BIG DOUBLE TUNNELS

Evening Post, Volume CVI, Issue 73, 24 September 1923, Page 2

BIG DOUBLE TUNNELS

ON MAIN TRUNK

IMPROVING PORIRUA LINK

SHORTER LINE ON BETTER GRADE.

The diagram printed on this page shows Wellington Harbour on one Bide and Porirua Harbour (West Coast) on the other, and the railway designs for making the intervening high country ■more amenable to heavy and fast traffic.

In order that the diagram shall not be loaded with detail, the high country has not been sketched in, but the courses of the streams shown (on the harbour side the Kaiwarra, Ngahaurang'a, and Kordkoro, and, on the West Coast side, the Ponrua- with its tributaries), suffirm" s Indlcate how the main ridge l\vhv .exb *in? single-track railway (\Velhngton-Ponrua section of thi \ l lMd Main Trunk) » dearly IT S} l h°Wn by theS black Hne with cross-dasb.es. "A" and "B" are alter-

native routes, along one of which a double track railway can be made by use of tunnelling. The existing line reaches its highest point between Khandallah and Johnsonville, where it is 521 feet above sea level. "A" route at its highest point is 320 feet, a, reduction of 201 feet. "B" route at its highest point ie 250 feet, a reduction of 271 feet, as against the existing railway, and of 70 feet as against "A." But other factors in "A's" favour appear to make it superior to "B."

THROUGH INSTEAD OF OVER. First glance at the diagram will show that the feature of the project is the long tunnels. "B" route has one of them. "A" route has two of them, which are in aggregate, though not individually, longer than J'B's" tunnel. Also, "A" may possibly have a third tunnel, of some six or seven chains, between the two tunnels shown on this route.

If the reader will follow the course on the diagram of the Wellington-Hutt-Wairarapa duplicated railway, he will see the "A" route branching off hillward at a Bpot about a mile on the Petone side of Kaiwarra (not far from where the City Council's new road to Khandallah leaves the Hutt road). Then he will see, almost immediately, the first "A" tunnel (60 chains, or threequarters oi a mile, long), and he will see that it brings the railway out on

the bank of the Ngahauranga stream behind the Wellington Meat Export Company's works. He will see that the stream and road are then crossed by a bridge, and that the railway continues its upward course, not by climbing the Ngahauranga Gorge, but by taking a. side gully (the game gully as carries the Newlands ioad. marked on diagram). It is here that the small tunnel (not marked on the diagram) will be put in, if put in at all; but as the shallow spur that invites the tunnel could also be dealt with by a sidling surface line, or even by a cutting, this «x or seven chains tunnel must be regarded as contingent.

A TWO-MILES TUNNEL.

The next feature on the diagram is the main "A" .tunnel, by means of which the railway will be taken underground from Newlands road gully (also called Wakefield gully) to a spot on the Ponrua, stream below the existing railway viaduct (shown on diagram). Thus the main divide is tunnelled. That the tunnel emerges on the Porirua side at a point well below the existing railway is sufficiently shown' by the fact that, continuing on its course, but now downward, route does not link up with the existing railway till a point has been reached about" half a mile below (north of), the present Tawa Flat railway station. This "A" tunnel will be 160 chains (two miles) long if a grade ot 1 in 100 is maintained. If the grade is to be improved to 1 in 120, the tun-

nel will be 176 chains (2 miles 16 chains) long. As the start.on the harbour side is practically from sea level, it follows that the grade dips southward—that is, irom the Porirua valley towards Wellington—so the upward haul will be the outward one. Either "A" or "B" gives a double-track route much superior to any double track that can be laid on the existing railway's route.

RECLAMATION FACTOR. iwo™??,^ "A" w'th "B," it iB seen that B does all its tunnelling in one piece, while "A^ tikes two lite, at the cherry. "B" keeps along the harbour side, m order that its one long tunnel may be able to break into the I'onrua valley at a point lower than the Ponrua portal of "AY final tunnel. And, ly doing that, "B," as shown above, has less climbing to do ?v • e?o' n and Jsable to kee P a grade of 1 m 120. But »B" buys this 8 advantage at the cost of keeping along the harbour-side to a spot about half way between Njjahauranga and Petone {near Rocky.-Point) before tunnelling miles of harbour-Bide running as compared with a limited amount of such running on "A" ; and if (as seems to be the case) the already congested Hutt road-and-rai way cannot take a .Main Jrunkdouble track without new reclamation from the sea, then "B" is loaded with much more reclamation than "A."

Also, "A's" small amount of reclamation,- being in the comparatively sheltered zone of Kaiwarra, would be relatively much less costly than "B's," and much less exposed to the action of wind and sea, whose impact on the sea-wall north of .Ngahauranga has at times been very destructive.

EQUAL COST : THE ADVANTAGES

OF "A."

It is understood that, when the "B" route is weighted with the expected reclamation co3ts, the estimate works out at a figure so little below the estimated cost of "A" ac to b« practically neglig. Assuming, then, that both are equal m cost, and that "A" requires traffic to go only 70 feet higher than "B," and that, by some extra tunnelling,. "A" can preserve the same grade (1 in 120), how do the two routes compare otherwise? • One took at the diagram will show that A is shorter and straighter. The distance figures printed on the diagram make A three-quarters of a mile shorter than "B," and two and a quarter miles shorter than the existing railway. A does not over-tax . the Hutt road, and needs little or no reclamation The longer /'B" tunnel could .not be attacked afc so many points as the two "A" tunnels, so they could be constructed at a higher rate of speed, 2" g lotiger '" the aggregate. •i ,'» B>. tunnel is 207 chains (2 miles, 47 chains) long. On . "A" . the

minimum length of tunnalling would bs 220 chains, the maximum 243 chains. On the minimum measurement "A" has, therefore, 13 chains (about 6 per cent.) more tunnelling than "B." :

BENEFITS OF DEVIATION!.

The grade of the present Main. Trunk railway (Wellington to Porirua) is 1 in 40 in combination with bad curves— calculated to be equal virtually to 1 in 34. To double that railway would need an expenditure of something over half a million, and still, froni the trunk railway point of view, it would lack much. Another £400,000, or thereabouts, would give "A" route with all its traffic advantages.. It i«. claimed that an immediate financial gain in the working of the railway would result. The diagram shows that Ngaio, Khandallah, and Johnsonville are cut right out of the through traffic, and the line there would remain for suburban service. Though the existing Taws, Flat railway station is not on the: through line, it could be rebuilt thereon so as to serve all purposes. ■ Deviation-and-duplication, as outlined would not only create a firat-claw express double track right through to. the Ponrua valley. A further gain would be that the considerable area-of flat land m the lower parts of that valley would be given a suburban railway service, and would be probably brought ai dose to the city as Seatoun and Lower ilutt.