Self-weight collapse of Loessic Soils. Prediction by simple models(Autocolapso de Suelos Loessicos. Modelos Sencillos de Predicción)

R.E. Terzariol, M.E. Zeballos, R.J. Rocca

Dans les comptes rendus d’articles de la conférence: GEO2011: 64th Canadian Geotechnical Conference, 14th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, 5th Pan-American Conference on Teaching and Learning of Geotechnical Engineering

Session: Behaviour of Unsaturated Soils

ABSTRACT: Córdoba city has several kinds of loessic soils, with differences in the settlements due to the collapse of the internal structure during wetting. Most of these soils require external load for collapsing, but approximately 15% of the area contains self-subsiding soils type. When these soils are loaded with any construction, problem gets worse. There are laboratory and field tests for characterizing these behaviors. Settlement patterns in buildings have been evaluated using full scale prototypes, with different types of foundations in the 1970s. The ground was wetted through septic wells and loose connections pipes. Settlement measurements were followed for over two years. Recently, some schools buildings have been founded in collapsible soils. One of them has a deep septic well whose infiltration caused a localized increase in the groundwater level, leading a deep collapse around it. The settlements were propagated to the surface, creating a crater that reached the outbuildings. For the purposes of this research, there were made surface leveling and drilling in the influence area. The interpretation of the pattern of settlement produced by the advance of saturation was done with a simple model. There are comparison with this real case and prototype testing.

RESUMEN: La ciudad de Córdoba presenta distintos tipos de suelos loessicos, con diferencias en los asentamientos debido al colapso de la estructura interna, cuando el suelo se humedece La mayoría de estos suelos requieren de carga externa para que el fenómeno se produzca y aproximadamente, en el 15% del área, son autocolapsables bajo su propio peso. Cuando estos suelos están cargados por alguna construcción, el problema se agudiza considerablemente. Para estudiar estos comportamientos y tipificarlos, existen ensayos específicos tanto de laboratorio como in-situ. Las características del asentamiento en las construcciones han sido evaluadas mediante prototipos a escala natural, con distintos tipos de fundaciones, en la década de 1970. Se hizo colapsar al suelo mediante la pérdida de un pozo absorbente y sus conexiones y se midieron deformaciones durante más de dos años. Recientemente, se han construido en Córdoba algunas escuelas que se fundaron en suelos autocolapsables. En el caso particular de este trabajo, se presentan los estudios realizados en una de ellas, en donde un pozo de infiltración profundo generó un aumento localizado del nivel freático produciendo el colapso en profundidad. Los asentamientos se propagaron a la superficie, generando un cráter que alcanzó a las construcciones aledañas. A los fines del estudio, se efectuaron nivelaciones en superficie y perforaciones en la zona de influencia. Para la interpretación se empleo el modelo de asentamientos producido por el avance de la saturación. Se comparan los resultados entre el caso real y los ensayos de prototipos previamente descriptos. 1. INTRODUCTION Silty soils with metastable structure (due to the increase of moisture content), occupy the central semiarid region of Argentina, particularly in Córdoba city. These metastable soils may suffer the collapse of their internal structure under the action of its own weight, without increment of external forces (Terzariol, 2010) when wetted (self weight collapsing soils). There are different kinds of loessial soil, showing differences in the settlements due to the collapse of the internal structure when the ground is wet. Most of these soils require external load in order to the phenomenon occurs, but approximately 15% of the area are occupied by selfcollapsing soils, (Rocca et al. 2006). To study these behaviors, and typify this kind of soils, there are specific tests both laboratory (Rocca et al. 1992) and in-situ (Terzariol et al 1999). A previous research in the National University of Córdoba, that includes test on prototypes allows analyze those results against the behavior of buildings in this research, where there are similarities and differences. The geotechnical background of the city, plotted in collapsibility maps indicate that the area has a potential settlement of about 45 cm., in case of saturation of the first twelve meters. During the years 1999 and 2001, in the west suburbs of Cordoba, it was built a school complex that consists of a kindergarten, a primary school and a high school. The geotechnical exploration carried before the construction shows a thickness larger than 30 meters of silty self-weight collpasing soils. The structures consists in precast reinforced concrete columns, beams and panels, simply supported each one or connetced mean welded steel plates. The structures are founded by frictional auger and cast in place piles, 15,00 meters deep. The sewage, after being treated, are dumped in the ground through a 30,00 meters deep well. The drainage well (yellow point), was located in proximity to a sports courtyard. Figure 1 shows the facilities.

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