66 TUNNELING 



ceeding chapters ; but they are generally either of the king-post 

 or queen-post type, or some modification of them. The king- 

 post truss may be used alone, with 

 or without the tie-beam, as shown 

 by Fig. 43 ; but generally a queen - 

 post truss is made to form the 

 base of support for a smaller king 

 post truss mounted on its top. 



FIG. 43. Trussed Center for Constructing; * . 



the Roof Arch. This arrangement gives a trape- 



zoidal form to the center, which approaches closely to the arch 

 profile. Owing to the character of the pressures transmitted to 

 the center, the usual tension members can be made very light. 



The combined center and strutting system devised by Mr. 

 Rziha has already been described in a previous chapter. In 

 recent European tunnel work quite extensive use has also been 

 made of iron centers consisting of several segments of curved 

 I-beams, connected by fish-plate joints so as to form a semi- 

 circular arch rib. The ends or feet of these I-beam ribs have 

 bearing-plates or shoes made by riveting angles to the I-beams. 

 Centers constructed in a similar manner, but made of sections 

 of old railway rail, were used in carrying out the tunnel work 

 on the Rhine River Railroad in Germany. The advantages 

 claimed for iron centers are that they take up less room, and 

 that they can be used over and over again. 



Setting Up Centers. According to the n:ethcd of excava- 

 tion followed in building the tunnel, the centers for building 

 the roof arch may have to be supported by posts resting on the 

 tunnel floor ; or where the arch is built first, as in the Belgian 

 and Italian methods, they may be carried on blocking resting 

 on the unexcavated earth below, Whichever method is em- 

 ployed, an unyielding support is essential, and care must be 

 taken that the centers are erected and maintained in a plane 

 normal to the tunnel axis. To prevent deflection and twisting, 

 the consecutive centers are usually braced together by longi- 

 tudinal struts or by braces running to the adjacent strutting. 



