Soils for Building Foundations
Soils for Building Foundations:
Generally, soil groups listed toward the top of Figure 2.2 are more desirable for supporting building foundations than those listed further down. The higher listed soils tend to have better soil engineering properties, that is, they tend to have the greater loadbearing capacity, to be more stable, and to react less to changes in moisture content.
Rock is usually the best material to find a building. When the rock is too deep to reach economically, designers must choose from the layers of different soils that are close to the surface and design the foundation to work satisfactorily on the selected soil.
Figure 2.5 gives some conservative values of load-bearing capacity for different types of soil. These values give only an approximation of the relative strengths of the different clays; The strength or absence of any particular soil depends on the presence or absence of water, the depth below the soil surface, and, to some extent, the way the foundation works on it.
In practice, the designer may choose to reduce the pressure of the foundations in the soil to less than these values Ability to build a colony. Soil stability is the ability to retain its structural properties under different conditions that may occur over the life of the building. Generally, stone, gravel, and sand are the most stable soils, soils are least stable and silt somewhere in between.
Under changing surface moisture conditions many soils change size, swell significantly and become dry when absorbed by water. In the presence of more elaborate clay soils, the foundation may need to be designed with underlying zero spaces where the clay can expand to prevent structural damage to the foundation.
When wet clay is pressurized, the water can be squeezed slowly, gradually decreasing in the corresponding volume. In this context, the long-term disposition of the foundation in such soil is a risk to be considered. Taken together, these properties make many soils the least predictable soil to support buildings. (In Figure 2.2, fine-grained clay groups with a fluid limit of more than 50 are usually affected by water content, exhibiting high plasticity and low strength when wet and expanded).
In areas of significant earthquake risk, soil stability during seismic events is also a concern. Sand and silt, which have high water content, are particularly susceptible to liquidity, ie, temporary change from solid to liquid state during periodic shaking. Soil liquefaction can lead to loss of support to the foundation of the building or excessive pressure on the foundation walls.
Soil drainage properties are important in predicting how water flows around the building site and around the building’s substructures. Figure 2.6 illustrates the differences in particle size or range of two gravel samples, which are coarse-grained. The left-hand model, with a variety of particle sizes, comes from fine-grained sand gravel.
On the right is a model of uniformly graded gravel, in which there is little variation in particle size. (Photographs by Joseph Iano) Almost all are of the same size, which has the highest possible volume of zero space between the particles, and the water passes through it very easily. When coarse-grained soils are composed of a variety of particles, the volume of zero space between the particles decreases, and such soils drain less efficiently.
Coarse-grained soils containing particles of all sizes are well-graded or poorly sorted, small-sized particles are known to be poorly graded or well-graded, and often one-sized particles are homogeneously graded (Figure 2.6).
Due to their small particle size, delicate soils drain water less efficiently: water passes slowly through very fine sand and silt, and not through most soils. The site of a building with mud or sludge near the soil is poorly drained and covered with sludge and puddles during the rainy season, but the gravel area is likely to remain dry.
Water quickly passes through the underground, gravel, and sand layers but accumulates on topsoil and fine silt. The best way to dry the basement is to cover it with a layer of uniformly graded or crushed stone. The water passing through the soil towards the building cannot reach the basement without falling into the bottom of this porous layer, where it can be removed in perforated tubes before it is collected (Figures 2.60-2.62).
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Placing the perforated drain pipes directly into the clay or sludge is a good idea because the water cannot flow through the untreated soil. Rarely are the soils below the building all the same. Under most buildings, a variety of soils are arranged in superimposed layers (strata) formed by different geological processes.
Frequently, soils in any one layer are a mixture of different clay groups, including fine-grained gravel with sludge and sand, poorly graded sand with clay, thin clay with gravel, etc. Determining the appropriateness of the soils of any particular site for support of a building foundation, then, depends on the behavior of the different types of soils and how they interact with each other and with the foundation of the building.