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Geotechnical protection in case of earthquakes

Domenii publicaţii > Stiinte ingineresti + Tipuri publicaţii > Articol în revistã ştiinţificã

Autori: Vasile Tudor

Editorial: Ad Astra, 2011.


Together with the volcanism and the floods, the earthquakes are the greatest natural catastrophes which cause terrible damages and human losses. In his book „Natural issues” the famous philosopher Seneca mentioned: „All dangers have a cure, and we can keep away from all of them, the lighting did never destroy entire populations, the plague depopulated the cities, but did not destroy them. But the catastrophe of the earthquakes is the most spread one, the most unavoidable, the most inexorable, the most general of all the dangers”. In spite of all the international material and financial efforts allotted for the study of earthquakes, their forecasting and the protection measures are still inefficient. The concerns of the seismic engineering have become state politics which is concerned with the drawing of the seismic zonation maps, with the elaboration of the planning norms for secure buildings, with the seismological education of the population and with the assurance of the human settlements. There is a large network of geophysical observatories and of seismological stations equipped with modern equipments which record a series of geophysical parameters that refer to gravitation, magnetism, electricity, radioactivity, as well as to the movements of the earth shell. The information achieved through personal effort or through international collaboration is analyzed on modern computers in order to decipher the causes of the earthquakes appearance, of the propagation of the seismic waves, but also for the identification of certain prediction methods and of certain methods of limitation of the destructive effects. On the way of confronting the ideas for the deciphering of the earthquakes mechanisms, people went beyond the naive conceptions, based on myths and legends achieving bold hypothesis about the continental drift and the expansion of the ocean bed, included afterwards in the theory of the global tectonics. It is considered that the lithosphere (the solid shell, with a thickness between 70 and 100 kilometers) is divided into tectonic plates which float on a sluggish stratum called asthenosphere which is moved slowly by the convection currents generated mainly by the calorific gradients. When the tectonic plates come close to one another great tensions may appear, which can cause earthquakes. The strength of the earthquakes is evaluated according to intensity and to magnitude. For intensity, the most well-known scale is the Mercalli scale – modified (MM scale), which classifies the earthquakes according to 12 degrees, on the basis of their effects upon people, upon the buildings and upon the ground. The magnitude scale was initially elaborated by C. Richter and perfected subsequently by B. Gutenberg, in order to eliminate the subjective estimations in the evaluation of the seismic waves strength, by being defined on the basis of certain amplitude recordings with standard seismometers. It is considered that the greatest earthquakes that happened on earth cannot theoretically outrival the value of M = 9 due to the resistance limit of the rocks. Furthermore, we present a geotechnical solution for the protection of the human settlements against earthquakes. At the moment, the protection of the buildings against earthquakes is achieved by means of various building solutions, based on superstructures of light materials, but resistant to the mechanical loads, complemented by flexible elements or even attenuation appliances which undertake the seismic shocks, however, they are expensive and difficult to apply on a large scale. The anti-seismic buildings need to have resistant fittings, without large openings and massive decorations, having the fundament structure adapted to the geological conditions of the area. The invention excludes these disadvantages by means of the fact that the geotechnical protection of the buildings against the earthquakes is achieved on zonal scale through underground structures situated in the way of the seismic waves, in the vicinity of the protected human settlement, which ensures the change of the propagation direction of the incidental seismic waves by means of the reflection and refraction phenomena, due to the modification of the mechanical properties of the environment which included it. By applying the invention, the following advantages can be obtained:
-common protection of all the buildings of the set up area against the earthquakes, along centuries.
-the increase of the land steadiness set up anti-seismically against the tectonic processes.
-the redemption of the investments for the human settlements anti-seismic set up by means of a considerable decrease of the damages and of the human losses.
The huge energy liberated in a seismic center propagates through mechanical waves of volume – longitudinal P, transversal S – until they reach the Earth surface, where the surface waves L appear through interference, as long waves, Love and Rayleigh type, which in case of high magnitudes produce fissures and damages of the constructions. The earthquake is a complex process – for the significant spectral components, obtained through Fourier analysis – it has the wavelength starting from minimal values of thousands of meters until maximal values of tens of kilometers. The mechanism of the deep earthquakes with intermediate centers is based on the litospheric plates dynamics which tend to achieve an isostatic balance on the asthenosphere under the influence of the convection currents generated mainly by the temperature differences. Huge tensional accumulations happen in the subduction areas, which cause seismic movements by means of their sudden discharge. Besides the tectonic processes, other causes which can generate earthquakes are related to volcanism, to tide waves, to rock failures, to the impact of the meteorites, to the sudden variations of the air pressure or they are caused artificially by means of powerful explosions. The amplitude of the seismic waves decreases as the covered distance increases due to the energy absorption processes of the earthly substance. The change of the seismic waves propagation direction happens on the surfaces which separate two environments with different physical properties, a phenomenon met for example in case of Moho discontinuity between the earth shell and the mantle. The waves reflection is characterized by a reflection angle equal with the incidence angle, while in case of the waves refraction, the relation between the sine of the incidence angle and the sine of the refraction angle is equal to the relation of the waves propagation speeds in the two environments. In general, the reflection and refraction of the waves are two phenomena which happen simultaneously, but if the waves propagation speed of the former environment is slower than the waves propagation speed of the latter environment, in case of incidence angles larger than the limit angle, the refraction phenomenon disappears, while the phenomenon of total reflection takes place in the separation surface. The earthquakes forecasting still contains a lot of unsolved equations, the only protection measures of the human settlements are offered by safe buildings or by the geotechnical solutions of anti-seismic zonal set up which redirects the huge energy of the seismic waves. As the anti-seismic structures have an immense gauge, the building solution achievable in practice is the network type which modifies the physical properties of the environment which includes it, mainly the rigidity and the average density. The general dimensions of a regular anti-seismic network have to be with at least one size higher than the wavelength, so that it will not be avoided by the seismic waves by means of the diffraction phenomenon, while the length of the network sides have to be at least 5÷10 times smaller than the wavelength, so that the reflection phenomenon can act in accordance to the gravity of the transmitted seismic waves. The mentioned wavelength corresponds to the earthquakes with maximum intensity which propagates in the anti-seismic set up area, that are taken into consideration for the drawing of the seismicity maps. The refracted seismic waves, or the ones that cross undiverted through a rigid network included in a propagation environment with reduced hardness, shade away when the length of the network sides is smaller, and when the distance traversed in the network is higher. The invention is applicable not only to the lands situated outside certain human habitats, but also to the cities, as the seismically protected area is larger than the area in which the underground structure is situated, largely extending behind it towards the incident seismic waves propagation direction.
The anti-seismic structure is composed of a regular triangular network, which continues in depth with the vertical columns 2, fixed in the ends of the equilateral triangles with sides 1 and placed parallel and equidistant in the mass of the seismic waves propagation environment. Sides 1 of the R regular network have a rectangular profile, while the vertical columns 2 are cylindrical-similar to the shafts obtained through sinking. The R network is situated at the border towards epicenter of the city perimeter, with a length of several kilometers or even of tens of kilometers, being situated approximately parallel to the leveled surface of the ground, at a depth h = 2÷3 m, in order to allow the performing of agricultural works. In case of grounds with a high slope, columns 2 can be placed in a declivous manner towards the vertical. If the geomorphologic conditions are favorable, the underground structure will be oriented towards the propagation direction of the surface seismic waves, under an incidence angle larger than the limit angle in order to produce the total reflection phenomenon. The geotechnical structure is usually made of ferro-concrete, but we can use any other material if it has the rigidity and the density different from the ones of the underground set up anti-seismically. The geometrical dimensions for the components of an underground anti-seismic structure vary in accordance to the maximum intensity of the earthquakes, to the distribution of the seismic centers and of the tectonic failures, but also to the nature of the building materials and to the area of the protected surface. For example, we present the following dimensional values of an underground geotechnical structure: a=30÷100 m, b = 1÷2 m, c=0,3÷0,6 m, d=0,3÷0,6m, l=50÷500 m, where a, b and c represent the length, the breadth respectively the thickness of side 1 of the R network, while d and l represent the diameter respectively the length of columns 2 of the underground structure. In other building cases, the R network is formed of regular square polygons or hexagons, in which case the material demand is reduced, but the resistance to the mechanical demands decreases in comparison to the regular triangular network. The present technique allows the building of an anti-seismic set up of the territory which includes the following stages:
– the planning of the project in detail by the specialists in seismology and geo-techniques, adapted to the concrete geo-morphological conditions;
– the building in the ground of the fosses for network R and the sinking of the shafts for columns 2;
– the building of the metallic fittings which are covered with concrete;
– the end of the works by covering with ground the fosses, and the leveling of the ground.
In order to simplify the execution operations, the underground geotechnical structure is mainly placed in the underground of light grounds, without big accidents, by avoiding the acclivous grounds made of hard rocks.
Although the presented technical solution requires important investments, it is a zonal alternative more efficient than the anti-seismic protection of each building. It is easy to imagine, the set up of certain parts of the seas and oceans continental shelf with similar water structures for the protection of the shores against tsunamis by means of the reflection, refraction and interference phenomena of the seismic waves.

Cuvinte cheie: protelisav, voxinventica, earthquakes