1TNNELS THROUGH SOFT UK" 



arch owing to its unstable foundation on the unexcavated soil 

 of the lower portion of the section. The amount of settlement 

 in iv vary from a few inches in tirm soil to over 2 ft. in loose 

 soils. To counteract the effect of this settlement it is the gene- 

 ral practice to build the arch some inches higher than its nor- 

 mal position. When the settlement is great enough to infringe 

 seriously upon the tunnel section, repairs have to be made ; and 

 the only way of accomplishing them is to demolish the arch and 

 rebuild it from the side walls. It is usually considered best not 

 to demolish the arch until the invert has been placed, so that 



no further disturbance is likely once 

 the lining is completed anew. 



The rotation of the arch about its 

 keystone, or the opening of the arch at 

 the crown, by the squeezing inward of 

 the haunches by the lateral pressures, 

 is another characteristic accident. Fig. 

 68 shows the nature of the distortion 

 produced ; the segments of the arch 

 move toward each other by revolving 

 on the intrndosal edges of the keystone, 

 which are broken away and crushed together with the operation, 

 while the extradosal edges are opened. It is to prevent this 

 occurrence that the horizontal struts shown in Fig. 65 are em- 

 ployed. The manner of repairing this accident differs, dej>end- 

 ing upon the extent of the injury. When the intradosal edges 

 of the keystone are but slightly crushed, the repairing is clone 

 as directed by Fig. <>0. When the keystone is completely 

 crushed, however, the indications are that the material of the 

 keystone, usually brick, is not strong enough to resist the 

 pressures coming nj)on it, and it is advisable to substitute a 

 stronger material in the repairs, and a stone keystone is con- 

 structed as shown by Fig. 70. The middle stone of this key- 

 stone extends through the depth of the arch ring, and the two 

 side stones only half-way through, their purpose being merely 



Pio. 68. Sketch Showing 

 Failure of Roof Arch by 

 Opening at Crown. 



