Lesson 5/Learning Event 1
Airfields and roads are designed from the subgrade up. Each layer of soil has a particular name
depending on where it is placed in the flexible pavement. The actual "name" and "thickness" of each
layer is dependent on the soil's California Bearing Ratio (CBR) Value. CBR is a measurement of the
shearing resistance of the soil, i.e., soil strength. The higher the CBR, the stronger the soil. Most soils
usually fall between CBR values of 1 (swamps) to 100 (gravels).
The shearing resistance is related to a soil's ability to withstand load, and it is especially important for a
soil which is used as a base or subgrade beneath a road surface, an airfield runway, or any other
structure. With the CBR method, the shearing resistance of the soil is measured under controlled
conditions of density and moisture. Once the design strength (CBR) is found, construction must be
accomplished within certain moisture and dry densities to achieve this strength.