IN THE PUBLIC MIND.

Auckland Star, Volume LX, Issue 215, 11 September 1929, Page 6

IN THE PUBLIC MIND.

THE HARBOUR TUNNEL. QUESTION OF DEPTH. V^ (To the Editor.) In the address given to the Waitemata Chamber of Commerce on the sth inst, Mr. Harkness states that one of the advantages of a harbour tunnel over a bridge would be that the approach of a tunnel could be constructed more cheaply than that of a bridge owing to the difference of the elevations of each project, a tunnel going down to only seventy feet and a bridge up to one hundred and eighty feet. According to the Admiralty chart, the channel of the Waitemata average's approximately six fathoms deep at low springtides, meaning that the bed of the harbour would be covered by forty-six feet of water at high tide, leaving only twenty-four feet of solid ground over the back of a tunnel, that is taking mean average highwater mark as datum. This would not be a safe proposition. I consider that a. tunnel would have to go down as low, or perhaps lower, than a bridge would have to go up. My reason is that the country rock over which the Waitemata flows has been distorted and faulted owing to violent movements caused by Auckland's once active volcanoes. Borings and tests taken to ascertain data in conjunction with tunnel construction are mostly very unreliable, whereas the borings of the foundations of a bridge would give accurate data, Thus we would be able to make a correct estimate of the cost of a bridge, but the cost of constructing a tunnel could not be definitely arrived at. The data obtained in regard to the sewer tunnel under the Brisbane River showed good, solid country, and yet this project had to be abandoned owing to the faulty nature of the country rock surrounding it. H.H.A.