Transactions of the ASAE Vol. 45(5): 1569–1577 E 2002 American Society of Agricultural Engineers ISSN 0001–2351 1569 MECHANICAL PROPERTIES OF GRANULAR AGRICULTURAL MATERIALS M. Moya, F. Ayuga, M. Guaita, P. Aguado ABSTRACT. The use of numerical methods in the design of silos requires properties that describe the behavior of materials that are commonly stored in silos. Variations in the values of these parameters can result in the risk of failures in this type of structure. Because of the lack of existing data in the literature, the goal of this research was to: (1) provide different testing methods for determining these properties, and (2) determine values of the mechanical properties that can be used in either traditional or more recent silo design methods. Keywords. Silos, Numerical methods, Angle of internal friction, Dilatancy angle, Poisson’s ratio, Oedometer modulus. he design of agricultural silos requires the designer to understand not only the principles of structural design but also the properties of stored materials. Many researchers throughout the world are studying the behavior of stored materials so that the loads in these type structures can be better predicted. In order to provide guidelines to silo designers, the new Eurocode (ENV 1991–4, 1995) for bin design was recently published. This design standard predicts bin loads using Janssen’s (1895) equation. However, this design technique is questioned by some researchers (Aguado et al., 1998; Ayuga, 1995; Guaita, 1995) since it cannot accurately predict dynamic loads. To improve the structural modeling of grain bins, the finite–element method has been applied to the study of agricultural silos (Jofriet et al., 1977). With this technique, some aspects of bin loading can be studied that previously were not adequately modeled. To accurately predict silo loads, it is necessary to know the property parameters generated by the bulk material that influence the loads on these structures. Earlier designers of grain storage structures assumed that the stored grain produced pressure within bins similar to hydrostatic pressure. This technique did not take into account the properties of the stored material nor the friction forces that occurred between the grain and the bin wall. At the end of the 19th century, other design methods for silos were developed (Airy, 1897; Janssen, 1895) that took into account some properties of the stored material such as specific weight (g), internal friction Article was submitted for review in August 2001; approved for publication by the Structures & Environment Division of ASAE in June 2002. The authors are Manuel Moya, Agricultural Engineer, and Francisco Ayuga, Professor, Department of Construccion y Vias Rurales, Polytechnic University of Madrid, Spain; Manuel Guaita, Associate Professor, Department of Ingenieria Agroforestal, University of Santiago de Compostela, Lugo, Spain; and Pedro Aguado, Associate Professor, Department of Ingenieria Agraria, University of Leon, Leon, Spain. Corresponding author: Dr. M. Moya, Dept. Construccion y Vias Rurales, Polytechnic Univ. Madrid, E.T.S.I. Agronomos, Ciudad Universitaria s/n 28040 Madrid, Spain; phone: +34–91–336–58–65; fax: +34–91– 336–56–25; e–mail: manuelmi@cvr.etsia.upm.es. angle ( f), and grain–to–wall friction coefficient (m). Some of these design techniques are still commonly used today. Therefore, it is possible to find these properties in the literature for different granular materials that are usually stored in silos. Nevertheless, in many cases, the literature lacks information on the varietal differences of these materials, the level of grain maturity, and the moisture content. Recently, numerical methods have been used in silo design. Using this technique, it has been possible to better model the behavior of material inside a bin. However, in order to use these methods, it is necessary to consider additional material properties, such as elasticity modulus (E), Poisson’s ratio (n), and dilatancy angle (y). Currently, limited information exists in the literature on these design parameters. Therefore, in most finite–element models created by silo designers to study different topics related to silo design, such as silo discharge, hopper eccentricity, etc., it has been necessary to use values determined by other researchers and even randomly select values, which has resulted in some wrong solutions in applying numerical methods. P ARAMETERS AND TESTING MATERIALS While it is possible to find values of the internal friction angle, the apparent cohesion, or the specific weight in the literature for predicting bin loads using traditional methods of analysis, some parameters that are used by the finite–ele- ment method, such as the dilatancy angle and Poisson’s ratio, are almost unknown. Therefore, it is necessary to determine these parameters experimentally to obtain more accurate results when using this calculation technique. One of the goals of this research was to determine Poisson’s ratio, the dilatancy angle, the modulus of elasticity, the internal friction angle, the apparent cohesion, and the specific weight. All these properties can be determined using different testing methods. Poisson’s ratio relates the strain in the lateral direction of a material to the strain in the longitudinal direction. This is somewhat analogous to the K value, which is the lateral–to– vertical pressure ratio inside a grain bin. Therefore, many of T