154 



TUNNELLING TimOUOH VARIOUS STKATA. 



it would havo nocossitatod a considorablo outlay in pumps, or 

 a delay of the works until the lioadings from tlio ends had 

 been driven of sufSciont length to tap the water ; but this 

 might easily havo occurred in a tunnel of greater length, 

 and would have been the more inexcusable, as it might havo 

 been avoided by a little care and foresight. 



An instance of tlio collapse of a tunnel owing to being 

 undorniincd by colliery workings is the case of Ilirwain 

 Tunnel, near Merthyr, on the Great Western Railway. 

 This tunnel had for years given great trouble, and bricks 

 were constantly falling out of the crown of the arch, and 

 repairs were on the same scale diligently proceeded with, 

 until at last the tunnel entirely collapsed for a length of 

 about 100 yards, and completely stopped all traffic for the 

 space of about twelve months. It was' sujjposcd that the 

 bricks in the crown of the arch had crushed, and an inside 

 lining had boon in progress for a considerable timo, there 

 having boon ample room owing to the tunnel having boon 

 originally built for broad gauge. 



The method adopted for making good the collapsed por- 

 tions was timbering down from the surface, taking out the 

 damaged work, and re-building with brickwork in cement. 

 The timbering required was very massive, and probably 

 amongst the most extensive of the kind that has ever been 

 carried out ; but it saved immensely in j)oint of time, be- 

 sides being able to thoroughly examine the damaged por- 

 tions, and rebuild in a stronger and more thorough manner 

 than could otherwise have been done. 



The tunnel had been giving trouble for years ; but it was 

 not suspected that the colliery workings were the cause of 

 the mischief, and it was hardly thought that they extended 

 under the site of the tunnel. Afterwards surveys of the 

 workings in the vicinity wore obtained, and found to ex- 



