74 



THE SEVERN TUNNEL. 



as well as tlio lowering of all tlio sliafta and pumps ; but tlio 

 worst job bo bad to do was in making the Tunnel on tlie 

 landward side of tbe big spring. Tbe conglomerate bed, 

 which constantly varied in thickness from four feet at Sud- 

 brook shaft to 'ifi feet at 5 miles 4, sliaft, wiis encountered 

 along half a mile of Tunnel from the big spring westwards. 

 It was very full of water, and very hard to excavate. In 

 some places the water poured in in such quantiliicis tbat it 

 was almost impossible to keep any lights burning, shade 

 them how you would. The execution of this part was very 

 costly. It -was by far the wettest part of the Tunnel, and 

 proved eventually to be that in which tbe water pressure did 

 the most damage to the brickwork, as described presently. 



The brickwork of the Tunnel throughout its whole length 

 has been built in the form shown in. the drawing. The fonn 

 is a very good one, approaching as nearly as circumstance 

 would admit to a cylindrical shape. The greater part 

 put in through very hard, rocky strata, and common bricks 

 only were used in its construction. 



Where the invert rests upon a rock bottom, the small 

 piece of level footing below the side walls should be omitted. 

 Also, if common bricks iilone are used, there is a w(!ak, point 

 in the three-cornered springing block, between the invert 

 and the bottom of the side walls, where the brickwork has 

 to sustain the greatest pressure of all, for a good part of it 

 can only be fdied up with broken bricks and spawls ; and as 

 a good brindlo brick will bear at least twice the crushing 

 weight of the best cement, this three-cornered piece is 

 necessarily weak. The proper remedy for this is to have 

 some special bricks made to suit the angle. One form of 

 special brick alone is necessary, by the use of which, to- 

 gether with common bricks, as shown on the drawing, this 

 three-corncT'cd piece, which is really the spiinging of both 



is 

 was 



J 



