Definition of Elastic Modulus
Elastic modulus is also known as Young's modulus after Thomas Young who published the concept in the early 18th century. A material is elastic if it can return to its original size or shape after being stretched or squeezed (as long as the application or action does not permanently deform the material). Nearly all materials are elastic to some degree, and elasticity, along with geometric shape, is part of an object's flexibility. An elastic modulus can be calculated for any solid material and is the ratio of stress to strain. A key concept to remember is that elastic modulus is not the same as strength. Strength is measured by the stress needed to break a material, whereas elasticity measures how well a material returns to its original shape. The lower the MPa or KPa (pascal), the more elastic the object or soil type is.
Factors Influencing Soil Modulus
Soil with closely packed particles tends to have a higher modulus. This can be determined by looking at the soil's dry density or porosity. However, two soil samples can have the same dry density but different structures, like loose or dense, and thus have different moduli.
Water content also impacts moduli. At low water contents the water binds the particles, increases the stress and suction between the particles and leads to a high soil moduli. This is especially apparent when considering the stiffness of dried clay. However, this does not hold true for coarse grain soils. If water content rises too much, the particles are pushed apart and the modulus is reduced.
If the soil has been subjected to stress in the past, it will impact the modulus. An over-consolidated soil will generally have a higher modulus than the same normally-consolidated soil.
Soils may see cementation, or a glue effect, between particles from either low water content or chemical cementation developed at contacts. This cementation leads to an increase in modulus.
Typical Moduli of Various Soils
The typical modulus of elasticity of silty soils is 35 to 150 MPa or 5,000 to 20,000 psi; clay soils is 35 to 100 MPa or 5,000 to 15,000 psi; and crushed stone is 150 to 300 MPa or 20,000 to 40,000 psi.
Soil modulus is useful for a variety of applications within geotechnical engineering including shallow foundations, deep foundations and slope stability or retaining structures. With properly designed slopes and retaining structures, the modulus tends to be higher than in foundation engineering because of lower levels of strain. Pavements call for knowledge of soil modulus. Pavement over which millions of cars will travel requires a soil bed with a higher modulus than a small road.