How the railways protect their lines from the ravages of snow with costly snow sheds
CIVIL ENGINEERING - 15
BUILDING A SNOW-SHED ON THE LOFOTEN RAILWAY, SCANDINAVIA.
SNOW probably is the most implacable foe against which the railway engineer possibly can be pitted. Fog will throw traffic all sixes and sevens, and will cause exasperating delays by demanding slow, cautious movement, but snow often ties up a railway completely, bringing about total suspension of the services maybe for days.
Accordingly the railway engineer has come to regard the snow fiend with every respect, and has spared no effort to devise ways and means of circumventing its ravages. It is not the blizzard which he dreads so much, though at times when the snow drifts and scuds over the ground it brings traffic to a standstill by piling big white banks in the cuttings through which a locomotive cannot plough its way, but the avalanche is his terror. It not only fills up his cuttings with snow, rock, timber, and other debris, but, unless precautions are adopted to mitigate its effects, is able to knock the permanent way out of all recognition, demanding not only clearing but reconstruction of the track before the trains can be run.
In districts where snow wages its relentless warfare against human handiwork with seasonal regularity, the engineer practises the precept that “Prevention is better than cure”. He makes no attempt to arrest the progress of the snow movements, but lets them go their way unimpeded, merely striving to divert them clear of the track, so as to expend their destructive energy harmlessly at some other place.
The Canadian Pacific Railway always has suffered severely from the buffetings of the avalanche, or
“snow-slide”, as it is called locally. These assaults are experienced particularly in the mountains upon the 140 miles between Sicamous Junction and Golden.
The mountain section of this railway traverses five — through two, and over three — mountain ranges. Coming eastwards from the Pacific the line, following the Fraser and Thompson Rivers, passes through the Coast and Cascade Ranges at a comparatively low level. At no point does the train rise to an elevation exceeding 1,200 feet above the sea. In these ranges a very heavy rainfall, varying from 70 to 105 inches per annum, is encountered. On the other hand there is almost a total absence of snow.
Continuing eastwards the railway passes over three other ranges in rapid succession. These are respectively the Gold Range, the Selkirk Range, and the Main Range of the Rocky Mountains. As a matter of fact the two first named ramparts are subsidiaries of the great North American mountain system, but the Main Range of the Rockies is so-called in order to distinguish it from the others. In these three ranges the railway attains considerably higher elevations than upon any other part of the mountain section, the summits of the passes being respectively 1,900, 4,300, and 5,300 feet. Here the annual snowfall is very heavy. On the railway the average fall is 25 feet in the Gold Range, 35 feet in the Selkirks, and from 14 to 15 feet in the Rockies. Thus it will be seen that the Selkirks receive the heaviest precipitation, and the 35 feet average often has been exceeded. The heaviest maximum snowfall recorded is 45 feet 7 inches, but there is an unconfirmed report that in one winter the fall reached 56 feet!
The reason why the snowfall is so heavy in the Gold and Selkirk ranges is because these are the first high mountains encountered by the moisture-saturated clouds which drift eastwards from the Pacific Ocean. These high ridges intercept the cloud movements, with the result that the moisture with which they are laden becomes precipitated — rain in summer and snow in winter. By the time the air currents have reached the Main Range of the Rocky Mountains they have been deprived of the greater part of their moisture, and thus, being comparatively dry, the snowfall on the last named range is much lighter, although the ridge is approximately 2,000 feet higher than the other ranges to the west.
Therefore it will be seen that, while there is a considerable volume of snow to be handled in both the main and its two subsidiary ranges lying immediately to the west, the snow-fighting efforts to keep the line clear have to be concentrated upon that section of the railway extending through the Selkirks, with the Gold Range as a good second.
INTERIOR OF SNOW-SHED on the Canadian Pacific Railway.
During the very first winter, when the railway builders were toiling among the crags and precipices of the Selkirks, laying the bond of steel, the severity of the snow movements was driven home upon the Canadian Pacific Railway engineers very compellingly. The permanent way is practically side-hill excavation through the range. As the grade runs at right angles to the paths of the snow-slides it is exposed to the full brunt of any movements. Accidents innumerable have been caused through the snow, but, owing to the vigilance and unremitting care displayed by the railway officials, casualties have been few, although now and again there has been a heartrending calamity. The railway, however, suffers more heavily from the delays which are set up by the line becoming choked with snow and debris. Thus the problem has been to reduce this adverse factor to the very smallest degree.
Snow-shedding was the obvious measure of protection, but it was admitted that such works would have to be of a remarkably massive design and solid construction to withstand the buffeting of the snow movements. The mountain slopes are steep and are littered with boulders and masses of huge rock, as well as being thickly clothed with timber. When the snow moves in a mass, and commences its downward descent, it gathers an immense accumulation of timber and rock, which it hurls downwards with terrific force. It is doubtful whether any but those who are brought face to face with these slides can form any idea of the enormous force they exert. Few engineers have acquired such knowledge of this phenomenon and its results as Mr. J. P. Forde, who for many years was engineer-in-charge of the mountain division of the Canadian Pacific Railway, and who, perforcedly, was brought into intimate contact with the snow movements and how to avoid or to mitigate their devastating caprices.
This engineer narrated that on one occasion a slide was timed in its descent. After attaining its full dimensions it travelled for a distance of 2,500 feet down the steep hill-side in thirty seconds. When it had come to rest it was measured, and was found to average 500 feet in width, 40 feet in depth, and 2,000 feet in length. As the snow at the time was packed closely it weighed about 50 pounds per cubic foot. Consequently, when the slide attained its maximum velocity it was travelling at a speed exceeding 60 miles per hour, while the total weight of the moving mass of snow, ice, rock, earth, timber, and so forth was about 1,000,000 tons!
SNOW-SLIDE SCENE AT ROGERS PASS. Looking out of snow-shed partly wrecked by slide.
Is it surprising under these circumstances that huge trees are tom up like weeds, and snapped in twain like carrots, or that huge pieces of rock are wrenched from the mountain side and tossed about like pebbles? At the same time one can appreciate the unequal odds against which the engineer is pitted, and the ingenuity he is compelled to display in order to protect the slender link of communication from annihilation. Of course, it would be impossible to build any kind of structure capable of withstanding the impact' of such a slide as that referred to. It is only possible to design the protective works in such a way as to achieve the desired end without offering any resistance to the movement.
The sheds are invariably built of timber, although recently ferro-concrete has been brought into service as a constructional material, as described elsewhere. Remarkable ingenuity and skill are displayed in evolving the type of shed best adapted to the prevailing conditions. No one type possibly could meet every situation. Thus the sheds are not only of great variety, but a single shed even may be of a composite character, the variations occurring at different points to secure the desired result to the best advantage.
T
he main idea in carrying out work of this nature is to plan the shed so that it fits as closely as possible to the ground where it is built. Accordingly the structure may be of apparent simple and light design; on the other hand it may appear to be intricate and unwarrantably heavy. The grade being laid on a shelf excavated out of the mountain side, the engineer strives to restore the former contour of the hill side, so as to carry the debris harmlessly over and clear of the line. If this is impracticable, then he designs his roof in such a manner that it offers the least resistance to the moving mass. Moreover, he studies the character of the snow-slide and its accustomed path attentively, modifying his details of design according to the velocities of the avalanche, dimensions, weights, and composition. In some places the length of travel is comparatively short, the bulk small, and for the most part comprising snow only. In another the descent will be sharp, the travelling speeds very high, with timber, loose rock, and detritus looming largely in the mass, increasing its weight and dimensions. Also he takes into consideration the contour of the ground on either side of the line, since if it rises up again on the lower side, he has to bear in mind the possibility of the slide falling back after it has passed over the shed.
DIFFERENT TYPES OF SNOW-SHEDS.
In the diagrams reproduced above different types of sheds are illustrated, and these are capable of modification to an indefinite degree. The “A” or “K” type is perhaps the most familiar from pictorial representation. Here, on the mountain side of the line, an immense rock crib is built, balks of timber dovetailed, bolted together, and fitted to the wall, being packed and loaded with massive pieces of rock, while the roof is finished off to the slope of the mountain so as to form a sharp continuation thereof. On the opposite side the uprights comprise huge posts spaced closely together, heavily braced and strutted, to secure rigidity and strength for the roof. By giving the latter a sharp fall, the moving mass can be thrown clear of the structure on the lower side, to tumble into the valley below.
In the “B” type the rock cribwork is placed on either side, forming virtually a wooden tunnel for the line. In this form the protective wall on the lower side serves to prevent the debris damming back into the grade as might occur owing to the ground not falling away.
In “C”, as the track runs through a shallow cutting it is necessary to build up the slope formation on the mountain side so as to lift the avalanche almost imperceptibly over the track.
The “E” and “F” or “J” types are modifications of this design, and are generally introduced at such places where, owing to the configuration of the ground, the slide becomes somewhat spent before reaching the line.
Type “D” is somewhat simpler, being adapted to those points where the line skirts a precipice, and where it is probable that the avalanche invariably attains a high velocity, so that it clears the track quickly, instead of dropping directly on to it.
Type “G” is useful where small pure snow movements are likely to be experienced, or where, owing to the open character of the location, the snow is likely to drift heavily.
The “H”, “I” and “L” types are more elaborate, and are modifications of one another. There is a double roof, with intervening rafts and bracing. These are used at points where the slides are apt to bring down masses of rock and timber.
The final type, “M”, is a simple means of throwing the snow clear of the line. On the mountain side the heavy rock crib is built up to support massive balks which are laid so as to point upward over the track. The lower ends of these timbers are buried, and the ground shaped to form a hollow. The descending snow rushes into the depression and up the inclined plane to fly into the air and to fall clear of the track, the clearance varying with the velocity of the avalanche. This is the system which has been adopted extensively, only in masonry, upon the Lotschberg Railway.
The snow-shed is a costly protection. The more elaborate and heavy types run up to as much as £40,000 per mile to build. In one or two instances this figure has been exceeded, especially in places where the timber has had to be hauled from a distance. While the engineer by snow-shedding protects the line from one danger he invites another. This is fire. A spark from a locomotive may set the structure ablaze, and, once the flames secure a strong hold, destruction of the work is certain, since the shed acts as a huge flue. But the forest fire is dreaded more than the spark from the passing engine. Among the Selkirks this terror of the forest wreaks widespread havoc every year. In order to reduce the losses from this cause the sheds are built in short sections, with long gaps between, so that the possibility of the flames “jumping” is reduced. Incidentally it is the forest fires which accentuate the severity of the avalanche. The trees come toppling down as their roots are burned away, or are scorched into lifelessness, so that they succumb readily to such an attack as snow movements or even of the wind. The sides of the mountains thus become littered with gaunt trunks, maybe one hundred feet in length, and when these are picked up by the slide and hurled downward, they strike an obstruction with the force of a battering ram.
In order to guard against the ravages of the fire-fiend water pipes are carried through the sheds, and at close intervals hydrants and lines of hose are provided ready for instant use. The sheds are patrolled day and night, so that an outbreak may be caught in the incipient stage. Telephone facilities enable the watchman to get into touch with assistance, so that fire-fighting forces can be hurried up if the conflagration gets beyond the man on the spot. During the summer season, when the forest fires are paging, the patrolling forces are doubled and trebled if necessary. The necessity of these elaborate precautions will be appreciated when it is remembered that a burning shed not only represents a heavy monetary loss, but what is far more important upon such a line as the Canadian Pacific with its heavy transcontinental business, provokes a serious delay to traffic.
A CANADIAN PACIFIC RAILWAY ROTARY SNOW-PLOUGH AT WORK. Note the stream of snow being thrown to the side.
The distances, or “fire breaks”, between the snow-sheds vary from 100 to 200 feet according to conditions. The possibilities of a snow-slide rattling down and smashing up the line in these open spaces is eliminated by the heavy “glance” cribs or “split fences” planted on the mountain side above the line, which serve to divide the avalanche, sending it flying over the adjacent sheds. Now and again the “glance” crib does not fulfil its avowed purpose completely, so that the open part of the line becomes choked, if not damaged, but such incidents are comparatively uncommon; the “split fence” seldom fails.
Nature appeared to resent the ingenuity of the engineer at the onset, since the line scarcely had been opened when it was subjected to an unusually savage assault during the winter of 1886-7. The snowfall was terrific, 81 feet falling within a week, while for three weeks it snowed incessantly. As a result the avalanche season was unduly lively, and the rumblings and groanings, roars and crashes, of the moving masses were continuous night and day. The conditions on the Selkirks are somewhat peculiar. There may be a heavy snow-fall. Then comes a warm spell, as the chinook wafts over the range, accompanied by heavy winter rain storms. The snow is half-melted, when a sharp spell of severe frost sets in, converting the slushy mass into ice. The line was opened for traffic before snow-shedding had been completed, and as a result many of the open stretches of side-hill excavation became filled with the debris of avalanches. When the frost gripped the debris the snow-fighters had a harassing time. Picks and shovels made no impression — the heterogeneous mass of snow, earth, rock, ice, and timber had to be blasted out in big chunks, and several days elapsed before a passage 40 feet deep, and just wide enough to admit the trains, was driven.
CLEARING A SNOW SLIDE FROM THE TRACK BY HAND. The quantity of timber in the slide prevented the use of any type of snow-plough.
Yet despite all the precautions which can be taken, the snow-shed at times comes to grief, being either crushed under the terrific weight imposed or carried away and ground to splinters. Rocks and timber in the snow are responsible for this destruction as a rule. They tear an opening in the roof, when the moving snow secures a purchase upon the structure, wrenching it to pieces. The imprisoned air also plays sad havoc in such cases. Unable to escape, and becoming heavily compressed, it exercises a terrific bursting strain upon the artificial tunnel. The timber creaks, groans and bends until it cannot withstand another ounce of pressure. Then it flies, with a crashing report. Widespread damage js inevitable, and the engineer anticipates a long ding-dong battle against time in his effort to restore communication.
The capriciousness of the avalanche is extraordinary at times. On one occasion a slide swept down a steep slope from a point some 4,000 feet above. It hit the roof of the shed with tremendous force. The top was torn off bodily, but instead of being carried down into the valley, was hurled some 200 feet up the mountain slope above the line. The interior of the shed was filled with muck, which continued to a depth of 30 feet above the walls of the structure. When the snow-fighters appeared and buckled into the clearing task they found huge cavities or “pockets” in the debris, where the air had been caught, and, unable to escape, owing to the velocity of the slide, had been compressed. The displacement of the roof was the most remarkable feature, and to this day the engineers cannot determine decisively whether it was tom off by the snow-slide or blown up the mountain side by the bursting effort of the compressed air.
But the peril of the snow-slide soon will be a thing of the past upon the mountain section of the Canadian Pacific Railway. The engineers have manifested remarkable ingenuity in devising means of compassing the avalanche, and have toiled hard to keep the line clear throughout the stress and storm of winter. The authorities recognise the unequal odds against which their staff is ranged, and have now decided to escape the snow movements once and for all. The worst stretch of the danger zone is to be tunnelled at a cost of about £4,000,000. Not only will this avoid the snow-swept reaches, but it will provide the line with an easier grade.
END OF SNOW-SHED AFTER SLIDE HAS BEEN CLEARED OUT. Observe the baulks of timber brought down by the avalanche.
You can read more on “The Canadian Pacific Railway - 1”, “Clearing the Line” and
“Snow Sheds - 2” on this website.
