200 terzaghi. SOFT-GROUND TUNNELING [Ch. 11 



METHODS OF SOFT-GROUND TUNNELING 



The methods used in constructing an earth tunnel must be adapted 

 to the stand-up time t s of the ground and to the performance of the 

 ground during the process of mining. The following paragraphs con- 

 tain a brief review of the methods, illustrated by Fig. 4. 



In firm ground, with a stand-up time t s of more than about one day, 

 no temporary support is required (Fig. 4a) . In raveling ground with a 

 t s value between 1 day and 5 minutes and in squeezing ground, steel 

 liner plates are mined in, one by one, and assembled into rings or 

 "courses" as shown in Fig. 4b. This can be done without giving the 

 ground an opportunity to move. The face does not require any lateral 

 support. In cohesive running or running ground, the leading edge of 

 the roof and side support must be kept ahead of the excavation. This 

 is accomplished by means of "poling boards" or "poling plates" (Fig. 

 4c) . The face must be supported by means of breast boards. As ex- 

 cavation proceeds, the breast boards are carried forward, one by one, 

 over a distance equal to the width of one course of liner plates, start- 

 ing at the crown. 



Really serious difficulties are encountered only in flowing ground. 

 It appears that the problem of mining through flowing ground was not 

 adequately solved until the beginning of the nineteenth century, when 

 coal-mining operations were started on a large scale. The technique 

 that was developed under the pressure of necessity is illustrated by 

 Fig. 4d. The roof and the sides are supported by "poling boards" 

 which are driven at a slight angle to the center line of the tunnel. The 

 face is breasted, and the tunnel bottom is covered with a timber floor 

 strong enough to prevent a heave of the ground located beneath the 

 tunnel. The procedure calls for expert carpenters, because no open 

 joints between boards can be tolerated. "Where the material is very 

 bad only small openings should be made and while one miner is making 

 it, another must stand ready with hay and filling material to stop the 

 hole as soon as enough ground has, for the time, been admitted; this 

 stopping must often be done while the man is splashed over with the 

 pouring mass and standing knee-deep in it, groping with his eyes shut 

 to perform his task. It is hardly necessary to say that none but the 

 best and most reliable workmen can be depended on to show the requi- 

 site coolness and dexterity at the right moment" (Drinker, 1878) . In 

 spite of skill and utmost precaution, catastrophes can not always be 

 avoided, and under difficult conditions the progress slows down to 1 

 foot per week, two twelve-hour shifts per day. 



