TVI'KS <F CKNTKUS AND MOLLS 



63 



FIO. 39. oround Moid for Constructing 



molds are used they are set up a short distance apart, and cords 

 are stretched from one to the other parallel to the axis of the 

 tunnel, by which the masons are guided in their work. Ex- 

 treme care has to be taken in 

 setting the molds to ensure that 

 they are fixed at the proper 

 grade, and are in a plane normal 

 to the axis of the tunnel. Where 

 only o:n? ground mold is employed, the finished masonry is 

 depended upon to supply the place of the second mold, cords 

 being stretched from it to the single mold placed the requisite 

 distance ahead. The leveling and centering of the molds is ac- 

 complished by means of transit and level. 



Two modifications of the form of ground mold shown by 

 Fig. 39 are employed. The first modification is peculiar to 



the English method of 

 excavation, and consists 

 in combining the ground 

 mold with the leading 

 frame for the lower part 

 of the side walls, as 

 shown by Fig. 40. The 

 second modification is 



FIO. 40. -Combined Oronnd Mold and Loading Frame ei|llj l ove( l w here the two 

 for Invert and Side Wall Maaonry. >1OVe 



halves or sides of the 



invert are built separately, and it consists simply in using one- 

 half of the mold shown by Fig. 39. When the last method of 

 constructing the invert masonry is resorted to, extreme care has 

 to lie observed in setting the half-mold in order to avoid error. 



Leading Frames. As already stated, leading f mines are the 

 patterns, or molds, used in building the side walls of the Hning. 

 Like the ground mold they are usually built of plank ; one 

 side being cut to the curve of the profile, and the other being 

 iinult' parallel to the vertical axis of the tunnel section. The 

 vertical side usually has some arrangement by which a plumb 



