RESÚMEN
La sobrecarga de un terraplén genera un incremento de las tensiones neutras o presión de poros en los suelos de fundación de obras en zonas costeras por la presencia de suelos finos o cohesivos. El exceso de presión de poros es disipado en tiempos inversamente proporcionales a la permeabilidad de los suelos. Con el caso de suelos arcillosos, este proceso puede tomar tiempos prolongados debido al proceso de consolidación, que pueden afectar la vida útil de las estructuras, de pavimentos u obras de arte. La determinación de los espesores de suelos compresibles y sus propiedades se tornan muy decisivas a la hora de determinar las alternativas para el mejoramiento de las condiciones de las fundaciones. El exceso de presión neutra generado por la sobrecarga en el suelo puede medirse con la ejecución de los ensayos de disipación de presión de poros con el piezocono (CPTU). Esta alternativa permite realizar ensayos a distintas profundidades del suelo de fundación y en puntos variables de la obra. Se presenta la metodología de medición y resultados prácticos de mediciones.
Introducción
La sobrecarga de un terraplén ejecutado sobre suelos compresibles genera el incremento de las tensiones en los suelos de fundación y de las tensiones neutras. Por la condición saturada en general de los suelos de fundaciones y por la presencia de suelos finos, las presiones neutras pueden ser investigadas con el equipo del CPTU (piezocono). Este ensayo consiste en la penetración estática de un cono que permite la determinación del exceso de presión de poros por medio de equipos conectados a un ordenador de registros digitales.
La medición del exceso de presión de poros se realiza con ensayo de disipación de presión de poros, deteniendo el avance de la introducción del cono y permitiendo la disipación de la sobrepresión de poros generada por el desplazamiento del cono al ser introducido en el suelo. Denominando a U2 la sobrepresión de introducción del cono, U1 a la presión disipada y estabilizada y Uo la presión estática, el exceso de presión de poros en el tiempo puede entonces calcularse por:
donde U es la presión de poros en el momento t de la medición.
La instalación de piezómetros eléctricos en puntos específicos de las fundaciones facilita la interpretación del incremento de la presión de poros por la ejecución de un relleno. Las mediciones en el tiempo van mostrando la variación de las tensiones asociadas a la construcción del relleno y al incremento de las tensiones.
En la Fig. 1 puede verse la curva de presión de poros generada por el relleno (5) que luego de finalizar la construcción del terraplén (2) y (3), tiende a disiparse y se orienta hacia la curva (1) de la presión estática. Por otro lado, la tensión efectiva (6) se incrementa en el tiempo, con tendencia a alcanzar a la tensión efectiva final del proceso de consolidación (8).
El CPTU o piezocono tiene la ventaja de ser un ensayo que puede ubicarse en forma relativamente fácil en cualquier punto de la obra y en la profundidad requerida. Puede considerarse como un equipo de instrumentación geotécnica móvil, a diferencia de los instrumentos fijos. El conocimiento de la presión de poros en un tiempo t desde el inicio de elevación del relleno tiene dos aspectos importantes para la construcción: a) La evaluación del incremento de la resistencia no drenada de los suelos de fundación en caso de suelos cohesivos, y b) La evaluación del avance de la consolidación y medición de la eficacia de sistemas de drenajes de las fundaciones para el aceleramiento de la consolidación, como ser los drenes verticales.
El piezocono o CPTU es una extensión del ensayo del cono estático, que por la medición de tres parámetros, a saber, la resistencia de punta, Qc, la fricción lateral, fs y la sobrepresión de poros, U2, permite la identificación del tipo de suelo por el comportamiento de estos tres factores y diversas correlaciones desarrolladas, como las relaciones Rf y Bq, [4], a partir de la permanente investigación desarrollada sobre los mismos. Para la ejecución del ensayo del CPTU se requiere de un equipo de reacción o penetrómetro, y equipos de lectura de las señales digitales de los transductores de presión residentes en el extremo del cono. Las lecturas típicas se presentan en los gráficos como se observa en la Fig. 2 que son lecturas de ensayos de 15.0 m. en la Bahía de Asunción.
Incremento de la Resistencia No Drenada de los Suelos Cohesivos
La construcción de un terraplén sobre suelos blandos compresibles puede realizarse siguiendo el método de construcción por estadios o etapas. En este método el ritmo de elevación del relleno está ligado al incremento de la resistencia no drenada del suelo de fundación debido a la consolidación. Así, los modelos de análisis pueden realizarse siguiendo el incremento de las capas constructivas de acuerdo al teórico aumento de la resistencia del suelo utilizando el Análisis de Resistencia No Drenada (U.S.A.) [3]. El método del U.S.A. está basado en el conocimiento de que la resistencia no drenada está asociada a la disipación de la presión de poros y la consolidación, aspecto que fueron abordados por diversos investigadores como Ladd & Foot (1974) y luego Ladd (1991). [1].
Dos expresiones que muestran la relación de la resistencia no drenada de las arcillas a las tensiones efectivas son:
En ambas expresiones, la resistencia no drenada, Su, está relacionada con el incremento de la tensión efectiva, que por ultimo depende de la disipación de la presión de poros. Estas fórmulas utilizadas en el análisis de estabilidad, pueden ahora ser perfeccionadas con la medición real de la condición de la tensión efectiva a una profundidad dada a partir de los ensayos que se pueden realizar con el CPTU en la sección que requiere el estudio y en un momento dado de la construcción. La utilización de drenes verticales es una medida generalmente utilizada para que el ritmo de consolidación aumente y se logre el aumento de la resistencia del suelo a corto plazo. Así, el grado de estabilidad del terraplén y sus taludes, pueden ser evaluados en la obra con la observación del exceso de presión de poros y el cálculo de la resistencia no drenada del suelo de fundación.
La aplicación del método de la construcción por estadios o etapas requiere siempre de la observación geotécnica acompañada de instrumentación como las placas de deformaciones, inclinometros y piezómetros, que sumados a los ensayos móviles del CPTU pueden en muchos casos prevenir que se produzcan fallas y decidir si el aumento de la siguiente capa de relleno puede ser realizado. El método Observacional en la Mecánica de Suelos [7] en este caso alcanza su máxima expresión.
Disipación de la Presión de Poros y Tiempo de Consolidación
La estabilización de las deformaciones de las fundaciones y del terraplén, requeridos muchas veces para la continuación de la obra y la construcción de obras de arte, pavimentos y estructuras, dependen del avance del proceso de consolidación.
La medición de las deformaciones con el seguimiento de la disipación de la presión de poros con el método de campo del CPTU, dan los parámetros requeridos para la toma oportuna de las decisiones.
El ideal es que el exceso de presión de poros sea nulo, y que se haya logrado el fin de la consolidación primaria. Esto puede ser verificado en forma práctica en puntos diferentes de la obra y así tomar las decisiones prácticas a partir de esa evaluación.
En la Fig. 3 puede observarse el grafico de disipación del exceso de presión de poros generados por la introducción del cono (U2), la curva de disipación hasta alcanzar la estabilización aproximada en el exceso de presión de poros causados por el terraplén, aun no disipados (U1), y el nivel de presión hidrostática (Uo) que corresponde al nivel freático. Es un ensayo de disipación de campo en la Bahía de Asunción a la profundidad de 8.96 m. a partir del cual se puede realizar la evaluación del exceso de presión existente y el tiempo para su disipación con la determinación del coeficiente Ch.
Conclusiones
La evaluación de la estabilidad de un terraplén y el tiempo para la estabilización de las deformaciones pueden estimarse en el proyecto y medirse en el campo en forma directa durante la construcción, con la utilización del CPTU.
Entre los factores que pueden mejorar la estabilidad de un terraplén está el aumento de la resistencia de los suelos de fundaciones. Este aumento se da por el proceso de consolidación durante el proceso constructivo y debe ser evaluado continuamente, para aplicar el concepto de aumento de resistencia de los suelos de fundaciones y la construcción por elevación de estadios o etapas de relleno.
El método constructivo por etapas con el método observacional geotécnico en mecánica de suelos y el análisis U.S.A. (por incremento de la resistencia no-drenada) con la incorporación de instrumentación móvil con el CPTU permite mayor flexibilidad a este método constructivo, a diferencia de un proyecto que solo cuenta con instrumentación fija geotécnica. Esto resulta en importantes beneficios para la obra y para la rápida y segura culminación de la misma.
El plan de instrumentación y monitoreo durante y después de la obra es un componente fundamental para la seguridad de las obras y el éxito del proyecto bien planificado. El proyecto a su vez debe ir acompañado por profesionales calificados en el diseño de la obra que puedan alertar de cualquier anomalía que pueda surgir en el proceso constructivo y en la etapa posterior a la conclusión de la obra.
REFERENCIAS
[1] – TAN, Yean Chin; “Embankment over Soft Clay –Design and Construction Control”. Geotechnical Engineering 2005. p. 1-13
[2] – Stanichevsky, Miguel. “Informe de Verificación del Terraplén Refulado de la Avenida Costanera Norte – Primera Etapa”. GEO-STAN S.R.L., Asunción, 2010. No publicado.
[3] – LADD, C.C. (1991) “Stability Evaluation during Staged Construction”. ASCE, JGED, Vol.117, No.4, pp.540-615.
[4] – SCHNAID, Fernando; “Ensaios de Campo e suas aplicacoes a Engenharia de Fundacoes”. Oficina de Textos, 2000. p. 68-70
[5] – BALIGH, M.M., AZZOUZ, A.S., WISSA,Z.E., & MARTIN .R.T. “The Piezocono Penetrometer” Cone Penetration Testing and Experience, St. Louis, Missouri, 1981.
[6] – HACHICH, W., FALCONI, F.F, SAES, J.L., FROTA, R.G.Q, CARVALHO, C.S., NIYAMA,S. “Fundacoes Teoria e Practica” PINI, ABMS/ABEF, 1996. p.124
[7] – PECK, Ralph. B. “Advantages and Limitations of the Observational Method in Applied Soil Mechanics” (1969). Geotechnique, V19, No.2, pp.171-187.
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