Pressure bulbs will overlap, as shown in Figure 1.4, if two or more silos are constructed too close to each other. Because pressures are additive, the resulting pressure bulb will be much larger and will extend to greater depths. The soil in the overlap zone will be subjected to higher stresses and the foundations over this region will settle more, causing the silos to tilt towards each other.
Most foundation in clay soils occur when a silo is quickly loaded for the first time. As filling proceeds, the loads are applied to the soil skeleton and to the pore water contained within the voids of the clay. Pressures generated in the pore water tend to reduce the friction between soil particles and hence decrease the shear strength of the soil. If, at the end of loading, the available shear strength is greater than the applied shear stresses, the structure will be stable. With time the excess pore water pressures will dissipate, the soils will consolidate and gain strength, and the structure will be stable for subsequent loadings.
1.8 Soil Investigations Needed
Bearing capacity and vertical settlement are directly related to the engineering properties of the soil at a site, so that a soil investigation is required to provide information on soil profile, location of the groundwater table, index properties, shear strength, and compressibility. Previous Digests provide more detailed information on engineering site investigations and soil testing.
To determine the bearing capacity of soil, shear strength measurements are required to a depth below the footings equal to two thirds the diameter of the foundation. The number of tests required as a function of depth should be sufficient to determine the strength of the various strata within this depth.
Shear strength may be measured by several methods: in the field, in situ, by means of a field vane or cone penetrometer pushed vertically into the soil to obtain the strength of specific layers at different depths; in the laboratory, by unconfined compression or triaxial strength tests performed on undisturbed samples of soil.
To estimate vertical settlement of the structure consolidation tests are required to a depth equal to twice the diameter of the foundation. These tests should be performed on representative undisturbed soil samples obtained from each of the major soil formations. It cannot be emphasized too strongly that extensive soil investigations are necessary to assure a good foundation design for silos constructed on clays. Normally, such investigations are carried out by a soils engineering consultant.