00 TUNNELING 



verging toward the top and supporting a cap-piece close to 

 the roof. On this cap-piece are placed the first longitudinal 

 crown bars carrying transverse poling-boards. Additional 

 props standing on the sill and radiating outward are inserted 

 as parts No. 3 are excavated. These radial props carry 

 longitudinal bars which in turn support transverse poling- 

 boards. When polygonal strutting is used, it may have the 

 construction described below as being employed in the Mont 

 Cenis tunnel, or may take the form of three or five segment 

 arches of heavy timbers. 



The roof arch, usually of brick masonry, is built before the 

 side walls, which are generally of rubble masonry, with its feet 

 supported temporarily by the unexcavated rock below. Plank 

 centers are usually employed, since the pressures they carry are 

 usually limited to the weight of the masonry. The method 

 of underpinning the roof arch with the side walls is that pecu- 

 liar to the Belgian method of tunneling. The drain is usually 

 constructed of brick masonry, and may be located at the center 

 or at one side of the tunnel floor. 



Tunnels excavated by drifts enable simple means of hauling 

 to be employed, and this is one of the reasons why the method 

 finds so much favor with European engineers. The tracks 

 are laid along the floor of the drift, and carry all the spoil 

 from parts Nos. 2, 3, and 4, as well as from the front of the 

 drift itself. As fast as the full section is completed, this single 

 track in the drift is replaced by two tracks running close to 

 the sides of the tunnel, or by a broad-gauge track with a third 

 rail. 



Mont Cenis Tunnel. The Mont Cenis tunnel was the first 

 of the great Alpine tunnels to be built It is 7.9 miles long, 

 and connects France and Italy by a double-track railway. Con- 

 struction was begun in 1857, and the tunnel was opened for 

 traffic in 1872. 



Material Penetrated. The material penetrated by the ex- 

 cavation consisted chiefly of limestone, calcareous schist, gneiss, 



