TRAFFIC TUNNEL

Evening Post, Volume CXII, Issue 82, 3 October 1931, Page 14

TRAFFIC TUNNEL

WORK WELL AHEAD

OPEN IN A FEW WEEKS

SPECIAL PROBLEMS

Though the Mount Victoria tunnel may have been a long time in the pigeon-holes from the year of the authorising of the loan, away back in 1920, until the commencing of the work in 1929, and though the main drive may have stood for a good many months after the contractors had finished with it, the work of laying the concrete roadway and installing special equipment is now being pushed ahead at a great rate, and the tunnel is'only a matter of a very few weeks off completion and opening to traffic. There is no doubt that when the loan was authorised a combined tramway and traffic tunnel was in the minds of everyone, but in the scramble for finance the tramway aspect was rather sent to the background, and since then a considerable body of opinion has turned against trams through'the tunnel, as has been evidenced by demands for inquiries and investigations. The City Council has ne\ Ter taken a vote on the point, though the question has been debated inconclusively often enough, but has instructed the engineers to lay down a flooring adequate for all heavy traffic and with provision for the quick laying of train tracks when the final decision is made, given the approval of the Public Works Department, and the necessary & s. d. FOE TRAMS—SOME DAY. The flooring, then, consists of a foundation slab of reinforced concrete the full width of the tunnel floor, with a second slab of plain concrete above that, with channels left for the placing of double tram tracks later on. These channels are' lined first with heavy bitununised paper, and are filled with concrete of considerably coarser mix than that used in the roading slab, so that, when .necessary, they can be quickly broken out. Cross-channels have been left at the usual spacing for tie rods, etc. The whole surface isto be given a bituminous, surfacing, and will be flush right across. ' The main concreting was finished yesterday, and only a few days' work remains to be done in the track channels." ' Surfacing will be commenced shortly, and should be a rapid job, for the. way will be clear ahead and wet weather will not bother the bitumen gangs in there. THE VENTILATION PROBLEM. This roading work is fairly straight ahead engineering, complicated a littlo by the special provision that is being made perhaps' for trams and perhaps not, but the ventilation problem presented altogether new work for the city engineering staffs. Though quite a lot of data is available from overseas, the conditions in each traffic tunnel vary greatly. It may be that forced ventilation will bo very seldom required, for there is generally a, pretty stiff natural draught through the tunnel; but where motor exhaust is concerned no risks can be ; taken..; The danger from motor exhaust is in the CO (carbon monoxide) content; this is a colourless and odourless gas, present iv; motor exhausts at all times, but particularly yi the exhaust from oold or slowly running engines; it becomes dangerous in slight concentrations when persons are exposed to it for any longth of time. The fact that there is a footpath for pedestrians therefore came into first consideration in the ventilation calculations, for the pedestrian will be the slowest traveller through the tunnel. V : TOUR POWERPUL rANS. There are four electrically x driven fans to ensure that there is an adequate clearing draught at» all times: two (one at either portal) i will foree air into the lower ventilation duct under the raised footway, and two (one at the top of each, vertical shaft) will suck air from, the upper duct, formed by the flat false ceiling-in the tunnel arch. Just how the engineers worked out the necessary fan capacity to cope with the probably most extreme need (and with a safety margin over that) is a long story, but, however they did it, they certainly arrived at1 a full-sized decision —those fans have real authority. Running at full power, , they force air along the ducts at "the rate of 2500 feet per minute, just under 30 miles an hour., which is a considerable draught. ■ During the testing of the recording instruments it was necessary for anemometers to be placed and read in the ducts with the fans running at full speed, the results being telephoned to the operator, a breezy job in all conscience. The recording devices, which are, of course, electrically operated, are quite simple in principle, which is of wind pressure upon movable plates iiung in the ducts and shafts. An interest attaches to the 35-h.p. fan motors in that they are the first of a new type of variable speed induction (alternating current) motor in New Zealand. As a rule electric motors running on alternating current run at fixed speeds, but these new machines run at anything between 290 and 580 revolutions per minute, according to the setting of the brushes of the motor. AUTOMATIC ANALYSIS OF AIR. The proportion of carbon monoxide will be determined by chemical analysis of the air drawn off from various points in the tunnel by special piping to the control room at the Hataitai end. This apparatus has not yet been set up, but will consist of a chamber in which chemicals will determine the CO content, and by which the proportion will be recorded electrically. According to the readings so obtained, one or more fans will be started up, speeded up, or slowed down. It may be possible to make the fan control entirely automatic, by adding refinements to the CO recorder to throw: in switches, through relays, as required, but that is a complicated business, which .'may or may not be considered after some months' trial of manual control. The air forced into the lower duct from either end will be distributed through the main tunnel chamber through a series of slits, narrow near the portals, wider further in, and similarly the air will be exhausted from the arch through many small, graduated openings. Provision has been made, should it later become necessary, for the driving of a third vertical shaft from the centre of the tunnel to give still greater ventilation. « It may be argued that this forced draught ventilation might wait until Wellington ,'s traffic increases to the density of that in cities overseas where traffic tunnels are in commission, particularly as the natural draught is generally to be relied upon, but carbon monoxide cannot be >played with: a breakdown in the tunnel, a collision, possibly a fire, might in the first day hold up a line of cars and lorries, all throwing out gas in dangerous quantities. Moreover, it is very likely that the heaviest traffic the tunnel will see for the next five or six years will be on the veryfirst Sunday after it is opened. There is not so very much to see, but every other Wellington _ motorist will insist on running through it that first Sunday, and the pedestrian traffic will be immense. Even so, the fans may not be necessary to maintain the tunnel air at a perfectly safe low level of CO, but they will be there, an insurance against even the possibility of trouble.