Application Of Compressed Air

9 Application of compressed air

The idea to apply compressed air to prevent groundwater from entering into excavated spaces goes back to Sir Thomas Cochrane, who obtained a patent in 1830.1 Tunnelling under compressed air is connected with the following hazards:2 1. Health problems 2. Fire due to increased oxygen concentration (fires ignite more easily, burn more rigorously and are more difficult to extinguish) 3. Blow outs. Compressed air is mainly applied in loose sandy or silty soils which are headed conventionally or with shield and is also applicable in the cover and cut method: Shield heading: The face is often supported with pressurised slurry or earth spoil. However, for maintenance one has to enter the excavation chamber. On this, slurry is removed and the support is accomplished by air pressure. Air pressure support can also be permanent within an appropriately closed part of the tunnel. Of course, locks must be provided for. The same applies to pipejacking (Fig. 9.1). Conventional heading: The tunnel is sealed by a bulkhead and pressurized with air. Air locks permit access through the bulkhead. At the tunnel face, hydrostatic groundwater pressure increases linearly with depth, whereas the air pressure is approximately constant. Consequently, the air pressure can only balance the water pressure at some level, above which the air pressure exceeds the water pressure. Air escapes through the unprotected face and through fissures and weak spots of the shotcrete lining. Of course, the proportion of the latter losses increases with the length of pressurized tunnel. It is very difficult to predict the air losses, which are reported to 1

R. Glossop, The invention and early use of compressed air to exclude water from shafts and tunnels during construction. G´eotechnique 26, No. 2, 253-280 (1976) 2 Changes in the air. Tunnels & Tunnelling International, January 2002, 26-29

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Fig. 9.1. Pipejacking with air pressure and locks at the start shaft.

amount to between 20 and 700 m3 /min. Table 9.1 shows some specific examples. The corresponding costs for compressors range from 10,000 to 200,000 ¤ per month.3 It should be taken into account that the permeability of partially saturated soil with respect to air increases with time. Compressed air is effective in all ground conditions (including fissured rock), provided the air losses can be controlled. For the U2-subway in Munich, the permeability of the ground was extremely high so that the overburden had to be grouted to avoid extreme air loss. City

Length (m) driven Air pressure under compressed air (bar) Munich 6,961 0.3 - 1.1 Essen 1,330 0.4 - 1.2 Taipei 400 0.8 - 1.4 Siegburg 240 0.6 - 1.2

Air loss (m3 /min) 25 - 580 52 - 250 50 - 180 50 - 450

Table 9.1. Examples of application of compressed air in combination with NATM4

Cover and cut: The application of air pressure in the cover and cut tunnel construction in groundwater is shown in Fig. 9.2: The cover is buttressed 3 S. Semprich, Tunnelbau unter Druckluft – ein immer wiederkehrendes Bauverfahren zur Verdr¨ angung des Grundwassers. TA Esslingen, Kolloquium ’Bauen in Boden und Fels’, Januar 2002. 4 S. Semprich and Y. Scheid, Unsaturated flow in a laboratory test for tunnelling under compressed air. 15th Int. Conf. Soil Mech. and Geot. Eng., Istanbul, Balkema, 2001, Vol. 2, 1413-1417.

9.1 Health problems

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on two diaphragm walls, subsequently the soil is removed whereas groundwater is kept off by means or air pressure. The needed air supply must be empirically estimated for the proper choice of compressors. The air leakage is composed of the losses at the face, along the tunnel wall and at the locks.

Fig. 9.2. Application of compressed air in cover and cut tunnelling

9.1 Health problems Increasing the air pressure implies that more air is dissolved into the blood. The surplus oxygen is supplied to the cells, whereas nitrogen remains dissolved and drops out in case of decompression. If the decompression occurs too fast, then bubbles can appear in the blood, the joints and the tissue causing decompression illness, which is accompanied with pain in the joints and can lead to embolism. Therefore, decompression has to occur gradually. The required time increases with pressure and retention period. In the German Standard5 tables (’diving tables’) indicate the required decompression time. Persons are allowed stay in pressures up to 3.6 atmospheres, their age is limited between 21 and 50 years. Complaints can appear even 12 hours after decompression. The only reasonable treatment is to put the person again into a pressurized chamber. In construction sites with more than 1 atmosphere air pressure a special recompression chamber for ill persons must be provided for. Persons working within pressurized air must always carry a red card with appropriate hints that help to avoid unneeded medical treatments in case of sudden illness. According to Table 9.2, in cases I and II 5

Druckluftverordnung of 4th October 1972 (BGBl. I p. 1909) last change of 19th June 1997 (BGBl. I p. 1384). See also: Work in Compressed Air Regulations 1996 and accompanying guidance document L96 (UK), BS 6164: 2001 ’Code of practice for safety in tunnelling in the construction industry’, and the draft CEN standard prEN12110 ’Airlocks-safety requirements’.

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the person must be withdrawn from the lock, whereas in case III a recompression is needed. The decompression time can be reduced by ca 40% with respiration of pure oxygen (with a mask). Note, however, that pure oxygen is toxic in pressures above 1 bar. Another disease, osteonecrosis, is manifested as corroboration of the joints. It can appear many years after the work under compressed air.

Fig. 9.3. Tunnelers in decompression chamber6

In the Netherlands exposures to air pressures up to 4.5 bar (in some cases 7 bar) are allowed for maintenance works in slurry shields.7 At pressures above 3.6 bar, nitrogen narcosis was observed: divers worked slower and made more mistakes. Special gas mixtures had to be inhaled via helmets. The following air pressure diseases are distinguished:

6

¨ ¨ Osterreichische Ingenieur- und Architekten-Zeitschrift (OIAZ), 142, 4/1997, p. 247 7 J. Heijbor, J. van der Hoonaard, F.W.J. van de Linde, The Westerschelde Tunnel, Balkema 2004

9.3 Blow-outs

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Symptom appears at I pain in the tympanum increasing pressure II rapture of the deep constant pressure III pain in sinus and joints decreasing pressure Table 9.2. Symptoms due to compressed air

9.2 Influence on shotcrete The increased moisture of compressed air accelerates setting. If the adjacent rock is permeable, the shotcrete will be percolated by large amounts of air, which lead to drying and shrinkage and, thus, to reduced strength. Countermeasures are: moistening of fresh shotcrete, additives to reduce its permeability and early sealing the shotcrete surface. If the adjacent rock is relatively impermeable, compressed air is favourable for the shotcrete.

9.3 Blow-outs If the pressurized air pipes through the soil, it can abruptly escape, thus causing a sudden pressure drop in the tunnel. The pressure drop is accompanied with a bang and the formation of mist and can lead to collapse of the tunnel. It is in most cases announced with increasing air leakage. To avoid such blowouts it is necessary that the total primary stress at the tunnel crown exceeds the air pressure by 10%. To achieve this, it is sometimes needed to bring in an embankment at the ground surface. Its width should be 6 times the diameter of the tunnel.