Influence of clay fraction and moisture on the behaviour of soil-aggregate mixtures

Mixing soil subgrades with a durable aggregate, such as limestone or sandstone, has been used to improve the bearing strength of highway subgrades. When the mixture contains too much clay or moisture, the bearing strength of the mixture may decrease to a level that may cause instability. Geotechnical tests were performed on compacted specimens with different percentages of clay and crushed limestone aggregate to develop mixture guidelines and to examine the influences of the percentage of clay fraction and moisture content on the bearing strengths of compacted clay-aggregate mixtures. Five different mixtures were tested: 100% clay; 25% aggregate-75% clay; 50% aggregate-50% clay; 75%-aggregate-25% clay; and 100% aggregate. Index tests, moisture-density relationships, and swelling tests were performed on each mixture. Bearing ratio (soaked specimens) tests and consolidated-undrained triaxial tests with pore pressure measurements were performed to examine the variation of strength with the percentage of aggregate of a given mixture. Stability, or bearing capacity, analyses of the subgrades constructed of the different clay-aggegate mixtures were performed using the effective stress parameters and a unique bearing capacity model based on limiting equilibrium. To prevent subgrade failure under a dual-wheel tire stress of 551 kPa, the percentage of aggregate of the aggregate-clay mixture must be at least 83 percent and the clay fraction (percent finer than 0.002 mm-sized particles) of the mixture cannot exceed about 11 percent.