countering clastic rocks as the depth of fili, at its maximum, is only 60-70 m. Therefore the tunnel will certainly be ovcrlain by at least 100 metres of rock. Inflow of water is to be expected in the limestones of the north side with the very con-siderable initial quantity of 300-400 litres per second.”
The geological advice did not correspond to the facts. After firing the explosive charge on the morning of 24th July 1908, with the heading at km 2.675, the tunnel caved in and an underground mudslide filled it completcly over a distance of 1600 metres with about 6000-7000 m3 of materiał. The quantity of water which flowed into the tunnel at first in all probability amounted to approximatively 4 m3/sec, but subsided very rapidly, so that at km 1.6 a flow of only 60-70 litres/sec was measured, i.e. hardly morę than the outflow prcvious to the accident. 25 men lost their Iives in the disaster (Fig. 22).
The day after the collapse a depression was noticed at the bottom of the valley of Gastem, which in the course of the day became still larger until it reached a maximum depth of 2-3 m and a diameter of 90 m. It was therefore immediately evident that the heading had entered from the limestone directly into the old Gastem valley, which in former times had been much deeper and had later on been filled with sand and gravcl and completely soaked with groundwater. Two explo-ration borings were immediately started in the Gastern valley along the linę of the old tunnel axis; they reached a depth of about 220 m without striking rock. It was therefore decided to bypass the site of the mudslide and to choose a linę which would ensure the tunnel being excavated entirely in rock.
It is interesting to notę that before the start of actual con-struction work, a geologist (L. Rollier) delivcred a report to the railway company, with regard to the undercutting of the Gastem valley:
“I believe that the alluvial fills, boulder clays (ground-moraines) and valley fills reach deeper than stated in the geological survey. But if the same should reach as deep as 200 metres, this can only be explained by regarding the ori-gin of the Gastern valley as the rcsult of glacial erosion. The vicws of the speciaiists in this respect are however widely divergent.”
Rock temperaturę
The maximum rock temperaturę encountered during the construction of the tunnel was 34° C. With the aid of forced ventilation the maximum air temperaturę was rcduced to 23° C. The air in the middlc of the tunnel has at present an average temperaturę of about 18° C. Thanks to electric traction it is no longer necessary to use forced ventilation and this parti-cular installation is no longer needed as the natural ventilation is fully sufficient.
The abutment and roof with masonry of the tunnel have been lined over the entire length; this has facilitated the main-tenance. Invert lining was only required ovcr a distance of 2661 metres.
Avalanche protection works
A few minutes after Goppcnstein the train will halt so that passengers may sec the Goppcnstein avalanchc protection works. The history of this work may be described as follows: The two entrances to the Lbtschbcrg-tunncl were and still are situated in the danger zonę of a number of ava-lanches. During the construction of the tunnel, a huge avalanche came down on the north side causing consider-able damage to the installations outside the tunnel entrance. The region where the avalanche had broken off was
Au lendemain de Feboulement on constata un affaissement dans la vallee de Gastern; il s’acccntua au cours des jours suivant$ et finit par atteindre une profondeur de 2-3 m avec un diametre de 90 m. II devenait donc parfaitement evident que le tunnel avait subitement debouche dans 1’ancienne val!ee de Gastem, autrefois beaucoup plus profonde, comblće de sable et de gravier aquiferes. On proceda immediatement a 2 sondages dans l’axe primitif, en partant de la vallee de Gastern. Ceux-ci furent menes jusqu’a la profondeur de 220 m sans atteindre le rocher. On decida donc de contourner remplaccmcnt de Feboulement et Fon choisit un tracę dont on pouvait etre assure qu’il serait cntićrement dans la roche.
II est interessant de relever qu’au debut des travaux, un gćologuc (L. Rollier) avait remis a la Socićte du Loetschbcrg un rapport dans lequel il s’exprimait comme suit au sujet de la traversee de la vallee de Gastern:
«Je crois quc les formations alluvionnaires, les moraines de fond et les depóts de rivieres vont plus profond que ne Fin-dique le rapport des geologues. On ne saurait cependant admettre qu’elles atteignent une profondeur de 200 m qu’en partant de la supposition que la vallee est due a Ferosion glaciaire. Sur ce point les opinions des specialistes divergent encore de faęon considerable.»
Temperaturę de la roche
La temperaturę maximale de la roche pendant la construction du tunnel atteignit 34° C. Par une bonne ventilation il a ete possible de maintenir la temperaturę de Fair au-desseus de 23° C. Aujourd’hui Fair dans le tunnel a une temperaturę moyenne de 18° C. Aprćs Fintroduction de la traction elec-trique, les installations devenues superflucs furent demontees, la circulation d’air naturcllc suffisant a assurer la puretedeFair. Le tunnel fut maęonne sur toute sa longueur (pieds droits et calotte), bien que ces mesures ne se soient pas averees absolu-ment necessaires. Elles se sont toutefois montrees utiles pour Fentretien. Un radier a ete necessaire sur une longueur de 2661 m seulement.
Quclques minutes aprćs Goppenstein le train fera halte pour permettre aux Congressistes de mieux voir les ouvrages de protection contrę les avalanchcs de Goppcnstein.
Ouvrages de protection contrę les avalanches
Les deux extremites du tunnel du Loetschbcrg etaient, et sont encore aijDurd’hui, en partie exposees a un certain nombre d’avalanches. Ainsi, lors de la construction du tunnel, une gigantesque avalanche descendit sur Fentree nord (sa zonę d’arrachement etait situee sur le Fisischafberg) et en-dommagea fortement les installations. La zonę de rupture de Favalanche fut immediatement consolidće et assuree par 130 murs de maęonnerie seche, remblayes a Farriere, et par 79 terras-ses; depuis lors, Favalanche ne cause plus de degats. Les avalanches qui menacent Fentree sud, ou se trouve la gare de Goppenstein, sont encore plus importantes et dangereuses. Ccttc gare est le point de rencontre de 6 grosses avalanches dont les zones de rupture sont situees a 1400 m au-dessus de la voie ferree. Des 1907 (la construction du tunnel fut commencee en 1906) il fallut se rendre a Fevidence qu’il serait necessaire d’ćdifier des ouvrages de defense dans les zones de rupture des avalanches, afin de proteger la ligne de chemin de fer. Les travaux ne furent pas immediatement mis
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