XXX. ABSTRACT OF PROCEEDINGS. 



admitted to be the wiser course to pursue, in estimating 

 the cost of such works, to provide for the excavation being 

 done by compressed air drilling and explosives, and filling 

 the space between the diameter of the tunnel and the rock 

 all round with concrete, in a similar manner to the work 

 done recently at New York in their pressure tunnels. As 

 it is some years now since they were completed, and 

 brought into use, and we have not had any report of their 

 being defective in any way, we cannot, I think, do better 

 than profit by their experience. 



The question of the resisting capacity of the Hawkesbury 

 sandstone, to the water in the tunnel under the pressure 

 it would be submitted to, due to the head at the Oataract 

 Reservoir, even though the tunnel be lined, was dealt with 

 both by Mr. Oorin and Mr. Oardew in a very interesting 

 way. 



Mr. Oorin was of opinion that the question whether such 

 a pressure as 6001bs. per square inch could be resisted by 

 concrete without reinforcement, even if backed up by the 

 Hawkesbury Sandstone, under the copsiderable pressure of 

 the superincumbent mass of rock, would have to receive 

 careful consideration, supported, perhaps, by experiment. 



Mr. Oardew said his experience was that at a depth of 

 150 feet below rock surface, as a minimum the rock is as 

 compact as ordinary concrete, and except where fissures 

 are met with concrete lining would be unnecessary in a 

 tunnel excavated by machinery. He said the sandstone 

 rock always contains a large percentage of water, and if 

 there are not pits or cleavage planes at that depth, the 

 hydraulic grade of the water table in the sandstone would 

 be flat, so that the rock would not absorb much water from 

 the tunnel under pressure, or if it did absorb any, it would 

 soon become surcharged and cease to do so. He was of 

 opinion that, if provision were made for a thickness of con- 



