TYPES OF CENTERS AND MOLDS 65 



arch masonry during construction, ami of the roof pressures 

 which are transferred to the center when the strutting is 

 removed to allow the masonry to be placed. The framework 

 of the center is supported either by posta resting upon the floor 

 of the excavation, or upon tin- invert masonry when this is 

 built first, as in the English and Austrian methods, or it may 

 be supported directly upon the ground where the arch masonry 

 is built first, as in the Belgian method of tunneling. 



In describing the various methods of tunneling in succeed- 

 ing chapters, the center construction and method of supporting 

 the center jieculiar to each will l>e fully explained, and only a 

 fc\\ ^cin-nil remarks are necessary here. Centers may be classi- 

 tie<l according to their construction and conijmsition into plank 

 centers, truss centers, and iron centers. 



One of the most common forms of plank centers is shown 

 by Fig. 42. It consists of two 

 half-polygon! whose sides consist 

 of 15 in. X 4 ft planks having 

 mdial end-joints. These two half. 

 _rons are laid one upon the 

 other so that they break joints, as 

 shown bv the figure, and the ex- iri - - PUllk f>n " r tnr 



ing tli- It... f .\rrh. 



trados of the frame is cut to the 



true curve of the roof arch. The planks commonly used for 

 making these centers are 4 ins. thick, making the total thick- 

 ness of the center 8 ins. IMank centers of the construction 

 ilx-.l are suitable only for work where the pressures to be 

 resisted are small, as in tunnels through a fairly firm rock, al- 

 though then: have been instances of their successful use in soft, 

 ground tunnels. 



Wh'-iv heavy loads have to he carrie<l, trussed renters are 

 generally employed, the trusses In-in-^ composed of heavy square 

 LMIIIS with scarfed and tenoned joints, tvinforced by iron plates. 

 I Afferent forms of trusses an- employed in each of the differ- 

 ent methods of tunneling, and each of these is described in sue- 



