Ch. 11] 



TREATMENT OF SWELLING GROUND 



197 



Settlement 



WPP«^ 



In fine-grained soils such as silt or soft clay or in mixed-grained 

 soils with a silt or clay matrix, the air does not enter the voids at all. 

 These soils are changed by compressed air from flowing or rapidly 

 squeezing into raveling or slowly squeezing ground. The reason is 

 illustrated by Fig. 2, representing a vertical section through the center 

 line of a tunnel in soft clay. The clay tends to flow along a curved sur- 

 face of sliding ab into the tunnel. This tendency is due partly to the 

 weight of the clay located above ab and partly to the seepage pressure 

 exerted by the water which per- 

 colates toward the heading. 

 Compressed air under a pressure 

 equal to the water pressure at 

 the level of point b not only 

 stops the seepage as shown in 

 Fig. 1, but it also resists the 

 tendency of the solid clay par- 

 ticles to descend toward the 

 heading. 



A third possibility for improv- 

 ing the ground conditions in 

 earth tunnels consists in clogging 

 the voids of the ground sur- 

 rounding the tunnel by grouting. 

 So far this method has been used 

 only for reducing air leakage through gravel strata and for reinforcing 

 the ground located between the roof of shallow tunnels and the 

 base of foundations located above the roof (Harding and Glossop, 

 1940). 



The limits which nature has imposed on the successful application 

 of the different methods for improving the ground conditions in earth 

 tunnels are discussed below in the section on "Methods of Soft-Ground 

 Tunneling." 



Fig. 2. Diagram illustrating the cause 



of the "squeeze" in tunnels through soft 



clay. The clay moves slowly along ab 



toward the heading. 



TREATMENT OF SWELLING GROUND 



Even a very dangerous swelling ground may behave like a firm 

 ground at the heading. However, a few hours or days after exposure 

 the ground begins to advance toward the tunnel, and the ground move- 

 ment continues until the tunnel is closed up, unless the movement is 

 stopped by the construction of a tunnel support. The movement is 

 associated with a considerable increase of the volume of the material 

 surrounding the tunnel. After the tunnel support is erected and back- 



