Ch. 11] 



METHODS OF SOFT-GROUND TUNNELING 



201 



On account of the costs and hazards involved in mining through flow- 

 ing ground, the attempts to simplify the procedure started very early. 

 They led to various refinements of the methods of drainage and, as 

 early as the middle of the nineteenth century, to the invention of the 

 compressed-air and the shield methods of tunneling. 



The original method of draining the ground surrounding a large 



g^\V//\^X\//\\V/y, 



4>lday p 

 | 



(a) Firm Ground 

 Liner plates 



// A\V/X\V/A \^aV/a\^. 



TH/ 1 

 I 



! 



(d) Flowing Ground (original method) t s =0 



(b) Raveling or Squeezing Ground 

 , Breasting 



4>5 min. 

 (e) Coarse-Grained Flowing Ground (shield method) 



mmzmm® 



t s <5 min. 

 (c) Cohesive Running Ground 



t s =0 



if) Soft Flowing Ground 

 (shield method) 



Fig. 4. Principal types of temporary tunnel supports. 



tunnel by means of drainage galleries located on both sides of the 

 tunnel was superseded by pumping from filter wells or wellpoints. The 

 wells are drilled on both sides of the tunnel from the surface of the 

 ground to a depth of 10 or 15 feet below the bottom of the tunnel, or 

 else they are installed on the bottom of the tunnel. New wells are 

 drilled as the heading advances. Both procedures have been suc- 

 cessfully used on tunnel jobs. However, drainage by pumping from 

 wells is impracticable unless the effective grain size D 10 of the most 

 permeable layers in the ground surrounding the tunnel is greater than 

 about 0.05 millimeter. Under exceptional conditions, finer soils can 

 be drained by means of the vacuum or electro-osmotic method (Ter- 

 zaghi and Peck, 1948, pp. 337-340) . 



The function of the compressed air in tunnels is illustrated by Fig. 1. 



