Accui.Ni> ASH i:i:i'Aii:s IN TI'NNKJ.S 



(4) Landslides which close the tunnel entrance are repaired 

 in a variety of ways. Fig. 138 shows a common method of 

 preventing the extension of a landslide which has been started 



FIG. 138. Timbering to Prevent Landslides at Portal. 



by the excavation for the entrance masonry. Fig. 139 shows a 



method often adopted when the slope is quite flat and the 



amount of sliding material is small. It consists essentially of 



removing the fallen material and building a new portal farther 



back ; that is, the open 



cut is extended and the 



tunnel is shortened. 



When the amount of 



the sliding material is 



very large, the contrary 



practice of lengthening 



the tunnel and short* n- 



ing the open cat, as 



Shown by tig. 140, Flo 130 _s hor tning Tunnel Cnwhed by LandHd 



may ta adopted. at ''"' 



Accident* After Construction. Accidents after the comple- 

 ii M of the tunnel may IK- divided into two classes: first, 

 those which entirely obstruct the passage of trains, of which the 

 collapse of the roof is the most common ; and second, those which 

 allow traffic to be continued while the repairs are being made, 



