Specific Gravity of Soil

Specific gravity ($G_s$) is a fundamental property of soil particles, defined as the ratio of the weight of a given volume of soil solids to the weight of an equal volume of water at a standard temperature (usually $4^\circ$C or $20^\circ$C). It essentially tells us how much denser the soil particles are compared to water.

The specific gravity of soil solids varies depending on the mineral composition of the soil particles. Different soil types, such as gravel, sand, silt, and clay, which are formed from different minerals, will therefore have different typical ranges for their specific gravity.

Specific Gravity Ranges for Different Soil Types

Here are the typical specific gravity ranges for the soil types mentioned in the options:

• Gravel, Sand, and Silt: These generally consist of quartz and feldspar minerals. Their specific gravity typically falls in the range of 2.65 to 2.70.
• Inorganic Clays: Clays are made up of finer particles and often contain different types of clay minerals (like kaolinite, illite, smectite) which can have slightly higher specific gravities than quartz or feldspar. Inorganic clays commonly have specific gravities in the range of 2.68 to 2.80. Some may even be slightly higher if heavy minerals are present.
• Organic Soil: Soil with a significant amount of organic matter has a much lower specific gravity because organic material is less dense than mineral solids. The specific gravity of organic soil can be significantly lower, often ranging from 1.2 to 2.0, and sometimes even lower.
• Silty Sand: This is a mixture. Its specific gravity would typically be somewhere between the range of sand/silt and any clay content, generally closer to the sand/silt range, maybe around 2.65 to 2.72 depending on the proportions.

Let's look at these ranges compared to the range given in the question (2.68 - 2.80):

Identifying the Soil Type

Comparing the provided range (2.68 - 2.80) with the typical ranges of the soil types:

• Gravel (2.65 - 2.70) is mostly below or just at the lower end of the given range.
• Organic soil (1.2 - 2.0) is significantly lower than the given range.
• Inorganic clays (2.68 - 2.80) fits perfectly within the given range.
• Silty sand (2.65 - 2.72 approx.) is mostly below the given range.

Therefore, the soil type that typically has a specific gravity range of 2.68 - 2.80 is inorganic clays.