Subgrade materials are typically characterized by their resistance to deformation under load, which can be either a measure of their strength (the stress needed to break or rupture a material) or stiffness (the relationship between stress and strain in the elastic range or how well a material is able to return to its original shape and size after being stressed). In general, the more resistant to deformation a subgrade is, the more load it can support before reaching a critical deformation value. Three basic subgrade stiffness/strength characterizations are commonly used in the U.S.: California Bearing Ratio (CBR), Resistance Value (R-value) and resilient modulus. Although there are other factors involved when evaluating subgrade materials (such as swell in the case of certain clays), stiffness is the most common characterization and thus CBR, R-value and resilient modulus are discussed here.
Stiffness and Strength Tests