The south end of the Chilean broad gauge
Geological report on the Lonquimay tunnel
In connection with the studies of the Ferrocarril Trasandino por Lonquimay the Dirección General de Obras Públicas asked the distinguished geologist Hans Brüggen* to give his opinion on the geology of the area in which the Las Raíces tunnel was proposed to be built, and of the probability of encountering difficulties during its construction, particularly in respect of encountering water which might hinder the works.
Mr Brüggen has presented the following report.
The proposed tunnel will pass under one of the offshoot ranges of the Cordillera Blanca, the name which is given round about Curcautín for the range which appears on the map of the Oficina de Mensura de Tierras as Sierra Nevada. A ring of fells separates the basin of the Río Agrio, an affluent of the Cautín, from the Río Punta Negra, an affluent of the Río Lonquimay.
The whole area is covered with virgin forests which, together with the thick cover of soil, hide the underlying rocks. If we add to this the impossibility of covering any distance without first opening-up tracks, it may be understood the difficulty of studying the solid rock though which the tunnel is to be driven, and the impossibility of drawing an exact section of the strata. In practice, I found solid rock in only three locations in the vicinity of the tunnel, as are indicated on the plan and on the section as shown with hatching.
In the three locations the rocks were the same, porphyrites†, which are the principal rocks of the Andes Range in which they reach a thickness of several thousands of metres.
† [Porphyrite is the rock which forms the Andes. It is similar to porphyry, but does not contain quartz.]
They are very compact and strong rocks which in their properties for boring a tunnel are identical to those of porphyry. They are the same beds as are found in the Palos Quemados tunnel of the Longitudinal del Norte. The lack of stratification at the three points makes it impossible to say anything about the location of the porphyrite, but given the distribution of the out-crops, it is very probable that this rock extends without major changes over the whole length of the tunnel.
Outwith the porphyritic strata, which may be considered as suitable for the construction of the tunnel, other strata appear on the higher levels of the fells. These are much more unsuitable, and have been indicated on the section by the letter t. These are poorly cemented large volcanic blocks which alternate with very soft sands and extensive beds of lava. These are the rocks which may be seen near the gateway which leads to Lonquimay.
Of greater interest is knowing if these beds extend downwards as far as the tunnel, or if their lower boundary is as shown in the section. In view of the fact that the forest impedes direct observation of the boundary in the area of the tunnel, we must try to obtain practicals deductions of the recent geological history in the vicinity of Lonquimay.
Looking at the Andes Range between Curacautín and Lonquimay, this appears as an extensive high plateau cleaved by deep valleys. The surface of the high plateau, which rises to little over 2 000 metres; only the modern volcanoes, such as Llaima, Lonquimay, Trolhuaca, etc., stand above this.
The high plateau is the erosion surface formed before the tertiary era which in that epoch was lifted to its present height, drowning at the same time the great depression of the longitudinal valley.
In view of this origin, it presented only gentle undulations, offering a view of an almost perfect plain. On the plain conglomerates and volcanic lavas were deposited, which are seen in the gateway which leads to Lonquimay. The result of this, which is also the lower surface of the volcanic rocks (t in the section), must be an almost perfect plane surface as has been drawn in the section.
The points of eruption where the lavas escaped are not yet know; it would be a very unlikely circumstance if one of these points were to be found in the area of the tunnel; but even in such an event, we may expect that the ancient volcanic chimney is full of hard and resistant lava, and not of loose volcanic sands.
With the lifting of the range in the tertiary era, the rivers started to dig their deep valleys, first in the volcanic beds, and then in the hard porphyrite located farther down. The quaternary glaciers reached the river valleys giving them their present form. In this very modern epoch, in the quaternary, there were new eruptions which continue until today, forming the high volcanoes such as Llaima, etc. Some very big flows of lava descended along the valleys, like the one that follows the Río Cautín as far near Curacautín. In general the centres of modern eruptions are far away from the tunnel, so that it is not necessary to worry about the presence of major quantities of loose volcanic masses in the tunnel.
The deepening of the valleys was not a single continuous process; there was a long interruption during which time the rivers widened their valleys. Then later, due to new lifting, they returned to deepen their valleys and the old wide valley floor remained as a raised terrace.
The terraces are covered with fluvial boulders and glacial clays as can be seen very well in the small gullies at 100 to
200 metres to the east of the northern entrance to the tunnel. The depth s of these modern accumulations are more than some tens of metres, but given the nature of the terrace sediments they will disappear within a short distance of the tunnel mouth.
From the plan and section it may be seen that the tunnel will pass under an affluent of the Río Agrio. There is little probability that they tunnel will cut through the loose boulders which form the subsoil of the valley.
A very important factor for the construction of tunnels is the amount of water which they encounter. Above we have seen that porphyrite will be the only rock encountered in the tunnel and in such an event it will not be necessary to fear a great amount of infiltration in view of the fact that they are impermeable rocks. There will always be some infiltration and this is more important when cutting through areas of broken rocks or faults. If such areas exist in the length of the tunnel, this cannot be known due to the forest cover; but there is no reason to worry that they will give rise to excessive flows of water.
Neither is the presence of thermal springs likely in the tunnel. The tunnel is located in the midst of a volcanic region which has a large number of thermal springs, such as the baths at Río Blanco, Manzanares, Trolhuaca, to which we may add others less well-known, such as those which emerge at the side of the Lonquimay road at the south mouth of the tunnel, and others which remain hidden in the extensive forests. But if we relate the number of thermal springs which are to be found and then tripled that, it would still result in very few per square kilometre, and with this a very small probability of coming across a thermal spring in the tunnel.
Santiago, 20 March 1929.
(signed) HANS BRUGGEN
* Hans Brüggen, Johannes Otto Brüggen (1887 Lübeck – 1953 Santiago), was a German Geologist. He came to Chile in 1911, and in 1917 became Professor of Geology at the University of Chile. There is a national prize in geology in the form of the Profesor Juan Brüggen medalla al mérito, which is awarded by the Colegio de Geólogos every third year.
Informe geológico sobre el túnel de Lonquimay, Hans Brüggen, Anales del Instituto de Ingenieros de Chile, March 1929.