154 



Tl NNELLNQ 



bottom outside of these timbers, forming an inclosure similar 

 to a cofferdam, from which the earth could be excavated with- 

 out disturbing the surrounding ground. The earth being 

 excavated, a layer of concrete 8 ins. thick was placed, and the 

 brick masonry invert constructed on it. In less stable ground 

 each of the above described cofferdams was subdivided by 

 transverse timbers and sheet piling into three smaller coffer- 

 dams. Here the masonry of the middle section was first con- 

 structed, and then the side sections built. Where the ground 





m. 



FIG. 80. Roof Arch Construction with Iron tenters, Baltimore Belt Line Tunnel. 



was worst, still more care was necessary, and the bottom had 

 to be covered with a sheeting of 1^-in. plank held down by 

 struts abutting against the large transverse timbers. The 

 invert masonry was constructed on this sheeting. Refuge 

 niches 9 ft. high, 3 ft. wide, and 15 ins. deep were built in the 

 side walls. 



Accidents. In this tunnel, owing to the quick striking of 

 the centers, it was found that the masonry lining flattened at 

 the crown and bulged at the sides. This was attributed to the 

 insufficient time allowed for the mortar to set in the rubble 



