THE PARNELL TUNNEL.

New Zealand Herald, Volume XLV, Issue 13780, 19 June 1908, Page 7

THE PARNELL TUNNEL.

SUGGESTED OPEN CUTTING

IS THE SCHEME FEASIBLE?

The suggestion made at the Chamber of Commerce dinner, on Wednesday night, by Mr. Barth. Kent, vice-president of the Chamber, in regard to the possibility of doing away with the Parnell railway tunnel, was naturally received with considerable interest, especially by the mercantile section of the community. Some inquiries which were made on the subject yesterday, however, show that attractive as the proposition of an open cutting may be, it is not one that may be expected to come within the range of practical engineering. Mr. Kent stated th.it there "was very little soil above the tunnel at the city end, and that there was, perhaps, no more than 20ft or 30ft at the other end. A Herald representative, "on looking up the distances in connection with the tunnel, ascertained that the depth of the excavation that would be required to make an open cutting, would be much greate- than assumed by Mr Kent. A few figures will make the position clear. The length of the tunnel is 17 chains. The depth from the surface of the ground to the rail level of the tunnel, where it commences at the city end, is about 33ft, and from this, as the hill is ascended, the distance gradually increases until it reaches about 85ft. From this it drops again abruptly a few yards from the Newmarket end to about 30ft. On taking approximate averages, it is seen that the average depth, for seven chains from the city end is about 50ft. The next two chains average about 70ft, and for the next seven chains, along the crown of the hill, it reaches the maximum of 85ft, the remaining chain averaging about 52tt. The point at which the tunnel passes under Manakau Road is within the 85ft stretch. The country in places is of a treacherous character and in the opinion of engineers the cutting would necessitate a. batter, of not less than one in one. The sloping back of the two sides of the cutting would thus represent a distance of about 170 ft at the top, or, say, a total width (allowing 30ft for a track bed carrying two pairs of rails) of 200 ft, which would be required to be spanned by a bridge. This structure would, in itself, be a considerable undertaking, whilst the excavation work would mean an extensive outlay. The cutting would also involve the acquisition of private land along a portion of its length to give the required batter. The present railway reserve over the tunnel .vould be wide enough at the Auckland end, where it has a width- of three eluiins, but along the crest of the hill the width, which varies, is in places no more than 100 ft. Apart from the cost of the bridge and the purchasce of extra land, it may be pointed out that, generally speaking, it is found cheaper to tunnel than to cut through a. hill when the height is over 40ft. It would lints appear, that the proposed open cutting is unlikely to be seriously entertained, and that when the necessity of a second pair of rails to connect the two ends of the already constructed double line is absolutely demonstrated to.the satisfaction of the authoritiesas it assuredly must bo are long—the duplication will have to he carried out by an enlargement of the tunnel, or, failing that, by means of a deviation that would avoid the hill altogether. At present about 100 trains pass through the tunnel daily, and this number will, of course, be increased after the completion of the Main Trunk line.