144 



TUNNELi.NG 



to resist the crushing forces which are greatest at the intrados. 

 Sometimes, when the pressures are unsymiuetrical, the arch 

 ring breaks at the haunches as well as the crown, as shown by 



FIGS. 69 to 71. Sketches Showing Methods of Repairing Roof Arch Failures. 



Fig. 71, which also indicates the mode of repairing. This 

 consists in demolishing the original arch, and rebuilding it 

 with stone voussoirs inserted in place of the brick in which the 

 rupture occurred. 



