Degree of Saturation Calculator (from Unit Weight)

An essential tool for geotechnical analysis, calculating soil saturation based on its water content, total unit weight, and specific gravity of solids.

Degree of Saturation (S)

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What is the Degree of Saturation?

The degree of saturation (S) is a critical parameter in geotechnical engineering and soil mechanics that describes how much of the soil’s void space is filled with water. It is expressed as a percentage. A soil with a degree of saturation of 0% is perfectly dry, while a soil at 100% saturation has all its pores completely filled with water, with no air present. This calculator helps determine the degree of saturation using commonly measured soil properties like water content and unit weight.

Understanding the degree of saturation is fundamental for predicting soil behavior. It directly influences a soil’s strength, compressibility, and permeability. For example, a fully saturated soil will behave very differently under load compared to a partially saturated one, especially concerning pore water pressure buildup. This metric is essential for the design of foundations, embankments, retaining walls, and in assessing slope stability.

Degree of Saturation Formula and Explanation

When you have the total unit weight (γ), water content (w), and specific gravity of solids (G_s), you can determine the degree of saturation (S). The calculation involves a few steps. First, the dry unit weight (γ_d) is determined:

γ_d = γ / (1 + w)

Next, the void ratio (e), which is the ratio of the volume of voids to the volume of solids, can be calculated:

e = (G_s * γ_w / γ_d) – 1

Finally, the degree of saturation (S) is found using the fundamental relationship:

S = (w * G_s / e) * 100%

Variables Table

Variable	Meaning	Unit	Typical Range
S	Degree of Saturation	%	0 – 100
w	Water Content	%	5 – 60 (can be higher for organic soils)
γ	Total (Moist) Unit Weight	kN/m³ or pcf	16-22 kN/m³ or 100-140 pcf
G_s	Specific Gravity of Solids	Dimensionless	2.60 – 2.80
γ_d	Dry Unit Weight	kN/m³ or pcf	13-21 kN/m³ or 80-130 pcf
e	Void Ratio	Dimensionless	0.3 – 1.5
γ_w	Unit Weight of Water	Constant (9.81 kN/m³ or 62.4 pcf)	N/A

Practical Examples

Example 1: Compacted Silty Sand

An engineer tests a sample of silty sand from a compacted fill and finds the following properties:

• Water Content (w): 12.0%
• Total Unit Weight (γ): 20.5 kN/m³
• Specific Gravity of Solids (G_s): 2.68

First, calculate the dry unit weight: γ_d = 20.5 / (1 + 0.12) = 18.30 kN/m³.

Next, calculate the void ratio: e = (2.68 * 9.81 / 18.30) – 1 = 0.439.

Finally, calculate the degree of saturation: S = (0.12 * 2.68 / 0.439) * 100% = 73.3%. This indicates a well-compacted, partially saturated soil. For more tools, check out our geotechnical engineering calculators.

Example 2: Soft Clay Sample

A sample of soft clay is retrieved from below the water table. Its properties are:

• Water Content (w): 35.0%
• Total Unit Weight (γ): 118.5 pcf
• Specific Gravity of Solids (G_s): 2.72

Using imperial units, the unit weight of water is 62.4 pcf.

First, calculate dry unit weight: γ_d = 118.5 / (1 + 0.35) = 87.78 pcf.

Next, void ratio: e = (2.72 * 62.4 / 87.78) – 1 = 0.935.

Finally, degree of saturation: S = (0.35 * 2.72 / 0.935) * 100% = 101.8%. A result slightly over 100% can occur due to measurement inaccuracies but indicates the soil is fully saturated, which is expected for a sample below the water table. You can learn more about soil mechanics formulas on our resource page.

How to Use This Degree of Saturation Calculator

Using this calculator is a straightforward process designed for accuracy and efficiency. Follow these steps:

1. Enter Water Content (w): Input the soil’s water content as a percentage. This value is obtained from laboratory drying tests.
2. Enter Total Unit Weight (γ): Input the total or moist unit weight of the soil sample as measured in the field or lab.
3. Select Unit System: Use the dropdown to choose your unit system for unit weight: metric (kN/m³) or imperial (pcf). The calculator automatically uses the correct unit weight of water (9.81 kN/m³ or 62.4 pcf) for its calculations.
4. Enter Specific Gravity (G_s): Input the specific gravity of the soil solids. This is a dimensionless value, typically determined by a pycnometer test. If unknown, 2.68 is a reasonable assumption for many soil types.
5. Review the Results: The calculator will instantly provide the Degree of Saturation (S) as a primary result. It also shows key intermediate values—Dry Unit Weight and Void Ratio—that are useful for other geotechnical analyses. The bar chart provides a quick visual reference for the saturation level.

Key Factors That Affect Degree of Saturation

Several factors in the field influence a soil’s degree of saturation:

• Depth of Water Table: Soil below the groundwater table is generally assumed to be fully saturated (S=100%). The zone just above the water table, the capillary fringe, can also be nearly saturated.
• Climate and Weather: Recent rainfall will increase the degree of saturation in near-surface soils, while prolonged dry periods or evapotranspiration will decrease it.
• Soil Type and Grain Size: Fine-grained soils like clays and silts have smaller pores and can hold more water through capillary action, often exhibiting higher degrees of saturation than coarse-grained sands and gravels in similar conditions.
• Compaction Effort: During construction, compaction reduces the volume of air voids in the soil. This process increases the unit weight and can increase the degree of saturation if the water content remains constant. Our geotech engineering calculators can help analyze compaction effects.
• Drainage Conditions: The ability of water to drain from the soil mass is critical. Poorly drained soils will have a higher degree of saturation than soils with good drainage.
• Vegetation: Plants draw water from the soil through their roots (transpiration), which can significantly reduce the degree of saturation in the root zone.

Frequently Asked Questions (FAQ)

1. What does a degree of saturation of 100% mean?

A degree of saturation of 100% means that all the void spaces between the soil particles are completely filled with water. There is no air in the soil matrix. This state is often referred to as “fully saturated.”

2. Is it possible to have a degree of saturation greater than 100%?

In theory, no. The degree of saturation represents the fraction of void volume filled by water, which cannot exceed the total void volume. However, in calculations, you might get a value slightly above 100% (e.g., 101%) due to small measurement errors in water content, unit weight, or specific gravity. This result should be interpreted as the soil being fully saturated. See our guide on civil engineering calculators for more on measurement precision.

3. What is the difference between water content and degree of saturation?

Water content (w) is a ratio of masses (mass of water / mass of solids), while degree of saturation (S) is a ratio of volumes (volume of water / volume of voids). A soil can have a high water content but not be fully saturated if it has a very high void ratio (e.g., peat).

4. Why is specific gravity of solids (G_s) important for this calculation?

Specific gravity links the mass of soil solids to their volume through the known unit weight of water. It’s a critical bridge that allows us to convert between the weight-volume (unit weight) and mass-mass (water content) relationships to find the volume-volume relationship (degree of saturation). You can find more details in our list of soil mechanics formulas.

5. How does the unit weight system affect the calculation?

The core formulas are the same, but the constant for the unit weight of water (γ_w) changes. The calculator automatically uses 9.81 kN/m³ for the metric system and 62.4 pcf for the imperial system to ensure the results are accurate for the chosen units.

6. What is a typical void ratio for sand vs. clay?

Loose sands typically have a void ratio between 0.6 and 1.0, while dense sands are closer to 0.3-0.5. Clays have a much wider range; soft, highly plastic clays can have void ratios well over 1.0, while stiff, overconsolidated clays might be closer to 0.4.

7. Can this calculator be used for organic soils?

Yes, but with caution. Organic soils (like peat) can have very high water contents (often > 100%) and a specific gravity (G_s) that is much lower than typical mineral soils (e.g., 1.5 – 2.0). Ensure you use an accurate G_s value for reliable results.

8. What does a “partially saturated” soil mean?

A partially saturated soil has a degree of saturation between 0% and 100%. This means its void spaces contain both air and water. Most soils near the ground surface are in a partially saturated state.

Related Tools and Internal Resources

Explore other calculators and resources to support your geotechnical analysis needs:

• Geotechnical Engineering Calculators: A comprehensive suite of tools for soil analysis.
• Soil Mechanics Formulas: A complete reference for fundamental geotechnical equations.
• Civil Engineering Calculators: A broad collection of calculators for various civil engineering disciplines.
• Void Ratio Calculator: A specific tool to calculate soil void ratio from other properties.