What is used to evaluate the performance quality of soil as a highway subgrade material?

The Group Index is commonly used to evaluate the performance quality of soil as a highway subgrade material because it accounts for various soil properties that influence its suitability for bearing loads in construction applications. It combines information on the plasticity and moisture content of the soil to provide a numerical value that reflects its capacity to support traffic loads without significant deformation or failure.

The Group Index formula incorporates factors such as the Plasticity Index and the liquid limit of the soil, effectively summarizing important characteristics like moisture retention and the soil's tendency to deform under load. In grading soils for highway subgrades, using the Group Index allows engineers to classify soils into groups that predict their behavior and structural performance with greater accuracy.

While other methods, such as the California Bearing Ratio and shear strength, are also relevant for assessing soil performance, they focus on more specific aspects or conditions of the soil, without offering the broader classification provided by the Group Index. The Plasticity Index, although informative regarding the plasticity and consistency of the soil, does not comprehensively evaluate its overall performance in subgrade applications.