194 tebzaghi. SOFT-GROUND TUNNELING [Ch. 11 



of the same sand may give rise to a "blow/' and, if this layer communi- 

 cates with a large body of water-bearing sand within a short distance 

 from the tunnel, a catastrophe may result. 



TUNNEL MAN'S GROUND CLASSIFICATION 



Unconsciously using the concept of the stand-up time as a criterion, 

 the tunnel man distinguishes between six principal categories of 

 "ground," namely, firm, raveling, running, flowing, squeezing, and 

 swelling ground. 



In firm ground the tunnel heading can be advanced without any 

 roof support, and the permanent lining can be constructed before the 

 ground will start to move. Typical representatives of firm ground 

 are loess above the water table and various calcareous clays with low 

 plasticity such as the marls of South Carolina. 



In raveling ground chunks or flakes of material begin to drop out 

 of the roof or the sides some time after the ground has been exposed. 

 If the process of raveling starts within a few minutes, the ground is 

 fast-raveling. Otherwise it is referred to as slowly raveling. Fast- 

 raveling conditions are likely to be encountered in residual soil or in 

 sand with a clay binder located below the water table. Above the 

 water table the same soils may be slowly raveling or even firm. 



In running ground the removal of the lateral support on any surface 

 rising at an angle of more than about 34° to the horizontal is followed 

 by a "run," whereby the material flows like granulated sugar until 

 the slope angle becomes equal to about 34°. Running conditions pre- 

 vail in clean, loose gravel and in clean, coarse or medium sand above 

 the water table. In clean, fine, moist sand the run is preceded by a 

 brief period of raveling. A ground with such behavior is cohesive- 

 running. 



Flowing ground moves like a viscous liquid. In contrast to running 

 ground, it can invade the tunnel not only through the roof and the 

 sides but also through the bottom. If the flow is not stopped, it con- 

 tinues until the tunnel is completely filled. The rush of flowing ground 

 into a tunnel is sometimes referred to as a blow. Flowing conditions 

 prevail in any ground with an effective grain size in excess of about 

 0.005 millimeter, provided that the tunnel is located below the water 

 table. Above the water table the same ground has the character of 

 a raveling or running ground. 



Squeezing ground slowly advances into the tunnel without any signs 

 of fracturing and without perceptible increase of the water content of 



