What is the term for the ratio of the volume of voids to the volume of solids?

The term that describes the ratio of the volume of voids to the volume of solids is known as the void ratio. This metric is crucial in geotechnical engineering, where understanding the properties of soils and other materials is essential for construction and environmental applications. The void ratio provides insight into the space within a material structure that is not occupied by solid particles, which can significantly affect the material's strength, compressibility, and permeability.

The void ratio is typically expressed as a dimensionless number, and it helps in characterizing the structure and behavior of materials under various conditions. High void ratios indicate a higher proportion of void space relative to solid material, which can imply lower strength and stability.

In contrast, other terms listed do not directly reflect the ratio of voids to solids. Bulk density refers to the mass of a material per unit volume, including both solids and voids; porosity measures the total volume of voids relative to the total volume of the material; and moisture content refers to the amount of water within the material, which does not specifically address the volume of voids versus solids.

Thus, the void ratio is a specific and critical term for understanding material composition in various engineering applications.