288 



through rock, this rock is of a seamy character, and in some 

 portions of the tunnel it disintegrates on exposure to the air. 

 In removing the timber to make place for the new lining some 

 of the roof was found close to the lagging, but often also con- 

 siderable sections showed breakages in the roof extending to a 

 height varying from 1 ft. to 12 ft. above the upper side of the 

 timbering. This dangerous condition of the roof made it neces- 



FIG. 149. Kelining Timber-Lined Tunnel, Norfolk and Western Ry. 



sary that only a small section of the timber lining should be 

 removed at one time. It made it necessary, also, that the brick 

 arch should be built quickly to close this opening, and finally 

 that all details of centers, etc., should be arranged so as to 

 furnish ample clearance to trains. The accompanying illustra- 

 tions show the solution of the problem which was arrived at. 

 Referring to the transverse and longitudinal sections shown 



