Diseño sísmico por desempeño de taludes proyectados en base a criterios de estabilidad estática
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2016
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es
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Universidad Andrés Bello (Chile)
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Licencia CC
Resumen
Tradicionalmente, y desde el punto de vista de la práctica de ingeniería, el diseño de taludes bajo la acción de cargas estáticas y/o sísmicas ha sido abordado por medio de criterios de estabilidad. Matemáticamente, lo anterior se expresa en términos de un factor de seguridad (FS), que corresponde al cuociente entre las fuerzas solicitantes y resistentes involucradas en el problema.
Para este tipo de análisis, es común utilizar los denominados métodos de equilibrio límite. Éstos consideran el balance de fuerzas y/o momentos en una masa de suelo, que se supone rígida–perfectamente plástica, sobre un conjunto de potenciales superficies de falla. De esta forma, se debe garantizar que para todas ellas, el FS sea superior al mínimo definido en las bases de diseño de proyecto.
En particular, la estabilidad sísmica de taludes se suele analizar por medio de métodos pseudoestáticos, en los cuales los efectos de un terremoto dado sobre la estructura son modelados a través de fuerzas horizontales y/o verticales que actúan en el centro de gravedad de la masa de suelo potencialmente inestable. Estas fuerzas se expresan como una fracción del peso de la cuña de suelo analizada por medio de coeficientes sísmicos, y deben poder representar razonablemente las características principales del terremoto de diseño; por lo que en la práctica resulta complejo estimarlas de forma apropiada.
Por este motivo, en lugar de diseñar sobre la base de fuerzas, suele ser preferible diseñar sobre la base de ciertos desplazamientos admisibles que no comprometan la estabilidad global del talud y permitan mantenerlo operativo luego de concluido el sismo. Esta forma de abordar el problema es lo que da origen a los llamados métodos de diseño por desempeño.
En este trabajo se propone una metodología simple para abordar el diseño sísmico por desempeño de taludes homogéneos que han sido proyectados bajo criterios de estabilidad estática. Se busca establecer relaciones entre el FS asociado a un determinado diseño estático y el correspondiente coeficiente sísmico crítico ky del talud. De esta manera, utilizando procedimientos basados en el denominado método del bloque deslizante de Newmark, como por ejemplo el propuesto por Richards y Elms (1979), el proyectista tiene la opción de anticipar el nivel de desempeño de un talud dado, o bien conocer los requerimientos de estabilidad que debe satisfacer un talud para un nivel de desempeño objetivo que se busque satisfacer bajo un sismo de diseño.
A partir de un conjunto de análisis numéricos realizados utilizando el software SLOPE/W (GEO-SLOPE International Ltd.), se presentan soluciones en forma de ábacos de diseño para distintos tipos de suelo, geometrías del talud, y niveles de sobrecarga distribuida uniformemente a nivel de coronamiento.
Typically, and in the context of engineering practice, the design of slopes under the action of both static and seismic loading has been addressed through stability criteria. Mathematically, this is expressed in terms of a factor of safety (FS), which is the ratio between the driving and resisting forces involved into the problem. For this type of analyses, it is common to use the so-called limit equilibrium methods. They consider the balance of forces and/or moments in a soil mass, which is assumed to behave in a rigid-perfectly plastic way, over a series of potential failure surfaces. In this way, it has to be guaranteed that for all of them, the FS is greater than the minimum value defined in the design criteria of a certain project. In particular, the seismic stability of slopes is usually analized by pseudo-static methods, in which the effects of a given earthquake over the structure are modeled through horizontal and/or vertical forces, which act in the center of gravity of the potentially unstable soil mass. These forces are expressed as a fraction of the weight of the soil wedge analyzed, and they have to properly represent the main design-earthquake characteristics; so in practice, it is complex to estimate them accurately. For this reason, instead of designing through a force-based approach, it is preferable to do it over the basis of allowable displacements, which do not compromise the overall system stability and allow keeping it operative after the design-earthquake occurrence. This way to approach the seismic design of slopes is known as performance-based design. In this work, a simple methodology is proposed to address the performance-based seismic design of homogeneous slopes that have been designed using static-stability design criteria. It looks for stablish relationships between the FS associated to a certain static slope design and the corresponding critical pseudo-static seismic coefficient ky. In this way, by using procedures based on the well-known Newmark’s sliding block method, as proposed for example by Richards and Elms (1979), the engineer has the option to anticipate the level of performance for a given slope. In addition, it is possible to know the stability requirements that a certain slope has to satisfy for some pre-defined target performance level in a design-earthquake scenario. From a series of analyses performed using the software SLOPE/W (GEO-SLOPE International Ltd.), solutions in the form of design charts are presented for different types of soil, slope geometries, and surcharge loads uniformly distributed over the top surface of the slope.
Typically, and in the context of engineering practice, the design of slopes under the action of both static and seismic loading has been addressed through stability criteria. Mathematically, this is expressed in terms of a factor of safety (FS), which is the ratio between the driving and resisting forces involved into the problem. For this type of analyses, it is common to use the so-called limit equilibrium methods. They consider the balance of forces and/or moments in a soil mass, which is assumed to behave in a rigid-perfectly plastic way, over a series of potential failure surfaces. In this way, it has to be guaranteed that for all of them, the FS is greater than the minimum value defined in the design criteria of a certain project. In particular, the seismic stability of slopes is usually analized by pseudo-static methods, in which the effects of a given earthquake over the structure are modeled through horizontal and/or vertical forces, which act in the center of gravity of the potentially unstable soil mass. These forces are expressed as a fraction of the weight of the soil wedge analyzed, and they have to properly represent the main design-earthquake characteristics; so in practice, it is complex to estimate them accurately. For this reason, instead of designing through a force-based approach, it is preferable to do it over the basis of allowable displacements, which do not compromise the overall system stability and allow keeping it operative after the design-earthquake occurrence. This way to approach the seismic design of slopes is known as performance-based design. In this work, a simple methodology is proposed to address the performance-based seismic design of homogeneous slopes that have been designed using static-stability design criteria. It looks for stablish relationships between the FS associated to a certain static slope design and the corresponding critical pseudo-static seismic coefficient ky. In this way, by using procedures based on the well-known Newmark’s sliding block method, as proposed for example by Richards and Elms (1979), the engineer has the option to anticipate the level of performance for a given slope. In addition, it is possible to know the stability requirements that a certain slope has to satisfy for some pre-defined target performance level in a design-earthquake scenario. From a series of analyses performed using the software SLOPE/W (GEO-SLOPE International Ltd.), solutions in the form of design charts are presented for different types of soil, slope geometries, and surcharge loads uniformly distributed over the top surface of the slope.
Notas
Tesis (Ingeniero Civil)
Palabras clave
Estabilidad de taludes, Coeficientes sísmicos, Análisis estático, Análisis pseudoestático, Equilibrio límite, Diseño por desempeño