What unit is generally used to express specific gravity in soil mechanics?

• A. grams per cubic centimeter
• B. kilograms per cubic meter
• C. dimensionless
• D. pascals

Answer: (C)

Specific gravity is a dimensionless quantity that is defined as the ratio of the density of a material to the density of a reference substance, typically water. In soil mechanics, this measure provides insight into the density characteristics of soil particles.

Since specific gravity compares the density of the soil (in relation to water) rather than providing a direct mass or volume measurement, it is inherently dimensionless. This means that specific gravity does not have any units; it is simply a ratio. As a result, being dimensionless makes it an ideal measure for understanding material properties without being influenced by the units of measurement.

The other options provided refer to units of measurement that describe mass or density directly but do not capture the nature of specific gravity as a ratio. For example, grams per cubic centimeter and kilograms per cubic meter are indeed common units when discussing density, but they do not convey the relative comparison that specific gravity implies. Pascals, on the other hand, are a unit of pressure and are unrelated to the concept of specific gravity.