We cannot overstress the importance of these observations; for they alone permit the building of effective structures and the properly considered investment of large sums of money. This comment has already been made in Chapters V, VI and VII; it therefore holds for all types of structures.
Within the observation area, or wherever there are no rocks, one must not neglect to find the lower limit of permafrost. This can undermine the establishment of stabilizing structures or wind deflecting structures; in this case, one often has to opt immediately for a solution such as deviating the avalanche at a lower altitude.
Placing snow stabilizing structures in a starting zone, even if one follows the guidelines to the letter, creates related practical problems in so far as snow will no longer be deposited as it was before. Only observations taken in winter will allow one to take the new measures required.
When the area seems to be satisfactorily under control, it must nevertheless continue to be observed during winter. Over the years, a catastrophic situation could develop, so that if the observations have been made properly it will be easier to devise the complementary preventions required (reinforcement or extension of the controlled area).
Defences are built in a series of successive stages. Most of the large avalanche control projects have been going on for decades. Some of them started in 1876 and were pursued for 40 years; they were then filed and considered completed. Today the files must be reopened, and the defence of the area must be taken up again from top to bottom with modern structures.
In another large project, which started in 1912 and lasted 60 years, each new stage was begun after an avalanche had caused severe material damage, even though the problem was regarded as solved. Up until the end of the work, unpleasant surprises kept cropping up.
The earliest structures, made of dry stone, were on the point of collapse and had to be completely rebuilt. Teams worked almost every summer for six decades.
Would the work have been completed more quickly had it been done in one enormous stage? (Assuming that the snow conditions were known 100 percent before hand.) We doubt it: even if we eliminate the financial problem and that of inflation, there remains the problem that above a certain altitude the working period is very short, and that one can only use a limited work-force on a steep slope and that, moreover, such large numbers of workers cannot be found easily.
Other areas where attempts were made to complete the defences quickly have now had to be worked on again because the old structures do not offer the safety necessary according to the guidelines, falling short either in their planning from the static viewpoint or in their slack layout. When correcting the faults in these areas, one finds that the density of structures required becomes economically too great.
It would thus seem that construction in successive stages is subject to fewer drawbacks, for mistakes are made on a smaller scale.
Only those who have never carried out such work have never had disappointing experiences. The majority of problems would have been avoided if it had not been necessary to carry out the first work immediately after a catastrophe, under the pressure of public opinion. One must nevertheless remember that, had all the defences held up, it would have proved the margin of safety to have been too high. In other words, the executor of the project could have been accused of having wasted the taxpayers' money.
If, during a very hard winter, one or two bridges collapse in an area without causing damage elsewhere, one can claim that the defences have been built economically.
The examples of Chapter VIII illustrate this better than long theories. It is absolutely essential to collect as many winter observations as possible to avoid setbacks. One of the mistakes that must no longer be made is fixing supporting pillars made of light metal into a concrete base. The expansion coefficient of aluminium is three times that of iron so cracks soon destroy the base.
The duration of works usually guarantees the correct upkeep of the structures whilst work is in progress. What will be done when it is eventually necessary to replace some permanent structures?
Here too there have been some bitter experiences. Some areas were controlled around 1945 with structures made with old railway lines forming the frame and round timbers for the superstructure. At the time they made do with a cable-way to transport material; 25 years later it was necessary to replace the half-rotten wooden elements. This time a road had to be built for access. When comparing the respective costs, it was found that if a track had been built immediately rather than the temporary cable-way, the cost of the latter would have paid for half of the road.
When dealing with cable-ways for transporting materials and workers, the restrictions in some countries are so stringent that it is easily worthwhile building an access road immediately. The upkeep of a track is without doubt cheaper than that of a cable-way. Furthermore, when the road goes through a forest, it can also be used to transport timber. In all cases, however, careful planning of the road is essential for obtaining the best network of communication (including that for the forest).
Cablecars can be used in other ways, such as for distributing materials around the area of defences, and over short distances. Depending on the case, this distribution can also be done by helicopter, providing that the difference in altitude between the materials pile at the end of the road and the site of the structure is not too great. Furthermore the work site must be very well organized.
Over the last few years, the technical problems presented by avalanche defence structures have been largely resolved, but questions relating to the organization and cost of the work have generally been left on one side. At the moment, knowledge of the works organization and financial aspects of avalanche defences is missing in civil engineering companies, among overseers of the works, and among the specialists carrying out the basic calculations for the defences.
Research done by F. Pfister on the analysis of these problems has produced a practical methodology which gives a better organization of the work site and can be used everywhere (this implies the definition of a series of criteria which have to be used for comparative purposes).
The results of this research show that two thirds of the total costs of installing avalanche defences in a starting zone consist of transportation and subsequent distribution of materials on site and the erection of the structures (half direct and half indirect costs).
If one wants to calculate the cost of erection, other parameters must be known, such as the market price of the structure, factors which determine production costs, etc. Nevertheless, these unknown parameters play a secondary role to that of the price of labour.
For this reason, the time taken for each job has been analysed and compared to the results of a survey, thereby allowing us to fix coefficients of work for each stage of the project. This allows one to show the interdependence of the various elements of the construction and to deduce the relative size of each in figures. A system of calculations which can be used in practice without much extra expenditure, and which is based on cost functions, allows one to obtain the best results as far as the financial aspect is concerned. This also constitutes a set of relations between all the factors analysed, and thus serves as a basis of a system for organizing the work. With the costs and their analysis completed by a determination of the time required for the jobs, one can then see how best to rationalize the operation. Mechanization is a case in point: in some cases the cost price can be reduced by using, when possible, a mechanical digger to sink the foundations and help erect the structures.
This research has also shown the importance of good planning of the work. Practical tests have stressed the value of the method sometimes known as "critical path" to find the most economic solution.
This aspect of the financial problems of the work must not be left aside since the money needed for its completion always comes out of the tax-payer's pocket.
The construction of avalanche defences is not an end in itself but a way of confronting the irregular danger of avalanches, especially those which are released above the tree line. This danger could obviously have been avoided, but man's negligence and/or the constant search for financial gain, have unfortunately permitted the construction of buildings in areas which have always been threatened.
The technical measures should in future be used to a greater extent for reconstitution of the protective forests which are no longer capable of playing their role satisfactorily. The causes of such a situation are numerous: various abuses by man, forces of nature such as hurricanes, fire and avalanches. The result is a weakening or a destruction of the upper part of the forest. Technical work in such areas will therefore aim to re-establish biological protection against avalanches. A survey conducted in the canton of Grisons (Switzerland) showed that:
- the sum of all areas of reforestation and of avalanche defences constitutes 1.5 percent of the total area of forest in the canton,
- the upper part of the forest is 1 400 km. long, and reckoned at a width of 200 m., constitutes 16.37 percent of the forested area of the canton,
- the theoretical costs of the interest rates needed for permanent avalanche defences on one hectare (Swiss Fr. 500000 - 600000) is Swiss Fr. 25 00030 000 per year,
- protecting forest in the upper area usually ends up paying for itself and that in some cases, so long as it is looked after and cherished, it can make a small profit.
The reciprocal influence of arboreous vegetation and snow is of enormous importance for both the growth of forest and for the formation of avalanches. Forest in good condition not only fixes snow to the slope, but also influences its deposition both by intercepting it in the canopy of its trees and by its influence on wind-blown snow. Snow constitutes a handicap to the growth of vegetation. Snow creep and glide and the settling of the snow cover damage the boles of trees and sometimes wipe out areas of saplings or isolated shrubby trees.
Another problem caused by snow, especially on north-facing slopes, is a shortening of the vegetative period and therefore a stunting of growth. Avalanches and snow-slides released above the tree line can practically destroy hitherto intact forests. and penetrate further each year until they have carved an avalanche path for themselves. This scourge must be combatted in good time, and this is usually possible by controlling a small starting zone.
The goal to aim for is:
a) To maintain and reconstitute the upper forest zones so that they can fulfill their protective function (see photo no. 136).
b) To reforest the wrongly denuded sub-alpine slopes to the upper limit of the forest so that the beneficial effect of the latter can be reinforced.
This goal requires, furthermore, that issues of grazing rights, the population of game, the density of paths and tracks, and of course that of avalanche defences that will be necessary in the long run, whether permanent or temporary, should all be regulated.
We will not go into the forestry details here, for they are outside the scope of this manual.
A mistake that has been made in most reforestation work of 50 to 100 years ago should be mentioned. Trees were planted without considering the way in which the wood from these forests could be exploited. Nowadays, tracks which can be used by vehicles have to be opened and this causes much damage. This is an additional reason why the roads must be planned along with the very first stage of the defence works, and why the roads should be opened immediately.