DESIGN CONSIDERATIONS - 3.2

 SOIL SUPPORT CAPABILITY 

The ability of the sub-grade to support loads transmitted from the pavement is one of the most important factors in determining pavement thickness. The sub-grade must serve as a working platform to support construction equipment and as a foundation for the pavement structure that supports and distributes traffic loads. 

Thus, it is essential to evaluate the strength of the sub-grade before beginning the structural design of the pavement. Figure 3-1 shows the spread of wheel load through the pavement structure and on to the sub-grade. 

If sufficient pavement thickness is not provided, the applied loads could cause greater stresses on the subgrade than it can resist. This may result in deflection of the pavement and ultimately in its failure. 

In street and highway construction, the subgrade provides the foundation for the pavement. Different types of soils have different abilities to provide support. A sandy soil, for example, will support greater loads without deformation than a silty clay soil. Thus, for any given traffic volume and weight of vehicles using the roadway, a greater pavement thickness must be provided on clay soils than on sandy soils.

SUBGRADE STRENGTH

Because thickness calculations depend on the strength of the finished subgrade, the soil must be tested for this information. Tests are based on bearing capacity related to the moisture and density of the soil. The California Bearing Ratio (CBR) is one of the most widely used methods of designing pavement structure. Once the CBR value is determined, the soil classification can be identified. Or, when the soil classification is known, a relative CBR value can also be identified.

The lower the CBR value of a particular soil, the less strength it has to support the pavement. This means that a thicker pavement structure is needed on a soil with a low CBR rating than on a soil with a high CBR rating. Generally, clays have a CBR classification of 6. Silty loam and sandy loam soils are next with CBR values of 6 to 8. The best soils for road building purposes are sands and gravels whose CBR ratings normally exceed 10.

The change in pavement thickness needed to carry a given traffic load is not directly proportional to the change in CBR value of the subgrade soil. For example, a one-unit change in CBR from 5 to 4 requires a greater increase in pavement thickness than does a one-unit CBR change from 10 to 9. 

A number of soil classification-strength systems are currently in use for roads and airports. A correlation chart follows for a general soil overview.