Department of Physical Properties of Plant Materials

Swelling pressure


The uncontrolled increase of moisture content may take place in stored grain due to grain respiration or as a result of wetting with ambient air during aeration. Increased grain moisture content leads to an increase in volume. Walls of the silo confine deformation of the grain in the horizontal direction that may lead to an increased lateral pressure. A theoretical model was developed to estimate the relationship between increased moisture content and increased lateral pressure. The model was based on the assumption that an increase in grain volume was equal to the volume of absorbed water, a decrease in grain elasticity due to moisture content increase was taken into account. Resultant volumetric strain εv is a sum of two independent components: εc strain generated by external pressure and εu strain resulting from grain swelling. The model adequately predicted the upper limit of pressure increase reached under a condition of high initial compaction of material, and in the range of water content increase below 0.02 %.


Experimental stand for examination of pressure increase caused by water absorption consisted of a model silo (0.61 m in diameter and 0.75 m high) with instrumentation to determine the radial distribution of vertical pressure on the floor and top cover of the silo, and the mean lateral pressure. The set up was equipped to generate, dose and apply moist air to the grain contained in the model silo.


Radial distributions of normal pressure σv on the floor of the model silo are shown in the figure for several values of increased water content. Pressure increase was found to be highest at the center of the floor and decreased towards the wall due to a decrease in rate of water adsorption along the path of air movement. The mean lateral pressure σn increased approximately to 125 kPa/kg/kg.
Horabik J., Molenda M.: Grain pressure in a model silo as affected by moisture content increase. International Agrophysics, 14(4), 385-392, 2000.