Soils containing organic matter typically have a specific gravity below 2.0.
Specific gravity is a fundamental property of soil particles, defined as the ratio of the density of the soil solids to the density of water at a specified temperature (usually 4°C). It is a dimensionless quantity used in various soil mechanics calculations.
Specific Gravity Values Based on Soil Composition
The specific gravity of soil particles (Gs) varies depending on the mineralogical composition and presence of other materials like organic matter.
According to the provided reference, standard soil particles generally have a specific gravity within the range of 2.65 to 2.85. This range is typical for mineral soils like sand, silt, and clay composed primarily of quartz, feldspar, and clay minerals.
However, the reference specifically notes that:
- "Soils containing organic matter and porous particles may have specific gravity values below 2.0."
- Conversely, "Soils having heavy substances may have values above 3.0."
This highlights that organic soils fall outside the typical range for mineral soils due to the nature of organic material.
Typical Specific Gravity Ranges
The specific gravity provides insight into the material composition of the soil solids. Here's a summary based on common soil types and the provided reference:
| Soil Type | Typical Specific Gravity (Gs) Range | Characteristic |
|---|---|---|
| Most Mineral Soils (Sand, Silt, Clay) | 2.65 to 2.85 | Particles primarily composed of quartz, feldspar, etc. |
| Soils with Significant Organic Matter | Below 2.0 | Contains decomposed plant and animal matter, less dense. |
| Soils with Heavy Minerals | Above 3.0 | Contains minerals like magnetite, hematite, or sulfides. |
Reference information derived from: Determine the specific gravity home.iitk.ac.in › ~madhav › expt2
Why Organic Matter Lowers Specific Gravity
Organic matter, such as peat or decomposed plant material, is significantly less dense than mineral soil particles. When present in large quantities within a soil, it reduces the overall average specific gravity of the soil solids. Porous particles, also mentioned in the reference, have internal voids that lower their bulk density compared to solid mineral grains, further contributing to a lower specific gravity for soils containing them.
Understanding the specific gravity of organic soil is crucial for geotechnical engineers as it affects calculations involving void ratio, saturation, and density, which are essential for predicting soil behavior under load and evaluating its suitability for construction.
