As discussed last month, during construction of the Cologne-Rhine/ Main and Nuremberg-Ingolstadt rail link, tunnels support had a particular significance.
In rock with insufficient stability for the required excavation cross-section, or when settlement was limited because of development, screens were called for, created by previously longitudinally installed pipes or by groutings.
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At several places along the new German rail link, motorways were underpinned using this method, protected by umbrella screens. According to the conditions these were either pipe screen without injecting, injection pipe screen or high-pressure injection screen.
After the advance support, the cross-sections can be driven under cover of the screened vault. It is more cost effective if pipe screens can be produced by the heading team with the tunnel jumbo (new drilling installations are necessary though) rather than manufactured by special companies. The required niches for installing the pipe screen are no longer necessary.
Pipe screen without injecting
Examples of pipe screen without injecting include the Bodex-system and the AT-Hüllrohrsystem.
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By GlobalDataProduction of the pipe screen uses an eccentric working drilling system where the drilling and the piping take place in one step. The pipe remains in the ground and is filled with concrete. For horizontal drilling, lengths of up to 30m are possible today where deviation depends closely on the predominant ground conditions. The space between the single pipes depends on the static aspects and is usually fractional.
Injection pipe screen
Injection pipe screens are similar in principle to pipe screens; there is a greater space between the single pipes as well as additional injecting of the in situ rock. The injection material is placed by special tubes which seals the drilling hole and prevents it getting wet. The method cannot be used in loose ground which requires a sleeve pipe to be installed using a cased bore.
Grout is placed in the surrounding rock through the openings in the sleeve pipe which are sealed by a rubber sleeve. At the Irlahull tunnel, a grouting pipe screen was used to underpin a motorway. Here the installation of the 15m long grouting pipe took place at an inclination of 7°, to within plus or minus 2%. Grout was placed through a valve in the pipe at a pressure of 10-15 bar.
Finally, the pipe was filled with a cement suspension.
High-pressure injection screens
In high-pressure injection the soil is cut, held in suspension and mixed by a cutting jet of a cementitious slurry. The loose ground becomes the aggregate for the concrete. The column-like concrete structure is produced at the same time as grouting and pulling back the drill rod. In general, the high-pressure grouting column can be overlapping or tangential, with the horizontal length limited to around 15m.
The high-pressure jet grouting-method (HDI-Verfahren) was used in the 282m long under-ground driven section of the Frankfurter Kreuz tunnel. The advance support consisted of 14.5m long, overlapping horizontal jet grouted piles.
Ground freezing
Ground freezing, mentioned only for reasons of completeness, is a well known method in civil engineering. It affords a temporary seal and improvement for the subsurface ground in the form of an advance support for non-groutable, but water-bearing conditions. Basically, screen covers are created.
In contrast to the grouting methods, freezing affects the geological conditions only during the construction period.
Summary
Using screens as advance support and the roof beam method, possibilities for the economical use of top heading methods are extended. These measures also contribute to increased safety at the face, which is of great importance today, because of the increasing shortage of experienced workers. Resorting to expensive side wall drifts will be confined to difficult conditions where there is high lateral pressure and special demands for settlement control. This way, it is possible to keep conventional tunnelling methods competitive with mechanised tunnelling.
Related Files
Franfurter Kreuz tunnel
Irlahull tunnel
