ASSHTO Soil Classification

Purpose of AASHTO Soil Classification System

• The classification system was originally developed in 1929 as paved roads and highways began to take form and become more abundant. Soil had to support the weight of paved roads. At the time, there was not much knowledge concerning the types of soil suitable for holding the weight of a paved road, so building paved roads or surfaces could be difficult. Classifying soil and soil mixtures provided highway construction planners with a way to plan upcoming projects.

Classification Chart

• The classification system consists of a chart that categorizes soil on a general scale, getting more specific within each category or subcategory. The left-hand column of the chart refers to the characteristics of soils. The top row of the column separates the soil into two separate categories. The row below that categorizes the soils in each category even further. All rows beneath the top two describe the sieve size through which standard categories of soils pass.

How the Chart Classifies Soil

• The most basic test for classifying soil by ASSHTO standards is to start by placing a dry sample of the soil in the topmost of a stack of sieves. Each subsequent sieve from top to bottom has smaller openings for the soil to pass through. A person or machine shakes the stack until every particle of soil has reached a sieve that it cannot pass through. The particles are carefully removed from each sieve and their weight is recorded. The graph of the percentage of weight in each sieve against the size of the holes in each sieve results in a curve called "the grain size distribution curve." This curve allows researchers to check values at specific points that are used to determine the ASSHTO classification of the soil.

Basic Classification

• The most basic classification on the chart has soil types arranged from A-1 to A-8. The numbers attached to the letter A refer to how good the soil is for highway construction use and whether or not it can hold the weight of a paved road. The lower the number, the higher the category, so A-1 is the best soil while A-8 is the worst soil.