The air vibrated with excitement when, in the night of 3rd December, two cranes loaded a Wirtgen cold milling machine with a weight of approx. 45 tons. The W 2200, owned by the company Kutter from the Swabian town of Memmingen, was hoisted over the railing of a bridge and then let down onto the terrain of the railway line approx. 7 m below. The scene of this event was the secondary road L132, which crosses the railway line from Türkismühle to St. Wendel in the Saarland. This section of track was to be reconstructed, including lowering of the subgrade level.
The sleepers on the 11 km long section had reached their usual lifespan of 25 to 30 years, and their replacement was combined with extensive modernization work. This included reinforcing the ballast underlying the sleepers to a minimum thickness of 30 cm. Additionally, the distance between the tracks and the overhead wires of this section was to be increased. As raising the overhead wires was impossible because of the bridge, the subgrade level had to be lowered instead.
Normally, the soil is removed using excavators or hydraulic hammers. The approx. 300 m long section below the bridge, however, consisted of extremely hard rock. Soil surveys revealed that it is composed of greywacke, grey shale and granite with compressive strengths between 50 and 100 MPa. As the level had to be lowered by as much as 50 cm, the maintenance and rehabilitation division of DB Netz AG began searching for a suitable construction method. “We have analysed all methods for removing this extremely hard soil. Blasting would have involved too much of an effort. Hydraulic hammers are not sufficiently accurate and would have delayed the construction progress too much”, says site manager Siegmund Dick, summarizing the problematic nature of the project. After a great deal of in-depth research, the experts were sure: The solution is a large milling machine equipped with special gear for rock milling. Siegmund Dick adds, “Despite the relatively high effort of having to load the milling machine by crane, this is definitely the most economical method of removing the rock and producing an even level within a short period of time.”
The soil underlying the first track of this section had successfully been removed two weeks before, and now the team was ready to commence the same operation in the opposite direction. First, the rails and sleepers were dismantled and the ballast removed in order to permit the operation of the cold milling machine. Additionally, the current-carrying overhead wire had to be slewed to the side to safely lower the machine, which has a working width of 2.20 m, from the bridge onto the level to be milled. To do this, the wires were cut off from power first. This, however, is possible only when there is no traffic on this section of the track. As a result, the crane loaded and lowered the machine during 1:00 and 4:00 o’clock in the morning, since this is the only time when there is no traffic on this section of the track.
After a team of power current engineers had made sufficient room to allow transport of the machine, the time had finally come: The milling machine took off from the ground, was turned by 90° and dangled above the railing, so to speak. Everybody present was thrilled by the scene. It is, after all, not a common sight that such a big machine is lowered several metres from a bridge to the ground. The pros handled this job with the necessary aplomb, however, and a few minutes later the red heavyweight was standing on its own four crawler tracks again. Now, it was time for Kutter’s well-tuned team to get started. The rock had to be removed over a total length of almost 300 m, across a width of 4 m and at a depth of between 30 to 50 cm.
Just for this construction site, Kutter’s W 2200 had been equipped with a special milling drum suitable for application in medium-hard rock and for routing work. When compared to the standard milling drums for road construction, the main difference lies in the type of quick-change toolholder system used. Wirtgen’s quick-change toolholder system type HT 7, which was used for this project, has been developed specially for cutting rock. It is very robust, as it is 50% larger than the standard systems HT 3 and HT 11. A further difference to the standard system: The toolholder system HT 7 offers different upper parts to accommodate tools with shaft diameters of 20, 22 and 25 mm. This feature allows milling machine users to be very flexible and well armed for the most wide-ranging applications when cutting rock. Quick-change toolholders with a diameter of 22 mm were used in St. Wendel. The cutting tools used – type W1-17/22 made by Betek – have a particularly long and robust carbide tip, since this application required maximum resistance to wear and tear as well as high breaking strength.
The advantages of the Wirtgen system took full effect, since not only the tools but also some damaged toolholders had to be replaced after milling the extremely hard rock. As the special drum is equipped with a quick-change toolholder system in lieu of welded toolholders, cutting tools and toolholders could be replaced with the usual on-board tools – and downtimes of the machine were very short.
Due to its engine power of 671 kW, the W 2200 is capable of milling asphalt at a maximum milling depth of 350 mm in one machine pass. What works well on classical road construction sites is, however, not feasible in such extremely hard rock. As had been expected, the soil was of a very inhomogeneous nature, which was clearly visible from its different shades of colour. Consequently, the milling pros approached the rock with a good deal of caution. On some sections, they worked at an average milling depth of 8 to 10 cm. On other sections, the milling depth had to be reduced even further. However, the team from Memmingen had anticipated that this would be the case. “We have milled different kinds of rock with this machine and the special drum. Sometimes, it’s more efficient to mill several passes at reduced milling depths than to cause premature wear and tear of the cutting tools and toolholders by working at high advance speeds and large milling depths”, machine operator Walter Schickling explains the modus operandi. As planned, the machine crossed the finish line on the following afternoon: It had produced an even subgrade level that is precisely 70 cm below the upper edge of the adjacent track. Kutter’s team had completed the job successfully – almost, because the cranes could only lift the machine back onto the bridge after 1:00 o’clock in the morning, but then the milling pros from Memmingen headed for home.
During the following week, the railway ballast was applied on top of the newly produced level at the required height of 30 cm. After the new concrete sleepers and track systems had been laid, the tracks were reopened to traffic on 10th December.
At this point in time, the W 2200 was in operation on quite a different kind of job site again: The machine was transported back to Memmingen on Sunday night. Having been converted to a “normal” road milling machine again, it was milling asphalt on a motorway – 350 mm deep.
Michaela Adams, Mario Linnemann
Press and Public Relations
Phone: +49 2645 / 131-0
Fax: +49 2645 / 131-499
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The large milling machine type Wirtgen W 2200 is equipped with sturdy loading and lashing lugs as a standard feature. As a result, just a few steps were needed to safely connect the 45-ton machine to the ropes of the cranes. Both cranes had a lifting capacity of 250 tons each.
Biting its way through hard rock quickly: Equipped with a special drum for cutting rock, the W 2200 removed material with compressive strengths of up to 100 MPa.
The milled material was deposited in the ditch on the side of the track. It was not possible to load the milled material on trains on the adjacent track because normal train services were continued.
During the milling operation, employees of DB Netz AG and Kutter checked regularly as to how much rock remained to be removed. The soil was milled off at total depths of between 30 and 50 cm in several machine passes. In the end, the difference in height between the subgrade level and the upper edge of the reconstructed adjacent track totalled 70 cm.
Clearly audible signal horns and lights warned the workers of approaching trains. For safety reasons, all machines had to stop work until the train had passed.