44 TUNNELING 



warning the engineer of the approach of danger; while oak and 

 other strong timbers resist until the last moment, and then 

 \ield suddenly under tin- breaking load. Soft woods, moreover, 

 are usuallv lighter in weight than hard woods, which is a con- 

 siderable advantage where so much handling is required in 

 a ivstrieted sp.ice. Round timbers are generally employed, 

 si:u-e thev are less expensive, and quite as satisfactory in other 

 respects as sawed timbers. In the English and Austrian 

 methods of strutting, which are described further on, a few 

 of the principal struts are of sawed timbers. 



The various timbers of the strutting are seldom 

 attached by framed joints, but wedges are used 

 to give them the necessary 

 bearing against each other. 

 Where framed joints are em- 

 Fio.i8.-joinin g Tmmei struts p i oye d they are made of the 



by Halving. J 



simplest form usually by 



J J FJG. 19. Round 



halving the joining timbers, as shown by Fig. 18. Timber Post 

 Fig. 19 shows a form of joint used where round g d ca P Bear- 

 posts carry beams of similar shape. The reason why 

 it is possible to do away with jointed connections to such a 

 great extent, is that the strains which the timbers have to 

 resist are either compressive or bending strains, and because 

 the timbei-s are so short that they do not require to be spliced. 

 Strutting of Headings. The method of strutting the head- 

 ing that is employed depends upon the material through which 

 the heading is driven. In solid rock strutting may not be 

 required at all, or only for the purpose of preventing the 

 fall of loose blocks from the roof, then vertical props are 

 erected where required, or horizontal beams are inserted into 

 the side walls, as shown by Fig. 20. These horizontal beams 

 may be used singly at dangerous places, or they may be placed 

 from 2 ft to 3 ft. apart all along the heading. In the latter 

 case they usually carry a lagging of planks, w r hich may be 

 placed at intervals or close together, and filled above "with 



