Bearing capacity of subgrade and influencing factors

The bearing capacity or strength of the subgrade is crucial to pavement design, since the thicknesses of the overlying layers are directly related to it. 

Subgrade bearing capacity depends on many factors, but mainly on the size of soil particles and soil cohesion, moisture content and degree of the compaction of soil or soil material used. 

The suitability of the subgrade to provide the platform on which the pavement is to be constructed can be determined by classifying the material using any authorised soil classification system (ASTM D 2487 2011, AASHTO M 145 2012 or others). However, the determination of the bearing capacity or strength of the subgrade is obtained only after conducting laboratory or in situ tests. 

The subgrade bearing capacity is expressed in terms of one of the following parameters California bearing ratio (CBR), modulus of reaction (K), resistance R value, resilient modulus (Mj) or stiffness modulus ( ). 

The determination of the moisture content during compaction is an easy task given that it is determined in the laboratory for maximum compacted density, employing the Proctor test (modified method), according to CEN EN 13286-2 (2012) or ASTM D 1557-09 (2012), or other suitable test methods (see Chapter 1). The only thing the engineer has to do is to ensure that the same moisture content was used during construction/compaction. 

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