Threshold Sand Content and the Behavior of Sand-Gravel Mixtures

The threshold sand content of mixtures of sand and gravel is defined as the sand content at which the mixture of sand and gravel transitions from behaving as a gravel to behaving as a sand. This transition occurs when there is insufficient room in the voids created by the gravel to contain all of the sand grains present in the mixture; at this sand content, the material begins to transition from sand grains contained in a gravel matrix to gravel particles contained in a sand matrix. Below the threshold sand content, the material behaves as a gravel; above the threshold sand content, the materials behave as a sand. This phenomenon of transitional behavior has been extensively studied for mixtures of sand and non-plastic silt, with the silt content at which the behavioral change occurs often being referred to as the “threshold fines content.” As increasing amounts of sand are added to a gravel, the soil mixture transitions from gravel to sandy gravel to gravelly sand and eventually to sand. This change in soil composition leads to changes in the maximum and minimum index void ratios (e_max and e_min) of the soil, as well as changes in the coefficient of uniformity, C_u. As sand is initially added to a gravel, the range of particle sizes increases, the index void ratios decrease and the coefficient of uniformity increases. These trends continue until the threshold sand content is reached. Mixtures that are below the threshold sand content typically have higher friction angles and stiffness. As the sand content continues to increase above the threshold sand content, the range of particle sizes decreases, the index void ratios increase and the coefficient of uniformity decreases. In this study, the changes in strength and stiffness that are predicted to occur at the threshold sand content were investigated for four mixtures of sand and gravel tested at different sand contents. The four mixtures studied consisted of a uniform sand mixed with a uniform gravel, a uniform sand mixed with a poorly-graded gravel, a well-graded sand mixed with a uniform gravel, and a well-graded sand mixed with a poorly-graded gravel. For each combination, specimens were prepared at a constant relative density at various sand contents and were tested in drained triaxial tests. From these tests, the change in shear strength and stiffness that occurred as the sand content increased were determined for each pairing of sand and gravel. At sand contents below the threshold sand content, the strength and stiffness were similar to that of the gravel. At sand contents above the threshold sand content, the strength and stiffness were similar to that of the sand. In each case, the changes in friction angle, ϕ’, and secant modulus, E_s, were found to be statistically significant and to occur at sand contents corresponding to the threshold sand content. This suggests that there is a change in the behavior of sand-gravel mixtures that occurs at the threshold sand content.