Elastic Limit of Soils
Soil mechanics refers to the engineering properties of soil and predicts how it will behave under certain circumstances. These properties describe the soil characteristics as construction materials and the suitability of the soil to withstand loading applications from varying types of structures. The elastic limit of a soil or any other kind of material is a measure of the maximum amount of stress that can be applied without causing a permanent deformation.
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The Elastic Limit
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The elastic limit is an engineering behavior of soil which can be determined in the engineering materials laboratory by the application and release of a series of increasing loads to the specimen until a permanent deformation is observed when the final loading increment is released from the soil specimen. Several characteristics of soil determine the elastic limit. These include the grain size distribution of the soil sample, how much water and air is contained in the void space, shear strength, bearing capacity, consolidation or compression, and the clay composition of the soil sample.
Soil Composition
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Soil is composed of three phases of matter. The solids may vary in size from clay particles to small pebbles or gravel to large rocks. Among the mass of solids are voids which may contain water or air. Under consolidation or compaction pressure, the void space may be reduced as the water and air are squeezed out of the soil. Newly formed or geologically young soft soils will have a higher percentage of porosity or void space, while older hardened soils are more consolidated and will have less porosity. The older soils are more dense and less likely to fail because of an exceeded elastic limit.
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Bearing Capacity
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This refers to the capacity of the soil to support the construction loads. It represents the maximum average contact pressure between the structure's foundation and the soil which would not produce a shear failure in the soil. The maximum pressure which the soil can support without failure is the ultimate bearing capacity. If the soil settling allows a structure's foundations to settle unevenly, differential settling occurs, which indicates that the elastic limit of that soil has been breached. As a result, cracking and shifting appears in the walls or foundation of the building.
Soil Strength
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Soil strength generally refers to a soil's resistance to shear forces such as gravity, friction, cohesion and pore pressure. Friction is the mechanical resistance of a soil to move due to the resistance from the roughness of the particle surfaces acting on each other. Cohesive forces apply especially to clay soils where the electrostatic forces among the fine particles makes them stick together more tightly. Pore pressure is provided by the water-containing voids which resist compaction or consolidation forces. If a soil becomes saturated with water because of excessive rain or flooding, the elastic limit of that soil may be exceeded in the form of a mudslide. It may also be observed in the cracking of a concrete floor or asphalt parking lot because of uplift pressure from the waterlogged soil below.
Elastic Modulus
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The elastic modulus value may also be called the modulus of elasticity. This is another way of representing the elastic limit taking different forces into consideration. It is a mathematical representation of the soil's tendency to be deformed in an elastic mode when a force is applied to it. There are several types of elastic moduli that serve as engineering reference points: Young's modulus, the shear modulus and the bulk modulus.
Young's modulus refers to the soil's tendency to deform along an axis when forces are applied along that axis. The shear modulus is also called the modulus of rigidity, which describes the soil's tendency to shear or change its shape. Shear refers to a tendency to break away suddenly as in a landslide.
The bulk modulus describes the tendency of soil to deform in all directions under a force.
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- Photo Credit Arid soil image by Igor Baryshev from Fotolia.com
