The usefulness of aggregate gradation

The position, the shape and the slope of the gradation curve provide useful information for the soil material. The position of the gradation curve determines more or less the soil classification group, while the shape and slope determine the distribution of the particles and the deficiency or sufficiency of certain size particles. 

The particle size of the soil material affects its mechanical behaviour. Coarse soil materials have a better mechanical behaviour than fine ones in terms of strength and resistance to loading. Additionally, the particle size determines the ability or possibility of water retention or absorption, as well as the possibility of frost damage or swelling. Very fine soils (particularly silt and clay) absorb and retain water. As a result, they swell and they are also susceptible to frost damage. Coarse materials do not have any of the abovementioned deficiencies hence, they are preferable and more suitable as foundation layer materials. More details regarding the suitability of soil materials, in general, can be found in Table 1.8. 

The relatively moderate slope of curves A, C, E and F indicates a uniform particle distribution and consequently the existence of all sizes particles. The almost vertical S -shaped curve (curve D) indicates a material with almost single-size particles. The double S -shaped curve (curve B) indicates a material in which certain particle sizes are absent. This type of curve is known as a gap graded curve. 

The uniformity of the soil material, desirable in most cases, is quantified by reference to the uniformity coefficient and the coefficient of curvature. The uniformity coefficient (Cu) and the coefficient of curvature (Cq) are determined by the following equations  

According to CEN EN ISO 14688-2 (2013), the shape of the grading curve with respect to Cy and Cq is characterised as shown in Table 1.2. 

---