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Purpose of soil laboratory tests
Laboratory tests on soils to determine their physical properties are an integral part of the design and construction of foundations and earthworks, as well as of specifications for their quality control.
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Soil as an engineering material
Civil and Environmental Engineering include the conception, analysis, design, construction, operation and maintenance of a wide variety of structures, facilities and systems, which are built on, in, or with soils and/or rocks (Mitchell & Soga, 2005). It is for this reason that the properties and behavior of these materials have a great influence on the success, economy and safety of the construction site. Therefore, to properly address the earth materials associated with any geotechnical project and/or problem, it is necessary to know, understand and appreciate the importance of geology, materials science, materials testing and mechanics, disciplines that come together in the soils laboratory.
Figure 1 Civil projects built on, in, or with soils (Source: www.google.com).
In this context, it is important to note that uncertainty is an important factor, always present in the entire geotechnical environment. Unlike other Civil Engineering disciplines that deal with engineered and manufactured materials (such as steel and concrete, for example), Geotechnics must deal with a wide variety of different materials, originating from complex geological processes, which are usually intermixed and with properties that are difficult to determine precisely, as they depend on various factors (such as grain size distribution, mineralogical composition, moisture content, variations in water table depth, and stress history and level, among others). In addition, the information available is usually limited and imperfect in any geotechnical study.
Considering the above, the role played by the soil laboratory is essential in any civil and/or environmental project, since the fundamental equations (theoretical and practical) that are part of Soil Mechanics, can be properly applied if, and only if, the physical parameters used in these equations, were properly evaluated in the laboratory.
Types of tests at soil laboratory
What are the types of tests performed in a soil lab? In general, the tests performed on soils in order to achieve their characterization can be divided into two main categories:
- Classification tests, from which the general type of soil and the engineering category to which it belongs are determined (here it may be useful to review the articles The Unified Soil Classification System (USCS) and The AASHTO Soil Classification System, published in this blog).
- Special tests to determine engineering properties, such as shear strength, compressibility, and permeability.
Figure 2 Soil characterization laboratory test category.
According to the above, it is evident that the classification tests are the basis for identifying and knowing the main engineering characteristics of the studied soil, while the special tests are essential for estimating the geomechanical properties of the ground, which will be used in the tests. constitutive models for geotechnical design.
Applications and standards
The development and constant updating of laboratory procedures and equipment over the years, has resulted in a greater understanding of the nature and likely behavior of soils as engineering materials.
Thus, some of the practical applications associated with the performance of laboratory tests that allow adequate characterization of the soil are the following:
- Reducción de la incertidumbre en el análisis de fundaciones y obras de tierra.
- Economía en el diseño, debido al empleo de factores de seguridad menores.
- Desarrollo de proyectos en sitios difíciles (terrenos altamente compresibles, potencialmente colapsables, expansivos, o licuables).
- Optimización de proyectos por el empleo de suelos como materiales de construcción (como por ejemplo, en presas de tierra o terraplenes).
- Control de calidad de movimientos de tierras.
Aún cuando existen normas locales en casi todos los países o regiones, la mayoría de los laboratorios alrededor del mundo realizan los ensayos siguiendo las normas publicadas por la American Society for Testing and Materials (ASTM) y por la American Association of State Highway and Transportation Officials (AASHTO).
En el caso de países de Europa Occidental, generalmente se sigue lo estipulado en el EUROCÓDIGO (el cual está basado, en el área de Geotecnia, mayoritariamente en las normas inglesas), mientras que en muchos de los países de Europa Oriental se observa en su normativa una fuerte influencia rusa. Sin embargo, huelga decir que estas normas no difieren significativamente de las normas americanas.
En todo caso, la realización de ensayos de laboratorio de calidad, y en la cantidad mínima necesaria para el proyecto que se desarrolle, permite alcanzar una mejor comprensión de las propiedades del terreno y, por ende, realizar diseños menos conservadores y controles de calidad adecuados, orientados a optimizar los costos asociados a fundaciones y obras geotécnicas de los proyectos.
References
- Das, B. (2002) “Soil Mechanics Laboratory Manual”. Fifth Edition. Engineering Press, Inc. California, USA.
- Head, K. (1980) “Manual of Soil Laboratory Testing – Volume 1: Soil Classification and Compaction Tests”. Pentech Press. London, UK.
- Mitchell, J. & Soga, K. (2005) «Fundamentals of Soil Behavior”. Third Edition. John Wiley & Sons. New Jersey, USA.
