Discussion. the Mont Cenis Tunnel

16 MONT CENIS TUXNEL. Mr. G. J. MORRISON said,he had lately seen another machine employed in a tunnel near Bristol, which he had charge of under Mr. Brunlees, M. Inst. C.E. The drill wasa hollow cylinder, of about 2& in.diameter, and its face was setwith diamonds. It was fastened on to the end of a tube 5 ft. or 6 ft. long, which was worked at from 180 revolutions to 200 revolutions per minute. The rate of forward progress was about 27t. in. per minute, and in the event of the drill being arrested by any hardsubstance which would endanger the setting of the diamonds, the advance was reduced by aself-acting friction clutch. The holes made by the drills were from 3 ft. to 3 ft. 6 in. deep, and 2& in. diameter. There were 8 drills in themachine, and from 22 holes to 28 holes were drilled in the face of the heading,which was about 10 ft. by 8 ft., or as nearly as possible the size of the advance gallery at Nont Cenis. In connection withtunneling machines, thetwo points of interest were :-l, the speed ; 2, the cost at which the work could be advanced. Therate at which the tunnel was advanced was not so rapid as at Mont Cenis. Afterthe tunnel Mr. Morrison referred to was commenced, it wasfound thatthe works with which it was connected would not be ready so soon as was at first expected, and therefore the construction was only advanced at the most economical speed; but even that was four times as great as could have been attainedby hand labour. The holes took from 3 hours to 4 hours drilling, the machine was then moved back, and the face of the rock was blasted with dynamite. That operation occupied about 4 hours, so that in 8 hours about 3 ft. were driven, being at the rate of 3 yards per day, or, working 6 days a week, instead of 7 days a week, as at Mont Cenis, at the rate of 18 yards per week. The rock was a hard carboniferous limestone, and no quartz was met within that partof the tunnelwhich was excavated by the machine. As regarded the question of expense, he thought if time was nottaken into account, tunnelingby machinery was a little dearer than tunneling by hand. In the case to which he referred the expense was not much greater.He thought it worthy of remark, that at theMont Cenis tunnel therewere 180 men employed on the advanced heading, divided into three‘ shifts ’ of 60 men each. With the machine he had described thcre were about 15 men employed, which at once made a considerable difference in theexpense. He hadtherefore not the least doubt that, under similarconditions, it would be found more economical in use than that employed at Mont Cenis. Although the rateof working was only about 3 yards per day, hewas confident that if they hadbeen pressed as regarded time, Downloaded by [] on [12/09/16]. Copyright © ICE Publishing, all rights reserved. CENIS TUNNEL.MONT CENIS 17 they could have tunneled 30 yards per weck without increasing the cxpense materially. With reference to the diamonds, it was at first supposed that it would be a very difficult taskto keep them fastened in theirplaces, but some of them had drilled as much as 820 ft. of holes without being worn out. They did not seem to wear, but sometimes they were broken. It was only occasionally there was one lost, and the expense of replacingthem was not more than half that of sharpening the steel drillsused in working by hand. Thediamonds were opaque. He believed they were brought from Brazil;and they cost from S3 to 54 each, according to size. A drill was worth 525 or S30. Withhand labour,even when payingthe men a premium, and when working day and night, the ‘advance ’ was scarcely morc than 3 yards per week. Major BEAUMOW,M.P., R.E., said,he was connected with the diamond boring machine, which had just been referred to, and he hoped on a future occasion, when more experience had been gained, to give some additionalparticulars; but for the prescnt Mr. Blorrison’s statement had rendered any further allusion to it un- necessary. The criticism which suggested itself in reference to thc Paper was twofold ;-l, with reference to the method of execution, andthe alterations that had been made inthe perforating machinery ; 2, with reference to the working of the tunnel after its completion. Referring to the first point, the peculiarity which distinguished ),l essrs. Sommeiller and Grattoni’s machine from other perforators was, that the percussive cylinder andthe rotary cylinder were distinct from each other. In most other machines the two motions were combined in one. He gathered from thePaper that this peculiarity remained, while in the general principle there was no notable alteration, the work being done by thc motion of a piston in a cylindcr, driven by compressed air; the power for the boring remained the same, so that practically the action of the machine was similar to what it was when the Author described it in his previous Paper. As regarded the next point, namely, the working of the tunnel when completed, there wcre one or two matters of considerable interest involved. It wasstated thatthe ventilation, notwith- standingthe prognostications tothe contrary, was satisfactory on the whole; that was to say, it occasioned no inconvenience to the passengers in the trains, although to the engine-drivers and those who looked after the tunncl, it was, to a certain extent, a source of inconvenience. About a year ago, BIajor Beaumont had r1872-73. N.S.] C Downloaded by [] on [12/09/16]. Copyright © ICE Publishing, all rights reserved. 1s MONT CENIS TUNNEL. an opportunityof testing it, whenpassing through the tunnelon an engine, and although thc whole height of thc Alps was above, he sustained no greater inconvenience than when in the tunnelson the Metropolitan railway, which were only under a street. The engine- driver said that the day was exceptionally favourable for ventilation ; but therewere times whenit was not so satisfactory; and that frequently he had beon much distressed in passing through the tunnel. Those variations of atmospheric conditions appeared to be occasioned not only by thedifference of level between the two ends of the tunnel, but also by the fact that the north, or French, end was in the form of a cone, up which the northwind drove with considerable velocity, and hence set the column of air in the tunnel in motion. It might be asked why the same thing did not occur at the opposite end ? The answer was, in the first place, on account of the difference of level, and secondly, owing to the configuration of the mountain at that point presentinga plain surface instead of a cone. The velocity of the air current in the tunnel depended on tho direction and force of the wind outside. To remedy the defects in the ventilation, a project was in course of execution. It con- sisted of laying an S-in. pipe along the tunnel,from which, by means of stop-cocks, compressed air could be introduced as required. With the moderate traffic now passing through, Major Beaumont thought that plan might answer, but when the trains ran more frequently, he considered that it would be insufficient. What took place was not the setting of a column of air in motion along an air-way, as was thc case in a coal-pit. The vitiated air was not removed, it was only diluted. In the event of thc traffic becoming as groat as might be justifiably presumed on an international line,thore would be a considerable increase of the noxious vapours in the tunnel. The question of the proper mode of ventilating long tunnels becamc very important, whcn it was considered that tunnels of 30 miles in length, such asthat under the Channel, were now proposed. He believed thc difficulty could be got over by verti- cally bratticing a tunnel, and letting thc trains always runin each section in the same direction that tlle current of air was traveling. Anartificial system of ventilation should also be provided. He mm aware that if the wind blow from the north, the artificial stream of air coming from the south would be impeded; but, as he had seen in other tunnels, that difficulty could be surmounted by having large doors at the end of the tunnel, and trusting entirely toartificial ventilation. Be considcrcd that the question of the ventilation of the Mont Cenis tunnel, andindeed of all long tunnels, wonld befound notto present any difficulty in practicewhich Downloaded by [] on [12/09/16]. Copyright © ICE Publishing, all rights reserved. MONT CENIS TUNNEL. 19 judicious arraagements might not get over. It appeared to him that a mistake had been made in the laying outof the Mont Cenis tunnel, which had been corrected in thecase of another long tunnel, namely, the St. Gothard tunnel. He alluded to the heavy gradient at Mont Cenis from the entrance of the tunnel on thc north sido up to thc summit level. It was true that it was but a little in excess of the ruling gradients of the line loading up to the mouth of the tunnel. It should, however, be remembered thatthe latter gra- dicnt,s were in the open air ; and hc believed that the amount of adhesive power of the wheels was less in the greasy atmosphere of a tunnclthan outside of it. In the case of the Mont Cenis tunnel it was possible that the adoption of those gradients was unavoidable, but, as a matter of principlc, the gradients inside of a tunnel should not be so heavy as those outsidc.

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