173 



PORT JACKSON SILT BEDS. 

 By Fred. B. Gipps, C.E. 





[Read before the Royal Society of N.S.W., October 5, 1887.'] 



About the end of last July, under engagement with the late 

 Harbour Tunnels Company, I undertook a series of borings across 

 Sydney Harbour, between Fort Macquarie Point and the bottom 

 of Beulah Street St. Leonards, and between Dawes Point and 

 Milson's Point, for the purpose of ascertaining the character of 

 what was supposed to be the rocky bottom on the lines of the 

 proposed tunnels. These tunnels were designed to provide for 

 both vehicular and railway traffic between Sydney and the North 

 Shore, at a cost of £450,000. In view of the large capital required 

 for this great engineering work, it was considered necessary to use 

 the utmost precaution in acquiring an accurate knowledge of the 

 depth of water and silt to the top of the rock, so that at least 30 

 feet of solid rock might be secured between the crown of the 

 tunnels and the bottom of the harbour. For this purpose a large 

 steam punt was moored on to a chain cable stretched across the 

 harbour from point to point, so that it could be moved forward or 

 backward as required. Divers were also engaged, in order to 

 examine the rock thoroughly, that any fissures might be detected 

 which previous soundings had not disclosed. Not anticipating 

 any great depth of silt from the readings on the Government Chart, 

 I was only provided at first with a sounding rod, made of different 

 lengths of one inch gas piping, with a half inch steel bar sharply 

 pointed, fixed on at the end. This was sufficiently stiff' to jump 

 through 44 feet of silt, but as this depth did not reach rock I was 

 soon obliged to use a stronger rod. My next trial was made with 

 two inch round and square bars of wrought iron jointed together 

 in different lengths as required, and weighted near the top end with 

 an iron rammer of 8 cwt. This rod was attached by chain to the 

 derrick at the stern of the punt ; its weight with the rammer was 

 about 15 cwt., and the drop at each turn of the cog wheel just one 

 foot. As the rod was not hollow, I was unable to obtain any true 

 sections of the different silt beds, but the silt collected at each 

 joint gave me a fair idea of their character at different depths. 



The specimens before you, together with the abstract of borings 

 and the annexed sections, will serve to convey to you the nature 

 and relative depths of these beds. The bottom bed is composed 

 chiefly of clay mixed with fine sand and mica. It crumbles very 



