by 1.8 m with every ring. Between the rings, there will be rubber seals. Concrete will be pressed into the gap between the rock and the lining. There is no
risk of a rock collapse or flooding, because the segments are put into position inside the watertight shield. In practice, this means that the groundwater level does not have to be lowered. It also means that the tunnel is always supported by either the TBM shield or the segment lining.

Bracing against the lining

The cutter head braces itself against the limestone by pressing 28 hydraulic jacks against the last tunnel ring. When the TBM has bored about 2 m the cutter head stops, the jacks are withdrawn and a new tunnel ring is assembled. About 10 tunnel rings are put in place every day. The jacks are also used to tow the service part forward.

The starting-block at Holma

When the TBM started to bore there was no lining to brace against. So a special “starting-block” was built. It gave the required resistance until the fi rst real tunnel rings were in place.

Back-up unit

Even though the most intensive work happens in the first 10 m of the TBM, the following 110 m are also very important. This is where the
power supply, water, ventilation, workshops, office, staff areas are located. It also brings the tunnel segments into the tunnel and waste
material out of the tunnel. The TBM is the work place for about 15 people on every shift.