Método de predicción de las direcciones principales de drenaje en el karst

Alternative Title

Kobie. Ciencias naturales

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Publication Date

1-1-1985

Volume Number

15

Abstract

The drainage in karstíc and classical aquifers is very different. In karst, permeability occurs because there is a fissures interconnection whose water circulation involves rock dissolution. For that reason, both are qualitatively different. In karst, the dissolution resulting from water circulatíon brings about an enlargement of holes or interconnected fissures, decreasing consequently, íts pressure drop. So, ín consequence, the hydraulic gradient of water flow grows, icreasing the water circulation and so, successively. By means of a feed-back effect, only the fissures which receive a stronger water circulatíon flow, get larger, to the detriment of the others. The result of that phenomenon is a tridimensional conducts net which explains the high transmissivitíes as well as both directional and discrete characteristic of karstic aquifers. When, the externa! conditions involve an hydraulic gradient decrease, the system evolves towards paleokarst which is characterized by a general tendency of conducts fillings. In the case of hydrothermal karst, a lot of ore minerals have the same origin, between them, we find a great number ín Central Europe. Dam building in karstic regions, where dam site offers excellent conditions, in civil engineer's point of view, presents frequently, serious likeage problems. Their corrections are resulting so expensive as unforeseeable. The aquifers, highly vulnerable to pollution and frequently, used as water supply for towns as well as endorreic drainages of sorne coastal karstic aquifers díscharging directly into the sea etc., explain the high socioeconomic costs involved for usíng this bit of nature, into man's benefíts. For all these reasons, sorne contribution which is an advance in the karst knowledge, will bring an improvement of solutions in its practica! application. After a lot of decades and 5 years of contrasting and testing, we think that this method gives an advance in the knowledge of karst, for this motive, we proceed to its diffusion. The method is based on two hypotheses, the first one is qualitative and the second one is·quantitative. 1) Karst is predetermined by tectoníc conditions suffered by the rock massif. So, it determines the disposítion of the tridimensional net of drainage conducts, according to its geological history. 2) · The most probable drainage directions are organized inside plans whích have the maximum component (n1 ) and the intermediate component (n2 ) of each stress ellipsoid. In consequence, they are perpendicular to the minor component of each respective ellipsoid. Work field investigations were limited to the application of structural geological techniques and also to the definition of stresses suffered by the massif, givin the definition of their respective ellipsoids. A better solution is the microtectonic analysis. So, it's only necessary, to classify tectoglyphs conjuctions which, we shall use to define the searched ellipsoids. These conjunctions are principally: -Stylolite - Veine (E-V). -Stylolite Fault (E-F). -Veine - Fault (V-F). -Conjugated Faults (F-F). By applying the stereographic projection, we shall resolve for each case, the ellipsoid in Wulff's net. By applying the second work hypothesis, we shall determine the drainage plan. In practice, it's more interesting to work with the greatest number possible of tectoglyphs conjunctions to determine the probability degree of each defined mode. For representing them, we shall apply the stereographic projection, but in this case, using Schmidt's net. The result is a tridimensional polimodal with a quantified probability percentage for each of its modes giving the principal directions of the subterranean drainage. This result is valid for predictions. To make easier field data processing, the method is available with three computer programs: -GEORED: In which, we see Wulff's stereographic net (equiangular) as well as Schmidt's net (equiareal) for sorne angle of inclination and also for severa! parallels and meridians densities. -GEODRE: In which, the position of the ellipsoid component (o1, o2, o3) as well as the drainage plans for each respective conjunction are calculated and drawn by plotter. -GEOPOL: In which, for a determined plans or pales family, the areas of equal concentration, for each wished area percentage, are calculated and drawn by plotter. The method was successfully applied in eleven examples, ten were carried out in very different geological conditions of Spain karstic regions and the eleventh was carried out into ice karst of the Svalvard Archipelago Glaciers. lts accuracy degree is very high, superior to 95 % in the cases, in which, it was possible to quantify. The purposes of the study of the Spanish examples were: The method contrast in the case of large caverns nets whose topography were possible to carry out; the leaks prediction in dam building; the pollution progression in aquifers; the subterranean discharge of fresh water into the sea etc ... The examples were chosen in order to bring a great variety into the sampling. They were carried out into the «Macizo Pirenaico», the «Cornisa Cantábrica», the «Paleozoico Asturiano», the «Sistema Central», the «Cadena Ibérica», the «Cordillera Bética» and they were located in the most various calcareous outcrops from the Cambrian to the Tertiary. The prediction given by the method has been confirmed in the majority of cases by means of coloration with fluoresceine, which has been used in sorne cases to modify the original engineering projects, in arder, to adapt them to the actual situation. The method contrast in the case of having available the topography of large subterranean nets, can be carried out by mea ns of the polimodals shown, in this particular case; in two dimensions. lf we apply Kolmogorov's test to the accumulated curve, we shall be able to quantify statistically, the validity of the described method. At first, the described method is applicable to the following fields: -Knowledge of the karstic drainage net. -Study and operation of karstic aquifers. -Pollution progression and contamination in karstic aquifers. -Leakages prediction in dams located in karstic regions. -Carrying out of diaphragm walls and grouting curtains for the impermeabilization in civil engineering jobs, in karstic regions. -Detection of fresh water leakages in coastal karstic aquifers and submarine springs. -Saline intrusion in coastal karstic aquifers due to overworking. -Prediction of the mineralization directions interrelated with paleckarst. -Water intrusion in open pit mines located in karstic regions. -Excavation under the water table in karstic aquifers. And is generally, also used for the quantitative estimation in the directional aspect of both karstification potential and validity, in the theorical studies of karst. The karstification process is very important and is not exclusively limited to a specific type of rocks. lt has been observed not only, in carbonatic rocks but also in quarzites of the Sarisarinama region (South part of Venezuela). We should be able to use this nature phenomenon as work hypothesis, thinking that nature has had enough time, during 1.600 millions of years, to carry it out, but it's not enough ... lf as work hypothesis, the concept of dissolution is amplified to the concept of salid likeage, for instance: colloidal dispersion or state change dueto fusion, then, it's possible to use the same posings for the following aspects. -Piping in clays core of earth dams. -Lavic tubes produced in sorne volcanic rocks. -Ice karstification in Subpolar Glaciers and its practica! consequence, the fresh water supply, for bases located in the Antartic periphery. lt has been tested with success in the Arctic Circle.

Document Type

Article

Language

English; Basque

Notes

pp. 15-164

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