Deviation Engineering RECLAMATION AND TUNNELS

Evening Post, Volume CXXIII, Issue 143, 18 June 1937, Page 23

Deviation Engineering RECLAMATION AND TUNNELS

The Tawa Flat scheme comprised some remarkable features. First there was the construction of tlio sea wall at; one of tlie deepest portions of tlie foreshore of the harbour. The water at this point varied in general from 25ft at ono end to lifts at the other, but some parla of the work were done in very deep watch There is a hole, near the Kaiwarra, end where tlie wall had to be 60ft high. The water is 35ft deep near there now. The; wall was four feet wide at the top! and reached colossal dimensions in parts of its base. It cost £220,000, and then a further extension was decided upon. The reclamation for which the wall was; built was largely the result of combined plans by the Wellington Harbour Board and the railway authorities. The extension of wharf accommodation northwards was indicated by the fact that there was, no- railway accommodation in the inner and southern parts of tlie harbour. The Thorndon Esplanade Being a reserve prevented immediate action, but the railways Heeded an extended yard, and eventually an agreement was entered into and embodied in an Act of Parliament. Finally by a compensating agreement, regarding the cost of the reclamation as a whole, in which the cost of the sea wall was balanced against that of the actual filling of the reclamation, the work was commenced. The wall presented no special difficulties. Il was commenced in tlie middle of 1923. The reclamation filling was started in June, 1925. by the dredge Wakariri, joined in April, 1926, by tho dredge Kaione, and was completed well in time. One of the features of the successful scheme was in fact the way in which the necessary parts of it progressed synchronously. The reclamation measures 69 acres, and its filling must have been beneficial to tlie harbour in deepening it by removal of so many millions of tons (2,250,000 cubic yards) of sandy spoil The reclaimed ground took some time to pack but amongst it tliere was a very large propor tion of hard sand ahd shell, and a surface ol spoil from the tunnels and brick from de molished buildings, broken concrete, and othei good material, enabled a gradual commence ment to be made with the erection of the rail way goods shed, etc., and later on the re-layinj of tlie station yard. This yard was one of tin important features of die scheme.

The old yard had to be entirely remodelled to suit tlie centralisation of the railway station and die altered northern exit through the tunnels.' This has been a work which has needed considerable forethought and ingenuity to keep the full rail traffic running while installing a completely'new system.

TUNNEL CONSTRUCTION.

Though the reclamation was a mammoth Work, public: interest centred in the piercing of the mountains to the north. There would have ifeen tunnels in any case and one of the Jreasons for the route adopted agaiust alternatives which put the tunnel entrances, further towards Petone was the .insecurity -.of the coastline in the harbour against the seas raised by strong Tsb'UthSify gale's." Up to"the 'spot where the tunnels now enter the hills, tlie harbour foreshore is largely sheltered from such gales and seas' by Point Jerningham, whereas further on:the.fulUsweep of seas from.Evans Bay strikes it. The route selected involved the construction of two tunnels, the first 63 chains long, and the second 2 miles 53 chains long, the second longest

in New Zealand, a total length-of 3 miles 36 chains. Between these tunnels the trains Hash out into daylight for fifteen chains and cross the Ngahauranga Gorge main highway by an overbridge on a slight curve. ,?.'?,

There is a double gain in the deviation. It shortens the distance of the old: route by two-aud-a-quarlcr miles, and it lops 323 feet off tlie climb. In the four miles from' Thorndon •"■ to Khandallah' the line rose from,sea level to .492 ft, and front Khandallah to Johnsonville, it climbed further to 518 ft above sea level in about half a mi 16. The highest point in the ,; deviation is 195 ft. The elimination bf "'lateral curves and the1 flattening of the grade was estimated to save; ,£40,000 a year in haulage. This can be understood when it is realised that the old grade at its steepest was lin 36, and the bulk.of the track was graded steeper than'l''in' 50, while tlie deviation is nowhere Steeper than 1 iii 110.

The shorter tunnel was commenced in June, 1928, pierced in July, 1929, and finished, includ. ing' excavation and concreting, in June, 1930. Widi the exception of some loose ground and water troubles, which.at one time caused anxiety

and delays, necessitating the driving of an adit for drainage in from near the Ngahauranga meat works, few difficulties were encountered. The longer tunnel was begun in August, 1928, pierced in August, 1931, and finished, including excavation, in November, 1933. ; ?

The material excavated from the tunnels was used to form the huge ramp which leads up from Kaiwarra to the bridge : oyei' the Hutt Road. From the two tunnels? and from cuttings, tliere were over 500,000 yards of spoil, more than ample for the, double; track ramp, which at its bulkiest-.is 30ft high,and 100 ft across the base. At one stage it was .necessary to run the spoil out through the drainage adit, to be taken in lorries to the ramp, but as the second tunnel grew after die completion of the first, there was a surplus, of spoil, which was deposited north of Glenside'. ■■■",•. '■■-■■■■-, -■-

SPECIALMETHODS,

Tliere Were -several ."Special features in the construction of the longer tunnel, notably die method used-.0-make -work possible on many faces at the same'limei The first of; these was the pioneer tunnej, ljOft x Bft, used in New Zealand for the first.time. j. This.was lower than, but parallel with the. actual tunnel, a chain or so "away, widi occasional' slanting- access --tunnels, arid besides acting most effectively ,in. draining away water: as soon, as it collected, allowed drilling and the.removal of spoil to proceed, at a number of places at once, while; it facilitated the transport of material. Again, a shaft was sunk below the viaduct, whicli made it possible to work three faces at once at the northern end

:uf lhe long tunnel. The most modern methods were used including big compressed air ''muckers," or shovels, electric locomotives for haulage, and concrete ."guns." The inside dimensions of the finished tunnels are 19ft 6in high, ancl 26ft across the widest-part of the arch, with ample room below for the double track. The excavated tunnel was in places considerably larger. The concreting was the most interesting of many interesting processes on the big job. A steel shield, or profile, the outside of which was the exact internal dimension of the finished tunnel, was wheeled into place on a gantry, its end flush with a finished portion, and concrete mix, brought in special shoot trucks from the mixer at tlie gorge, was driven in under tremendous pressure. These trucks had a specially simple valve, and when the air pressure was put on, the mixture, containing metal lo pass through a two-inch ring, was driven through steel pipes wilh a hoarse roaring bellow where it was coughed out above the profile. When tlie loose mix reached nearly to the end of the shield, a vertical collar was fixed some distance in, which gave a vertical face for the profile fo rest' against for the next filling. The interior finish given by this method was excellent.

Instead of letting a contract for the tunnelling the Government decided to have it carried out by the Public Works Department, of which at the time Mr. F. W. Furkert was chief engineer. The result fully justified this decision. The engineer in charge of the work was Mr. W. Uowncr. ,