62 TUNNELING 



CHAPTER VII. 



TYPES OF CENTERS AND MOLDS EMPLOYED 



IN CONSTRUCTING TUNNEL LININGS 



OF MASONRY. 



THE masonry lining of a tunnel may be described as con- 

 sisting of two or more segments of circular arches combined 

 so as to form a continuous solid ring of masonry. To direct 

 the operations of the masons in constructing this masonry 

 ring, templates or patterns are provided which show the exact 

 dimensions and form of the sectional profile which it is de- 

 sired to secure. These patterns or templates will vary in 

 number and construction with the form of lining* and the 

 method of excavation adopted. Where the excavation is fully 

 lined on all four sides, the masonry work is usually divided 

 into three parts, the invert or floor masonry, the side-wall 

 masonry, and the roof-arch masonry. At least one separate 

 pattern has to be employed in constructing each of these parts 

 of the lining ; and they are known respectively as ground 

 molds, leading frames, and arch centers, or simply centers. In 

 the following paragraphs the form and construction usually 

 employed for each of these three kinds of patterns is de- 

 scribed. 



Ground Molds. Ground molds are employed in building the 

 tunnel invert. They are generally constructed of 3-inch plank 

 cut exactly to the form and dimensions of the invert masonry 

 as shown in Fig. 39. To permit of convenience of handling in 

 a restricted space, they are generally made in two parts, which 

 are joined at the middle by means of iron fish-plates and bolts. 

 Either one or two ground molds may be employed. Where two 



