640 PROCEEDINGS OF SECTION J. 



The sand tunnels were close timbered at the tops and sides by 

 poling boards or runners behind " sets " placed three feet to three 

 and a-half feet apart, centre to centre, but it was not found 

 necessary to timber the bottom either in tunnel or open cutting, 

 The " sets " consisted of a cap piece and two props of round 

 hardwood timber fourteen inches in diameter. The props were 

 notched into the cap fdece and I'ested on longitudinal foot plates 

 and had spreaders between, at half the height, to take the side 

 pressure, which was found to be great, being sometimes suificient 

 to force the ends of a nine-inch diameter spreader an inch or so 

 into the props as if it had been morticed in. The caps were 

 further supported by corner struts from the props butting unto a 

 short spreader piece underneath the " cap." Cross-spreaders under- 

 neath the props were tried at first, but they had to be abandoned 

 and the longitudinal foot-plates were substituted. The tunnel 

 floor and roof were level in cross-section, and the span between 

 the props of the cap piece was thirteen and a-half feet, and at the 

 foot of the props seventeen feet, the height of cap above floor being 

 fifteen and a-quarter feet. The " runners " were cut to nine feet 

 lengths, and the tunnel face was advanced six feet or seven feet 

 at a time, and only the most experienced and most careful miners 

 procurable were allowed on this work. It took some days for the 

 timber to get the full pressure it had to support, and during this 

 time loud creaking noises were frequent, and were sometimes so 

 great as to cause the miners to leave the face and retire back in 

 the tunnel for a few minutes. On one occasion a cap piece fourteen 

 inches in diameter at the smallest end was crushed where it rested 

 on the prop; but whenever any indication of weakness wasobserved, 

 additional timber was put in and the work was completed without 

 any accident to those employed ; and the only mishap as regards 

 the work was a slight settlement, opposite a pumping well, on a 

 short length of the sewer constructed in open cutting, amounting 

 to one and a-quarter inch at the lowest point. For a short length 

 on either side of this lowest point there was a longitudinal crack 

 along the crown of the arch and the invert, and along the' 

 springing level at both sides. The cracks along the crown and 

 invert were about one-eight of an inch in width, and the cracks 

 along the springing level were very fine. It was thought that this 

 accident must have been due to some defect in the sub-duct box 

 allowing sand to enter below the foundation level. Levels were 

 taken at intervals and the work carefully watched for a consider- 

 able time after the settlement was discovered, and it was found 

 that no further settlement took place. It was not considered 

 advisable to take up this short length of sewer lest further 

 damage should be caused by pumping ; but the trench was opened 

 out again and the sewer was strengthened by widening the 

 concrete to the two sides along the damaged length and turning a 

 new concrete arch on the top of the old one. The cracks in the 



