Over the years silo builders have improved the design and construction of the above-ground portion of silos but, in contrast, very little has been done to improve the foundation. Towers have generally been erected on foundations prepared by the farmer, who often lacked the necessary technology for adequate design and construction. The practice was reasonably successful when silos were small because the bearing pressures applied to the soils were low. As bigger silos were erected, however, and the applied foundation pressures approached the bearing capacity of the soils, many structures settled considerably, some tilted, and some overturned completely. This Digest outlines the problem and indicates the need for a soil investigation to determine the allowable bearing capacity and compressibility of the soil and thus enable proper foundation design.
Many tower silos constructed on clay soils have ring-shaped concrete foundations.
To reduce costs, concrete floors are seldom provided. When the silos are filled with silage, part of the load is transmitted through the cylindrical walls to the footings and the remainder is carried directly by the soil inside the ring foundation. The underlying clays compress vertically under the weight of the loaded structure in such a way that the applied loads are distributed uniformly to the soil over the whole area enclosed by the circular foundation. Here, two footings of different size carry the same uniform load, but the pressure bulb under the larger foundation is much larger and deeper. In each case the maximum vertical pressure occurs immediately below the footing and diminishes to 10 per cent of this value at a depth equal to twice the diameter of the foundation.