TUNNELS THltorcil HARD ROCK 



EXCAVATION BY DRIFTS: MONT CENIS TUNNEL. 



General Description. The method of tunneling through 

 hard rock by drifts is preferred by European engineers. Both 

 UK- Mont Cenis tunnel built in 1857-70, and the great Simplou 

 tunnel now under construction, are examples of tunneling by 

 drifts. In this method the sequence of excavation is showu 

 diagrammatically by Fig. 54. As soon as the top portion of 

 the section has been opened, the roof arch is built with its feet 

 resting on the tops of parts No. 4. These parts are removed 

 by breaking down the outer portion 

 between the sides of part No. 1 and 

 the lines a b and a 1 l> 1 first, and 

 thru by driving transverse cuts 

 th rough to the sides of the section 

 at intervals, and filling them with 

 the masonry of the side walls. 

 These short sections or pillars of 

 masonry serve to carry the arch 

 while the rock between them is 

 bein^ excavated and the remainder 

 of the side walls built. In hard 

 rock the successive parts Nos. 1 to 

 4 are driven several hundred feet in advance of each other. 



The drift is usually strutted by means of side jxwU carrying 

 a cap-piece placed at intervals, and having a ceiling of longi- 

 tudinal planks resting on the successive caps. In hard nn-k 

 the roof of the section does not, as a rule, require regular 

 strutting, occasional supports In-ing placed ;it intervals to pre- 

 vent the fall of isolated fragments. When the rock is disinte- 

 grated or full of seams, a regular strutting may be neoes 

 and this may Iw either longitudinal or polygonal in tyj-. 

 \Vln-ii longitudinal strutting is employed, a sill is laid acrom 

 the roof of the drift, and upon this are set up two struts con- 



Fio. M. Diagram Showing Se- 

 quence of KkfitviitK-n- ill Drift 

 M.-tli.-l ..f Tiiniii'liiitf K<K-k. 



