9

Application of compressed air

The idea to apply compressed air to prevent groundwater from entering intoexcavated spaces goes back to Sir Thomas Cochrane, who obtained a patentin 1830.1

Tunnelling under compressed air is connected with the following hazards:2

1. Health problems2. 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 headedconventionally or with shield and is also applicable in the cover and cutmethod:

Shield heading: The face is often supported with pressurised slurry or earthspoil. 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 closedpart of the tunnel. Of course, locks must be provided for. The same appliesto pipejacking (Fig. 9.1).

Conventional heading: The tunnel is sealed by a bulkhead and pressurizedwith air. Air locks permit access through the bulkhead. At the tunnel face,hydrostatic groundwater pressure increases linearly with depth, whereasthe air pressure is approximately constant. Consequently, the air pressurecan only balance the water pressure at some level, above which the airpressure exceeds the water pressure. Air escapes through the unprotectedface and through fissures and weak spots of the shotcrete lining. Of course,the proportion of the latter losses increases with the length of pressurizedtunnel. It is very difficult to predict the air losses, which are reported to

1R. Glossop, The invention and early use of compressed air to exclude water fromshafts and tunnels during construction. Geotechnique 26, No. 2, 253-280 (1976)

2Changes in the air. Tunnels & Tunnelling International, January 2002, 26-29

198 9 Application of compressed air

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 specificexamples. The corresponding costs for compressors range from 10,000 to200,000 ¤ per month.3 It should be taken into account that the perme-ability of partially saturated soil with respect to air increases with time.Compressed air is effective in all ground conditions (including fissuredrock), provided the air losses can be controlled. For the U2-subway inMunich, the permeability of the ground was extremely high so that theoverburden had to be grouted to avoid extreme air loss.

City Length (m) driven Air pressure Air loss

under compressed air (bar) (m3/min)

Munich 6,961 0.3 - 1.1 25 - 580

Essen 1,330 0.4 - 1.2 52 - 250

Taipei 400 0.8 - 1.4 50 - 180

Siegburg 240 0.6 - 1.2 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 tunnelconstruction in groundwater is shown in Fig. 9.2: The cover is buttressed

3S. Semprich, Tunnelbau unter Druckluft – ein immer wiederkehrendes Bauver-fahren zur Verdrangung des Grundwassers. TA Esslingen, Kolloquium ’Bauen inBoden und Fels’, Januar 2002.

4S. Semprich and Y. Scheid, Unsaturated flow in a laboratory test for tunnellingunder compressed air. 15th Int. Conf. Soil Mech. and Geot. Eng., Istanbul, Balkema,2001, Vol. 2, 1413-1417.

9.1 Health problems 199

on two diaphragm walls, subsequently the soil is removed whereas ground-water is kept off by means or air pressure. The needed air supply mustbe empirically estimated for the proper choice of compressors. The airleakage is composed of the losses at the face, along the tunnel wall and atthe 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 dissolvedand 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 de-compression illness, which is accompanied with pain in the joints and can leadto embolism. Therefore, decompression has to occur gradually. The requiredtime increases with pressure and retention period.In the German Standard5 tables (’diving tables’) indicate the required decom-pression time. Persons are allowed stay in pressures up to 3.6 atmospheres,their age is limited between 21 and 50 years. Complaints can appear even12 hours after decompression. The only reasonable treatment is to put theperson again into a pressurized chamber. In construction sites with more than1 atmosphere air pressure a special recompression chamber for ill persons mustbe provided for. Persons working within pressurized air must always carry ared card with appropriate hints that help to avoid unneeded medical treat-ments in case of sudden illness. According to Table 9.2, in cases I and II

5Druckluftverordnung of 4th October 1972 (BGBl. I p. 1909) last change of 19thJune 1997 (BGBl. I p. 1384). See also: Work in Compressed Air Regulations 1996and accompanying guidance document L96 (UK), BS 6164: 2001 ’Code of practicefor safety in tunnelling in the construction industry’, and the draft CEN standardprEN12110 ’Airlocks-safety requirements’.

200 9 Application of compressed air

the person must be withdrawn from the lock, whereas in case III a recom-pression is needed. The decompression time can be reduced by ca 40% withrespiration of pure oxygen (with a mask). Note, however, that pure oxygen istoxic in pressures above 1 bar. Another disease, osteonecrosis, is manifestedas corroboration of the joints. It can appear many years after the work undercompressed air.

Fig. 9.3. Tunnelers in decompression chamber6

In the Netherlands exposures to air pressures up to 4.5 bar (in some cases7 bar) are allowed for maintenance works in slurry shields.7 At pressuresabove 3.6 bar, nitrogen narcosis was observed: divers worked slower and mademore mistakes. Special gas mixtures had to be inhaled via helmets.The following air pressure diseases are distinguished:

6Osterreichische Ingenieur- und Architekten-Zeitschrift (OIAZ), 142, 4/1997,p. 247

7J. Heijbor, J. van der Hoonaard, F.W.J. vande Linde, The Westerschelde Tunnel,Balkema 2004

9.3 Blow-outs 201

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 adjacentrock is permeable, the shotcrete will be percolated by large amounts of air,which lead to drying and shrinkage and, thus, to reduced strength. Counter-measures are: moistening of fresh shotcrete, additives to reduce its permeabil-ity and early sealing the shotcrete surface. If the adjacent rock is relativelyimpermeable, compressed air is favourable for the shotcrete.

9.3 Blow-outs

If the pressurized air pipes through the soil, it can abruptly escape, thus caus-ing a sudden pressure drop in the tunnel. The pressure drop is accompaniedwith 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 blow-outs it is necessary that the total primary stress at the tunnel crown exceedsthe air pressure by 10%. To achieve this, it is sometimes needed to bring in anembankment at the ground surface. Its width should be 6 times the diameterof the tunnel.