174 TUNNELING 



be strutted by planks laid close, and held in position by interior 

 framework ; the seams between the plank should be packed 

 with straw. 



(8) The masonry lining should be built in successive rings, 

 and the work so arranged that the water seeping in at the sides 

 and roof is collected and removed from the tunnel immediately. 



Excavation. The best and most commonly employed method 

 of driving tunnels through quicksand is a modification of the 

 Belgian method. At first sight it may appear a hazardous work 

 to support the roof arch, as is the characteristic of this method, 

 on the unexcavated soil below, when this soil is quicksand, but 

 if the sand is well confined and drained the risk is really not 

 very great. Next to the Belgian method the German method 

 is perhaps the best for tunneling quicksand. In these compari- 

 sons the shield system of tunneling is for the time being left 

 out of consideration. This method will be described in suc- 

 ceeding chapters. Whenever any of the systems of tunneling 

 previously described are employed, the first task is always to 

 open a drainage gallery at the bottom of the section. 



Assuming the Belgian method is to be the one adopted, the 

 first work is to drive a center bottom drift, the floor of which 

 is at the level of the extrados of the invert. This drift is im- 

 mediately strutted by successive transverse frames made up of 

 a sill, side posts, and a cap which support a close plank strut- 

 ting or lining, with its joints packed with straw. Between the 

 side posts of each cross-frame, at about the height of the 

 intrados of the invert, a cross-beam is placed ; and on these cross- 

 beams a plank flooring is laid, which divides the drift horizon- 

 tally into two sections, as shown by Fig. 102; the lower section 

 forming a covered drain for the seepage water, and the upper 

 providing a passageway for workmen and cars. The bottom 

 drift is driven as far ahead as practicable, in order to drain the 

 sand for as great a distance in advance of the work as possible. 

 After the construction of the bottom drainage drift the excava- 

 tion proper is begun, as it ordinarily is in the Belgian method 



