64 TUNNELING 



bob can be attached, as shown by Fig. 41, to guide the work- 

 men in erecting the frame. The combined leading frame and 

 ground mold shown in Fig. 40 has already been described. 

 The use of this frame is possible only where the 

 masonry is begun at the invert and carried up on 

 each side at the same time. This mode of con- 

 struction is peculiar to the English method of 

 tunneling ; in all other methods the form of sep- 

 arate ground frame shown by Fig. 41 is employed. 

 FIG. 4i.-Lead- f;he g roun ci frames are lined in and leveled up by 



ing Frame for 



Constructing transit and level ; and, as in setting the ground 



Side Wall Ma- r , , , , 



80nry frames, care must be taken to secure accuracy m 



both direction and elevation. 



Arch Centers. The template or form upon which the roof 

 arch is built is called a center. Unlike the ground molds and 

 leading frames, the arch centers have to support the weight of 

 the masonry and the roof pressures during the construction of 

 the lining, and they, therefore, require to be made strong. 

 Owing to the fact that the pressures are indeterminate, it is 

 impossible to design a rational center, and resort is had to those 

 constructions which past experience has shown to work satis- 

 factorily under similar conditions. In a general way it can 

 always be assumed that the construction should be as simple 

 as possible, that the center should be so designed that it can 

 be set up and removed with the least possible labor, and that 

 the different pieces of the framework and lagging should be as 

 short as possible, for convenience in handling. 



Tunnel centers are usually composed of two parts, a mold 

 or curved surface upon which the masonry rests, and a frame- 

 work which supports the mold. The curved surface or mold 

 consists of a lagging of narrow boards running parallel to the 

 tunnel axis, which rests upon the arched top members of two 

 or more consecutive supporting frames. The supporting frame 

 is built in the form of a truss, and must be made strong enough 

 to withstand the heavy superimposed loads, consisting of the 



