Giant gaskets seal underground highway tunnel

When city planners in Madrid, Spain, realized that the traffic congestion on the arterial roads around the capital was paralyzing the city, they set the stage for what would become one of the largest and most complex civil engineering projects in Spanish history.
By 2012, nearly EUR 8 billion will be invested by the city to solve the traffic problem on a road project known as the M30. One of the many solutions in the project was a 3,650- meter highway tunnel that crosses the city center underground, less than 6.5 meters away from a busy subway line at certain points. Finished in mid-2007, the M30 “Bypass South-East” has three lanes in each tunnel and sidewalks on each side. At the time of its construction, the excavated dimensions of the tunnel, 15.2 meters in diameter, were the largest ever required. This demanded an entirely new breed and size of tunnel boring machine, also called a TBM or “a mole.”

“Two machines were used, one at each end of the tunnel. TBMs are usually custom-made for each individual tunnel project,” says Erwin Brakenhoff, Sales Director, Trelleborg Bakker in the Netherlands. “Costing in the millions of dollars, a TBM has the advantage over other methods in terms of safety and timing; in addition, it is extremely precise, which in turn, reduces the cost of lining the tunnel. This makes TBMs more suitable for use in built-up areas.”

Alfredo Avello, Director of Abyo, a supplier of tunneling equipment in Spain, calls the M30 tunnel project a team success. “This project was led by Madrid Municipality, and two joint ventures formed by leading Spanish contractors excavated the tunnels. In addition, other highly qualified Spanish contractors executed very important complementary surface works,” says Avello. “It has also been a pleasure to work with Trelleborg, which supplied gaskets for sealing the segments in both tunnels.”

Knowing where and what to dig
Before digging a tunnel for trains, trams, cars, cables, sewage, or drinking water (tunnel engineers use the term driving), there must be a comprehensive understanding of the soil and water conditions the tunnel will be running through. This is ascertained either by sampling the rock or through advanced imaging technology that scans the earth by computing how sound waves travel through the ground.

How it digs
In Madrid, the two tunnel boring machines, TBMs, dug through soft and hard clays and gypsum at a maximum depth of about 75 meters, which was well above the water table. On average, about 100 meters of tunnel was excavated per week, with a top performance of 180 meters. A TBM weighs almost 4,500 tons and is 114 meters long. Like a mole, it inches along, excavating a hole. In Madrid, extremely powerful hydraulic jacks, with a pressure of 316,000 kilo Newtons, pushed the TBMs forward supported by the finished part of the tunnel. One TMB was equipped with a so-called Earth Pressure Balance Shield, which maintained a constant pressure on the face being excavated to avoid collapse. The principle is similar to building a sandcastle on the beach, where the more pressure that is applied to a tower, for example, the more robust it becomes.

Removing the soil
The second TBM had a twin cutter head system with an inner and an outer cutting wheel that could be turned in clockwise or counter clockwise directions, independently of each other. The inner cutting wheel, which was seven meters in diameter, could also be made to protrude ahead of the outer wheel if necessary. The advantage of this unique system helped the machine balance the enormous torque at start-up, which otherwise could cause it to twist around its axis. Overall, the twin cutter head was fitted with 57 disc cutters, 332 bits, 24 scrapers and a center cutter. Three screw-shaped conveyor systems on the TBM removed the excavated material, or muck, and transported up to 2,750 tons per hour to the main conveyor belt system installed in the tunnels for onward disposal.

Lining the tunnel walls
During the boring process, the lining or tunnel wall is built up of segmented tunnel rings inside the tail of the TBM. So as the machine progresses, it leaves behind a ready-made tunnel. In Madrid, two-meter wide and 600-mm thick tunnel segments, each weighing about 14 tons, were used for each ring. Each ring consisted of 10 segments connected by bolts, in between which gaskets supplied by Trelleborg provide a secure waterproof seal. The Madrid tunnel was excavated at an exact diameter of 15.2 meters. However, the final road tunnel is 13.45 meters in diameter. The difference between the two dimensions is the depth of the segments and a layer of grout between these and the outer wall of the tunnel. “The gaskets may only be relatively small in size, and a minor cost in terms of the total cost of the tunnel, but their function is essential in keeping the tunnel watertight,” says Erwin Brakenhoff, Sales Director, Trelleborg Bakker in the Netherlands.