48 TUNNELING 



pol ing-boards parallel to the axis of the tunnel. This system 

 of strutting is peculiar to the English method of tunneling. 

 The longitudinal timl>ers rest on this finished masonry at one 

 end, and are carried on a cross frame or by props at the other 

 end. At intermediate points the longitudinals are braced 

 apart by struts in planes transverse to the tunnel axis. This 

 construction makes a very strong strutting framework, since 

 the transverse struts act as arch ribs to stiffen the longitu- 

 dinals; but the use of transverse poling-boards requires the 

 excavation of a larger cross-section than is necessary when longi- 

 tudinal poling-boards are employed, and this increases the 

 cost both for the amount of earth excavated and the greater 

 quantity of filling required. 



In polygonal strutting the main members are in a plane 

 normal to the axis of the tunnel. They form a polygon whose 

 sides follow closely the sectional profile of the excavation. 

 These polygonal frames are placed at more or less short inter- 

 vals apart, and are braced together by short longitudinal struts 

 lying close to the sides of the excavation, and running from 

 one frame to the next, and also by longer longitudinal members 

 which extend over several frames. The polygonal system of 

 strutting is peculiar to the Austrian method of tunneling, and 

 is fully described in a succeeding chapter. One of its distinc- 

 tive characteristics is that 

 the poling-boards are in- 

 serted parallel to the tunnel 

 axis. Polygonal strutting 

 is generally held to be 

 stronger than longitudinal 

 strutting under uniform 

 loads, but it is more liable 



I- 10. 26. -Shaft with Single Transverse to distortion when the 



Strutting. 



loads are unsym metrical. 



Strutting of Shafts. Tunnel shafts are strutted both to 

 prevent the caving-in of the sides and to divide them into 



