Specific gravity
Specific gravity is a fundamental property that provides insights into the physical properties like porosity and moisture content of materials used in construction, such as soil, aggregates, and cement. This plays a key role in material selection and structural design.
Specific gravity: definition, methods and significance
Specific gravity is a dimensionless quantity defined as the ratio of the density of a substance to the density of water at a specified temperature. It is also known as relative gravity. Specific gravity ratio is calculated using the density of water at 4°C, ie. 1000 kg/m³. It provides information whether an object will sink or float in water. In geotechnical engineering, engineers use this property for foundation design for structures, stability calculations of soil embankments, and settlement estimations for engineered soil fills. Using specific gravity, engineers evaluate how materials perform under various loads and environmental conditions, ensuring the safety and durability of structures.
How is specific gravity determined?
The different methods for determining the specific gravity of soil samples are based on their particle size.
Pycnometre method
The process involves measuring the weight of soil solids and the volume they occupy.. Following that, a comparison is made with the mass of an equal volume of water. Pycnometre method assesses the specific gravity of both fine-grained and coarse-grained soils.
Density bottle method
The density bottle method, commonly used for fine-grained soils, includes drying the soil sample, passing it through a 2 mm sieve. Then, a subsample of this is oven-dried and transferred to the density bottle.
Glass jar method
The glass jar method deals with soil containing granular solids or coarse powders that may react with water. The technique relies on the principle of displacement of water- the phenomenon where an object submerged in water pushes aside a volume of water equal to its own submerged volume. This is also known as Archimedes’ principle.
Le-chatelier method
Le- chatelier method measures the specific gravity of finely powdered solids like cement using a specialized apparatus known as the Le Chatelier flask. Temperature control is crucial as water density varies with temperature, affecting the mass measurements.
Differences between specific gravity and density
Specific gravity |
Density |
|---|---|
Ratio of a substance’s density to water’s density |
Mass per unit volume of a substance |
No units |
kg/m³ |
Typically water at 4°C |
No reference needed |
Aids in material selection, compaction and settlement |
Used in structural fluid and material design |
Unaffected by pressure |
Varies with pressure |
Specific gravity formula
The formula for specific gravity is:
Specific Gravity(SG)=Density of the substance/ Density of water
Alternatively, it can be expressed as:
SG= ρsubstance/ ρwater
Where:
- ρsubstance is the density of the substance (in kg/m³ or g/cm³)
- ρwater is the density of water (1000 kg/m³ or 1 g/cm³ at 4°C)
Factors affecting specific gravity
Since specific gravity is not static, this property helps engineers and scientists who rely on accurate measurements to maintain the performance of their projects.
Moisture content
Water content affects the weight and volume of soil, altering its specific gravity. If the material is saturated, volume and mass increases slightly, leading to lower specific gravity.
Particle size and shape
Smaller particles tend to have a higher surface area, which increases the volume occupied by particles. This leads to lower bulk density and a lower specific gravity. Round or smooth particles reduce the voids between the particles, thereby raising the specific gravity.
Porosity
When the porosity of a soil sample is high, it indicates the volume of voids within the soil is large. This decreases specific gravity as the weight of the soil is reduced due to the increased presence of air occupied within the voids. It affects critical soil properties such as the soil’s ability to retain water and its compressibility.
Temperature
As the temperature of a material increases, its particles expand slightly. This leads to decrease in density, and consequently, a reduction in specific gravity.
Importance of specific gravity in geosynthetics
Materials with higher specific gravity such as geogrids are denser and provide additional strength while reinforcing soil or when used for erosion control. Its higher density allows it to resist flotation in saturated conditions, ensuring stability. Lower specific gravity of geocells reduce the pressure exerted on underlying soils, making it suitable for reducing loads such as in soft or unstable soils.
Geotextiles are dependent on the material’s specific gravity to manage water permeability and water passing through fabric. The higher specific gravity of the material enhances strength and robustness. If the specific gravity of geosynthetic material is less than 1, it will float on water. This property is important in the design of geomembranes used for liner systems in landfills or pond liners. Geosynthetics with higher specific gravity value tend to have better resistance to chemical degradation, UV exposure, and thermal oxidation. Lower value also helps to lower transportation costs and a reduction in carbon footprint, by reducing the overall weight.
Challenges in determining specific gravity
Specific gravity cannot be ascertained in all conditions. Typically, there are environmental conditions which affect this measurement.
Temperature influences
Fluctuations in temperature alter the density of both liquids and solids, leading to variations in bulk specific gravity (BSG) readings, hence compromising material selection. To ensure accuracy, tests should be conducted at standardized temperatures, typically around 39.2°F (4°C).
Moisture content
When moisture content increases, the weight of the soil sample increases, impacting the specific gravity readings, particularly for porous materials. Also, impurities or additives present in the material further skew the results.
Measurement techniques
Systematic errors occur due to the inherent limitations of the measurement techniques used. For example, using a pycnometer leads to inaccuracies if air bubbles are trapped within the sample. Furthermore, a saturated surface dry condition is crucial, as inadequate drying leads to erroneous weight readings.