News

Milling Work in the Berghofen Tunnel, Dortmund
Even the hardest rock gives way to the special bit from BETEK

The air in the tunnel is filled with noise and dust. A Wirtgen cold milling machine cuts away the dense rock from the floor of the tunnel, metre by metre. Everyone breathes a sigh of relief because the work is going well. The successful completion of the Berghofen project is a story about the right tool, special machine equipment and outstanding expertise in the field of milling technology.

A tunnel built according to the latest safety guidelines

People from Dortmund know all about the extremely heavy traffic on the B 236 in the part of the city known as Berghofen. The Federal road is being put underground in a tunnel which is 1,300 metres long. The groundbreaking took place on 13 August 2003. Some €80 million will have been invested in the construction work when the new B 236 is opened to traffic in 2007. Drivers will then move through a tunnel equipped with the very latest in safety technology.

At its widest point, the Berghofen Tunnel will measure 22.3 metres and be over ten metres high. It is subdivided into three different sections: a north section 405 metres long, a south section 256 metres long and a middle section 649 metres long. The middle section is being constructed in an "open-pit mine" and will be covered over upon completion. The north and south sections will be created as "mines".

Blasting? Working with jackhammers? Milling!

In Berghofen we have to deal with rocks such as Rhine shale, carbonate rock and sand. The tunnels themselves are excavated using local section machines. Then a concrete base is positioned for the height determination. Now the material must be removed to create the floor of the tunnel and an even plane. The conventional procedure is to use excavators and jackhammers, perhaps local section machines as well. If the substratum is especially hard, blasting may be required. All of these procedures have two serious disadvantages. For one, the substratum which has been removed must be cleared away and loaded for transport, a laborious and time-consuming task. For another, the resulting surface is not smooth and additional time-consuming and costly work is required. The construction company in charge of the project decided in favour of economical milling and engaged a Southern German milling specialist, the company Kutter from Memmingen.

One milling machine, two men and 80 BETEK bits

The machinists Schickling and Kuhn from the company Kutter arrived at the construction site with a Wirtgen W 2200. This cold milling machine has a special milling drum which is suitable for handling challenges like this. One of the differences is found in the holder system HT 7 from Wirtgen. It was developed especially for the milling of rock, is very sturdy and 50% larger than the standard systems HT 3 and HT 11. In addition, half of the upper part of the holder can be removed, thereby increasing the line spacing from 15 mm to 30 mm. An increased line spacing minimizes milling resistance and the bit can cut through the material better. Specially developed covers protect the lower parts from wear. Instead of the 180 bits usual when milling asphalt, 80 standard bits from BETEK were now on the holder.

A W 2200 with mining equipment in action is an impressive sight. The rock is milled away in the middle of the machine. The rubble is immediately loaded onto the waiting lorry by a conveyor system. This results in concrete savings in costs - and of course saves time.

First, special bits from BETEK "crack" the rock

The milling work began as planned. The compression strength of the rock was 85 mpa. But because there were very few cracks in the rock, the usual standard bits quickly reached the limits of their capabilities. This was the point when the company Kutter turned to the bit manufacturer BETEK. Consulting engineer Moosmann immediately headed for the site and quickly recognized what the problem with the standard bits was. A conversation with the tool experts at BETEK soon made it clear that only a custom solution would be able to handle this milling task. Using state-of-the-art 3-D CAD systems, BETEK engineers set to work designing a bit. It goes without saying that years of experience went into this special development. In addition, the design was tested with FEM calculations. The tungsten carbide powder for the bits was prepared in the new, modern spray tower in a mixture ratio calculated precisely for this processing task. At the same time, the precision moulds were being built in the tool construction shop owned by the tungsten carbide specialists from Aichhalden. The special bits were sintered in what is the most modern sinter-HIP facility available today.

Only a few days had passed when BETEK employee Moosmann drove off to Dortmund with the special bits, neatly packed in the practical BETEK toolboxes. 80 bits were immediately installed. The tension was palpable when the W 2200 started up. It milled its way - one metre, two metres, three metres, four metres. Expressions of growing relief appeared on the faces of the group. Success - the bits "cracked" the extreme substratum!

The solution: the right tungsten carbide and an ideal tool geometry

The new BETEK bit W1 K 17 V22, in contrast to the standard, displays a geometry which was designed specifically for this milling task. The work was able to continue without any tool breakage. However, wear was above average due to the low cracking in the rock and the hardness of some of the rock zones. The machine operators moved forward at a rate of 2 to 5 metres a minute; the dense rock prevented any faster progress. In comparison: a milling speed of up to 20 metres a minute is possible in asphalt, which is of course much easier to mill.

The cold milling machine removed the rock layer by layer. Every pass of the milling machine removed between 5 and 10 cm. Another comparison here: the milling machine reaches a milling depth of 35 cm in asphalt. A total milling depth of 1.2 to 1.4 metres had to be reached - for a smooth and even plane. 12,000 cubic metres of rubble were created during the up to 28 milling passes.

Events at this construction site confirmed the experience so familiar to BETEK from everyday work with tungsten carbide. It is impossible to avoid problems during milling work because of the broad range of ambient conditions. Some of the milling materials simply cannot be handled using standard equipment. This is precisely the point where the tungsten carbide specialist's service defines the norms. Working closely with the machine manufacturer and the milling company, the enterprise develops solutions which make the extraordinary possible - such as the Berghofen Tunnel in Dortmund.