Los Tipos de Suelos Condicionan las Repercusiones de los Seísmos o Terremotos

Si usted vive en un área con altos riesgos sísmicos, obviamente no podrá evitarlos. Sin embargo, sí pudiera estar en sus manos paliar el efecto sobre la vivienda en la que habita y la vida de sus seres queridos. Con tal propósito, basta elegir el tipo de suelos sobre los que se asentará, así como la posición fisiográfica en la que se edificará. Si los terremotos son bastante imprevisibles, las repercusiones pueden paliarse con mayor precisión, aunque nunca exista certeza absoluta, por supuesto. A ha hora de escribir este post,  hace muy pocos meses que en España se sufrió el terremoto de Lorca (2011) y días atras en la Isla de Hierro (Archipiélago Canario). Por centrarnos en Lorca, no se trató de un evento de gran magnitud. Sin embargo las repercusiones fueron mayores de las que hubiera cabido esperar. La prensa se hizo eco del debate sobre las causas de tal desastre. Obviamente no puede compararse con los que suelen aparecer en la prensa mundial. Sin embargo, retorné por unas pocas horas al tema de la “edafología de los desastres naturales”. Y, para mi sorpresa, detecte abundantísima información en suahili, y prácticamente ninguna en español-castellano. Pues si, la cuestión estriba en que distintos tipos de suelo-regolito responden de forma diferente a los temblores, siendo las repercusiones sobre unos mucho más graves que en otros.  

 loma-prieta-cal-state-la-lomaprietasofsoil

Tipos de Suelos y Terremotos. Fuente Cal State LA.

Desgraciadamente soy mortal, y disto en demasía de atesorar una formación enciclopédica. En España, la docencia de la edafología no suele incluir estos temas.  Más aun, debido a que las zonas de riesgo sísmico se encuentran muy localizadas y que aun en ellas los terremotos de gran magnitud son afortunadamente escasos, no suele ser un tema que despierta un gran interés en la opinión pública, hasta que algo ocurre, claro está. Ahora bien muchos de los lectores de esta bitácora procedéis de Latinoamérica, en donde tales desastres son muy comunes y graves en ciertas regiones.

Efectivamente, tanto las propiedades de los tipos de suelos como las de sus regolitos subyacentes condicionan las repercusiones de los terremotos que pueden acaecer en un territorio. De este modo, entre enclaves próximos la susceptibilidad de que ocurran tragedias es mayor en unos lugares que en otros, incluso en distancias pequeñas. En consecuencia, el ciudadano que tuviera tal posibilidad, podría escoger lugares menos “vulnerables” que otros ante el impacto de estos eventos. Con independencia de la fisiografía que condiciona las avalanchas y deslizamientos, las propiedades de los materiales edáficos son muy variadas. Así, por ejemplo, las rocas blandas y porosas /y más aun si se encuentran rellenas de agua) resultan ser más vulnerables que las duras y compactas. La resistencia a la tensión, la velocidad con que se  transmiten las ondas y la licuefacción son elementos muy a tener en cuenta. De hecho, abajo podréis observar una clasificación de suelos-regolitos en función de su vulnerabilidad ante el efecto de los seísmos. También en sentido estrito, las propiedades de los tipos de suelos o edafotaxa “clásicos”resultan relevantes.  Sin embargo, es palmario que la ordenación urbanística no suele basarse en las recomendaciones que al respecto suele ofrecer la ciencia. Basta viajar, observar el terreno y la localización de los asentamientos.

Ni puedo ni me atrevo a realizar un resumen de todo el material que he detectado en Internet. Os dejo pues los contenidos y enlaces de varias páginas Web.  Reitero que por desgracia tal documentación se encuentra escrita en inglés. Ahora bien, no es de difícil lectura. Incluso en algunos casos se dan instrucciones a los ciudadanos para que inspeccionen debidamente el terreno  (suelo y fisiografía) antes de comprarlo para edificar su casa.  Espero que tal material sea útil con vistas a que los docentes divulguen sus contenidos tras una previa traducción a la ciudadanía. La física de suelos no es precisamente uno de mis puntos fuertes. Lo lamento sinceramente!.

Juan José Ibáñez

 typos-de-suelos-y-terremotos-diy-guides

Tipos de Suelos y Terremotos. Fuente Tiy Guides

Soil Type and Shaking Hazard in the San Francisco Bay Area 

Ground shaking is the primary cause of earthquake damage to man-made structures. When the ground shakes strongly, buildings can be damaged or destroyed and their occupants may be injured or killed.

Seismologists have observed that some districts tend to repeatedly experience stronger seismic shaking than others. This is because the ground under these districts is relatively soft. Soft soils amplify ground shaking. If you live in an area that in past earthquakes suffered shaking stronger than that felt in other areas at comparable distance from the source, you are likely to experience relatively strong shaking in future earthquakes as well. An example of this effect was observed in San Francisco, where many of the same neighborhoods were heavily damaged in both the 1906 and 1989 earthquakes. The influence of the underlying soil on the local amplification of earthquake shaking is called the site effect.

Other factors influence the strengh of earthquake shaking at a site as well, including the earthquake’s magnitude and the site’s proximity to the fault. These factors vary from earthquake to earthquake. In contrast, soft soil always amplifies shear waves. If an earthquake is strong enough and close enough to cause damage, the damage will usually be more severe on soft soils.

Soil Types and Shaking Amplification

One contributor to the site amplification is the velocity at which the rock or soil transmits shear waves (S-waves). Shaking is stronger where the shear wave velocity is lower. The National Earthquake Hazards Reduction Program (NEHRP) has defined 5 soil types based on their shear-wave velocity (Vs). We have modified these definitions slightly, based on studies of earthquake damage in the Bay Area. The modified definitions are as follows:

Soil type A Vs > 1500 m/sec Includes unweathered intrusive igneous rock. Occurs infrequently in the bay area. We consider it with type B (both A and B are represented by the color blue on the map). Soil types A and B do not contribute greatly to shaking amplification.

Soil type B 1500 m/sec > Vs > 750 m/sec Includes volcanics, most Mesozoic bedrock, and some Franciscan bedrock. (Mesozoic rocks are between 245 and 64 million years old. The Franciscan Complex is a Mesozoic unit that is common in the Bay Area.)

Soil Type C 750 m/sec > Vs > 350 m/sec Includes some Quaternary (less than 1.8 million years old) sands, sandstones and mudstones, some Upper Tertiary (1.8 to 24 million years old) sandstones, mudstones and limestone, some Lower Tertiary (24 to 64 million years old) mudstones and sandstones, and Franciscan melange and serpentinite.

Soil Type D 350 m/sec > Vs > 200 m/sec Includes some Quaternary muds, sands, gravels, silts and mud. Significant amplification of shaking by these soils is generally expected.

Soil Type E 200 m/sec > Vs Includes water-saturated mud and artificial fill. The strongest amplification of shaking due is expected for this soil type.

Caveats

Surface geology provides only a rough estimate of the site effect.

Map boundaries are accurate only to within about 50 meters.

Soft soils tend to overlie stiffer soils and bedrock. Sites on thin layers (less than 4 meters) of soft soil overlying stiff soil will behave more like sites on stiff soil.

Some inaccuracy is introduced by assigning NEHRP soil-types to a geologic unit on the basis of the average velocity for that unit. For example, there is a widespread (in the bay area) unit consisting of Quaternary sand, gravel, silt and mud. It has been assigned a soil-type of C, based on its average velocity. While the average velocity is within the range of soil-type C, some of the slower-velocity soils within the unit fall into the range of soil-type D. Because the unit is undifferentiated in our digital geologic data set, we have no basis for identifying the slower-velocity soils.

Soil Types Affecting Earthquakes

The bodies of the Earth lay on tectonic plates that slide around underneath the surface. These plates when collide will create earthquakes that shake the ground and cause damage to both the earth and to the nearby structures and lifeforms. For property owners who want to lower the risk of having a problematic living place, they should learn how to assess certain soil characteristics that make the property more resistant to the earthquake’s shaking. Property owners can construct their buildings on solid bedrock too.

Here are among soil types that can be affected through earthquakes:  

Sedimentary basins and deep valleys have loose soil that goes very deep thus shaking the most as these areas have a lower altitude.

There are many cities built in valleys in basins that could lead to earthquakes and sometimes to something more catastrophic.

For earthquakes occurring in soft soil, high raise buildings and bridges sustain most of the damage. Soil at higher altitudes does not carry as many vibrations and have more potential for landslides.

Areas that have lots of rock and highly compacted soil are the best earthquakes resistant. The hard rock areas will resist the shaking and do not break easily. The areas that are artificially filled with loose sand and once very wet, would suffer the most during an earthquake.

The more condensed and compact the soil, the more hardened the rock that the soil contains resulted less than the soil transfer vibrations. Looser soil tends to transmit vibrations that lead to more destruction during earthquakes. Over time, it can create more damage as the vibrations are also longer on looser soil.

Soil liquidification is the loss of strength in saturated soil after a buildup of pore water pressures during the dynamic loading.

Soils have the ability to resist force that comes horizontally or known as shear resistance.

Soils subjected to earthquakes can lose their shear resistance that will cause the soil to flow around in semi-liquid form, which can cause a lot of damage to structures resting on soil.

This liquidification is the most destructive effects in low-laying areas with poor compacted artificial fill. It can come up from the ground through cracks and make the sand deposit all over and deform the land permanently.

Soils and Earthquakes

What is Soil?

A surficial material formed by chemical, physical, and biological weathering.

What Variables Control the Soil in an Area?

Climate and weather

Topography

Time

Parent material

Vegetation (dependent on the climate, weather and water)

Biological and chemical agents

How Do Soils Vary?

Grain size and hardness (There are 3 basic particle sizes that create the 3 basic soil types: sand, silt, and clay.)

Color

Grain size and shape

Chemical composition

Amount of pore spaces – open spaces filled with air

Amount of moisture

Permeability

Why is Soil Important to Consider in an Earthquake?

Although structures are supported on soil, most of us rarely consider soil, its differences, and its subsequent effect on structures in an earthquake. Some soil is hard, like rock, and can support over 40 tons per square foot (Levy & Salvadori, 1992), while other soil is weak, like loose sand. Different soil properties can affect seismic waves as they pass through a soil layer. In some areas, there may be many different types of soils layered one upon another before hard rock is encountered. Sometimes, ground shaking will be amplified. This will influence what needs to be done to structures to help them fare better in an earthquake. Also, a phenomenon known as liquefaction or ground failure can occur in moderate to major earthquakes.

What is Liquefaction?

When there is ground water less than 30 feet from the surface in soils that contain layers of sand, the pressures generated by repetitive squeezing of the earth by several seconds of seismic wave vibrations will cause the ground water to flow up and out. When this occurs, the sand grains, which have no strength except when touching each other, are forced apart. The ground then takes on the properties of a semi-solid. When it happens over a large area, houses and buildings with inadequate foundations may actually sink slightly. When liquefaction happens in a small area, liquefied sand can be ejected to the surface through fissures in the overlying layers. Soil failure, as described earlier, will have a larger impact on pipelines and pile foundations, and other structures below the surface of the earth.

Does Liquefaction Always Occur During an Earthquake?

No. Liquefaction occurs only under ideal conditions as a result of an earthshaking event and is controlled by the following variables:

Grain size of the soil

Duration of the earthquake and amplitude and frequency of shaking

Distance from the epicenter

Location of the water table

Cohesiveness of the soil

Permeability of the layer

Where Do I Begin?

Investigation of the soil is a good place to start. Get a sample from your yard or the yard next door. Examine it with a magnifying glass. Draw a picture of how the soil looks under the magnifying glass. Start a soil collection.

What color(s) are the grains?

What general color is the soil?

What size(s) are the particles?

Do the particles have rounded or sharp edges?

Is there anything living in the soil?

Is it moist or dry?

Soil Profile

A soil profile is a cross section of soil layers with different characteristics. You can make one with a clear plastic tube.

Carefully dig a hole as deep as the tube is long.

put a bit of soil from the bottom of the hole into the bottom of the tube.

take soil a few inches from the bottom and place it in the tube.

Continue this procedure until the tube is full of soil.

Next, evaluate and note the soil profile characteristics with the following questions:

Do the colors of layers vary?

Where is the darkest soil? The lightest?

Where are the most stones?

What can you learn by looking at the different layers?

Is the soil further down in a hole always the same as it is at the top?

Materials:

fine, well sorted sand (i.e., most grains the same size)

flexible plastic cup

8-12″ pie pan

1 oz. or larger sinker

beaker,125 ml water

Procedure:

Carefully cut off the bottom portion of the plastic cup (within 1 1/2 cm from the bottom).

Invert cup and place in the middle of the pie pan.

Pressing down on the cup, slowly pour the sand into the inverted cup to a level approximately 12 cm from the top. Make sure the sand is level, but do not try to compact it.

Gently place a sinker or comparable object on the surface of the sand.

Holding onto the cup, slowly pour 125 ml of water outside the cup into the pan. Record the time it takes for the water to migrate or move upward to saturate the sand (permeability).

Firmly holding the cup in place, tap forcibly on the side of the cup. What happens to the sinker?

What did you learn from this experiment?

Additional Resources:

Earthquake Engineering Research Institute. (January 1994). Earthquake basics: Liquefaction what it is and what to do about it.

Hilston, P., & Hilston, C. R. (1993). A field guide to planet earth: Projects for reading rocks, rivers, mountains, and the forces that shape them. Chicago, IL: Chicago Review Press.

Jennings, T. (1989). The young scientist investigates: Rocks and soil. Chicago: Chicago Children’s Press.

Levy, M., & Salvadori, M. (1992). Why buildings fall down: How structures fail. NY: W. W. Norton.

Model developed by: Len Sharp, Robert Allers, Borys Browar, Daniel Parke, John Rice, Richard Thomas.

Ver también el contenido de este pdf.

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Geo_p037.shtml

Is soil structure an important factor in earthquake dynamics? Investigate soil liquefaction and how different soil types respond to earthquake movements. Are movements more dramatic in sandy/loamy or clay type soils? Which soil structures are most stable? Which are the most volatile? (MCEER, 2005)

MCEER, 2005. “Soils and Earthquakes,” Multidisciplinary Center for Earthquake Engineering Research [accessed 2/25/06]

Earthquake and Soil liquefaction Kikipedia

http://en.wikipedia.org/wiki/Soil_liquefaction

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