TIMBERING OR STRUTTING Tl NNEL8 43 



CHAPTER V. 



METHODS OF TIMBERING OR STRUTTING 

 TUNNELS. 



THE purpose of timbering or strutting in tunnel work is to 

 prevent the caving-in of the roof and side walls of the exca- 

 vation previous to the construction of the lining. As the 

 strutting has to resist all the pressures developed in the roof 

 and side walls, which may be exceedingly troublesome and 

 of great intensity in loose soils, its design and erection call 

 for particular care. The method of strutting adopted depends 

 upon the method of excavation employed ; but in every case 

 the problem is not only to build it strong enough to withstand 

 the pressures developed, but to do this as economically as 

 possible, and with as little hindrance as may be to the work 

 which is going on simultaneously and which will come later. 

 Only the latter general problems of strutting i*culiar to all 

 methods of tunnel work will l>e considered here. For this 

 consideration strutting may be classified according to the 

 material of which it is built, under the heads of timber struc- 

 tures and iron structures. 



TIMBER STRUTTING. 



Timber is nearly always employed for strutting in tunnel 

 work. So long as it has the requisite strength, any kind of 

 timber is suitable for strutting, since, it hoing only temporarily 

 employed, its durability is a matter of slight importance. 

 Timber with good elastic properties, like j.ine or spnii 

 preferably chosen, since it yields gradually under stress, thus 



