202 terzaghi. SOFT-GROUND TUNNELING [Ch. 11 



Compressed air under adequate pressure has the same beneficial effects 

 as successful drainage. It transforms flowing into running, raveling, or 

 slowly squeezing ground. If the air pressure is lower than about 18 

 pounds per square inch, corresponding to a head of water of 36 feet, 

 the compressed air has no detrimental physiological effects. If the air 

 pressure is increased beyond 18 pounds per square inch, its effects on 

 the human organism become more and more harmful; the number of 

 working hours per shift must be reduced; at the same time the wages 

 per shift go up, and an air pressure of about 50 pounds per square 

 inch, corresponding to a head of water of 100 feet, is the highest the 

 organism of a normal person can stand. Hence the use of compressed 

 air is limited to tunnels located at a depth of less than 100 feet below 

 the water table. Furthermore, the use of compressed air requires the 

 installation of a compressor plant and of airlocks, which are expensive. 

 For this reason, in short tunnels or when a tunnel is driven over a short 

 distance through flowing ground, it may be more economical to drain 

 the ground, to consolidate the ground ahead of the working face by 

 means of injections, or, as a last resort, to try the original procedure 

 for mining through flowing ground, shown in Fig. 4c?. 



The shield method of tunneling is illustrated by Figs. 4e and 4/. A 

 shield is merely a ring-shaped biscuit cutter which is shoved ahead by 

 means of hydraulic jacks. If the shield travels through coarse-grained 

 flowing ground, it is necessary to transform the flowing into running or 

 raveling ground by means of compressed air. The ground must be ex- 

 cavated ahead of the shield, the roof located beyond the cutting edge 

 must be supported by poling boards or other convenient means, and 

 the face must be breasted as shown in Fig. 4e. 



In a shield tunnel the pressure on the breast boards can easily be 

 transferred onto the heavy steel beams that subdivide the central 

 opening into smaller panels. Furthermore if a run or a blow occurs, the 

 miner has at his disposal ready-made supports against which he can 

 brace his emergency bulkheads. However, a shield is very expensive, 

 and the shield method of tunneling requires a very much heavier tem- 

 porary lining than does the method of hand mining, because the lining 

 must be strong enough to sustain the heavy longitudinal pressure ex- 

 erted by the jacks. Hence, if the ground conditions call for excavation 

 ahead of the shield, it is commonly more economical to construct the 

 tunnel without the assistance of a shield. 



In soft, flowing ground such as river silt, no excavation ahead of 

 the shield is required. It suffices to push the shield ahead by means 

 of hydraulic jacks. The ground ahead of the shield is displaced and 



