Ch. 11] CHANGING GROUND CONDITIONS IN TUNNELS 195 



the ground surrounding the tunnel. Although the squeeze may not 

 even be noticed, the loss of ground due to squeeze may be important 

 enough to produce a conspicuous trough-like subsidence of the ground 

 surface above the tunnel. Squeezing conditions are met with in every 

 tunnel through soft or medium clay. 



Swelling ground, like squeezing ground, moves slowly into the tun- 

 nel, but the movement is associated with a very considerable volume 

 increase "of the ground surrounding the tunnel. Swelling conditions 

 are likely to develop in tunnels through heavily precompressed clays 

 with a plasticity index in excess of about 30, and in sedimentary 

 formations containing layers of anhydrite. 



PROCEDURES FOR CHANGING GROUND CONDITIONS 



IN TUNNELS 



All the serious difficulties that may be encountered during the con- 

 struction of an earth tunnel are directly or indirectly due to the perco- 

 lation of water toward the tunnel. Therefore most of the techniques 

 for improving the ground conditions are directed toward stopping the 

 seepage. 



In cohesionless or slightly cohesive ground, such as clean or silty 

 sand, a mixture of soil and water enters the tunnel through every gap 

 in the lining. Such "flows" or "blows" can be avoided by three dif- 

 ferent means. The water table can be lowered to a level located be- 

 low the bottom of the tunnel by drainage ; the tunnel can be filled with 

 compressed air under a pressure equal to the water pressure at the 

 bottom of the tunnel, or the voids of the ground surrounding the tun- 

 nel can be clogged by the injection of suitable materials such as ce- 

 ment or mixtures of chemicals. 



The water that occupies the voids of a porous material above the 

 water table is retained in the voids by capillary forces. (See, for in- 

 stance, Terzaghi and Peck, 1948, pp. 115-129.) Hence, if the water 

 table is lowered by drainage to a level below that of the bottom of the 

 tunnel, the water ceases to percolate toward the tunnel, though the 

 voids of the ground may remain almost or entirely filled with water. 

 Drainage changes flowing ground into raveling or running ground 

 which can easily be handled. 



Another method of improving the properties of soft ground involves 

 the use of compressed air. The principle of this method is illustrated 

 by Fig. 1. Figure la is a vertical section through the heading of a 

 tunnel in flowing ground. The tunnel is filled with compressed air 



