Soil Percent Saturation Calculator

Calculate soil saturation using unit weight, water content, and specific gravity.

What is Soil Percent Saturation?

Soil percent saturation, often denoted as ‘S’, is a critical parameter in geotechnical engineering and soil science. It represents the degree to which the void spaces (pores) within a soil mass are filled with water. A soil with 0% saturation is completely dry, while a soil at 100% saturation has all its voids filled with water, leaving no room for air. Understanding and calculating soils percent saturation using unit weight and specific gravity is fundamental for assessing soil properties like strength, compressibility, and permeability.

This value is crucial for civil engineers designing foundations, retaining walls, and earth dams. For example, a high degree of saturation can significantly reduce the bearing capacity of soil, potentially leading to structural failure. It is a key input for analyses related to soil compaction and stability.

Soil Percent Saturation Formula and Explanation

The degree of saturation (S) is not typically measured directly but is calculated from other fundamental soil properties. The primary relationship used involves the water content (w), the specific gravity of the soil solids (G_s), and the void ratio (e). The core formula is:

S = (w * G_s) / e

To use this, we first need to determine the void ratio (e) from the moist unit weight (γ), specific gravity (G_s), water content (w), and the unit weight of water (γ_w). The void ratio is found using this derived relationship:

e = (G_s * γ_w * (1 + w)) / γ – 1

Variables Table

Variable	Meaning	Unit (Auto-Inferred)	Typical Range
S	Degree of Saturation	%	0 – 100
γ	Moist Unit Weight	kN/m³ or lb/ft³	15-22 kN/m³ (95-140 lb/ft³)
w	Water Content	%	5 – 40
G_s	Specific Gravity of Solids	Dimensionless	2.60 – 2.80
e	Void Ratio	Dimensionless	0.3 – 1.5
γ_d	Dry Unit Weight	kN/m³ or lb/ft³	13-19 kN/m³ (80-120 lb/ft³)

Practical Examples

Example 1: Partially Saturated Clay

A geotechnical engineer is assessing a site with clayey soil. Field and lab tests provide the following data in metric units:

• Inputs:
  • Moist Unit Weight (γ): 19.2 kN/m³
  • Water Content (w): 25%
  • Specific Gravity (G_s): 2.72
  • Unit System: Metric (Unit Weight of Water, γ_w = 9.81 kN/m³)
• Calculation Steps:
  1. Calculate Void Ratio (e): e = (2.72 * 9.81 * (1 + 0.25)) / 19.2 – 1 = 0.741
  2. Calculate Saturation (S): S = (0.25 * 2.72) / 0.741 * 100 = 91.8%
• Results:
  • Percent Saturation (S): 91.8%
  • Void Ratio (e): 0.741
  • Dry Unit Weight (γ_d): 19.2 / (1 + 0.25) = 15.36 kN/m³

Example 2: Damp Sand in Imperial Units

An engineer is testing a sample of damp, sandy soil using imperial units. For more information on sandy soils, see our guide on testing sandy soils.

• Inputs:
  • Moist Unit Weight (γ): 118 lb/ft³
  • Water Content (w): 8%
  • Specific Gravity (G_s): 2.65
  • Unit System: Imperial (Unit Weight of Water, γ_w = 62.4 lb/ft³)
• Calculation Steps:
  1. Calculate Void Ratio (e): e = (2.65 * 62.4 * (1 + 0.08)) / 118 – 1 = 0.513
  2. Calculate Saturation (S): S = (0.08 * 2.65) / 0.513 * 100 = 41.3%
• Results:
  • Percent Saturation (S): 41.3%
  • Void Ratio (e): 0.513
  • Dry Unit Weight (γ_d): 118 / (1 + 0.08) = 109.26 lb/ft³

How to Use This Soil Percent Saturation Calculator

1. Select Unit System: Choose between Metric (kN/m³) and Imperial (lb/ft³). This sets the standard unit weight of water for the calculation.
2. Enter Moist Unit Weight (γ): Input the total unit weight of your soil sample as measured.
3. Enter Water Content (w): Provide the water content as a percentage. Do not use the decimal form (e.g., enter 15 for 15%).
4. Enter Specific Gravity (G_s): Input the specific gravity of the soil solids. This is a dimensionless value.
5. Interpret Results: The calculator instantly provides the Percent Saturation (S), Void Ratio (e), and Dry Unit Weight (γ_d). The phase diagram chart also updates to visually represent the soil composition.

Key Factors That Affect Soil Percent Saturation

Several environmental and physical factors influence the result when calculating soils percent saturation using unit weight and specific gravity.

• Rainfall and Infiltration: Recent or ongoing precipitation is the primary factor that increases soil saturation.
• Drainage Conditions: The ability of water to drain from the soil mass. Poorly-draining soils like heavy clays (learn about clay properties here) will have higher saturation levels than well-drained sandy soils.
• Water Table Level: Soils below the groundwater table are, by definition, 100% saturated. The proximity of the soil layer to the water table is critical.
• Soil Type (Porosity): The size and connectedness of void spaces affect how much water a soil can hold. Fine-grained soils (clays, silts) generally have higher void ratios than coarse-grained soils (sands, gravels).
• Evaporation: In arid or hot climates, surface evaporation can significantly reduce the saturation of the upper soil layers.
• Compaction: The degree of compaction reduces the void ratio, which in turn affects the saturation for a given water content. Explore our compaction effort calculator for more details.

FAQ

1. Can the degree of saturation be over 100%?

No. By definition, 100% saturation means all void spaces are filled with water. It is physically impossible to have more water than the total volume of voids.

2. What is the difference between water content and saturation?

Water content is the ratio of the weight of water to the weight of solids, while saturation is the ratio of the volume of water to the volume of voids. They are related but measure different aspects of the soil’s moisture.

3. Why is Specific Gravity (G_s) important?

Specific gravity relates the density of soil solids to the density of water. It is a fundamental property needed to convert between mass and volume of the solid particles, which is essential for calculating void ratio and saturation.

4. What does a void ratio greater than 1.0 mean?

A void ratio (e) greater than 1.0 simply means that the volume of the voids is greater than the volume of the solid particles. This is common in highly plastic clays and organic soils.

5. How do I choose the correct unit weight of water (γ_w)?

This calculator handles it for you. In metric, the standard value is 9.81 kN/m³. In imperial, it’s 62.4 lb/ft³. These are constants based on the density of water.

6. What is a typical specific gravity for soil?

For most mineral soils, G_s ranges from 2.60 to 2.80. Quartz-rich sands are around 2.65, while clay minerals can have higher values. Using an accurate value from a lab test is always best practice.

7. Does temperature affect the calculation?

Yes, slightly. The unit weight of water changes with temperature. However, for most geotechnical purposes, the standard values (9.81 kN/m³ or 62.4 lb/ft³) are used as the variation is typically negligible.

8. Can I use this calculator for organic soils?

While the formulas are technically the same, organic soils can have very high water contents and specific gravities that fall outside typical mineral soil ranges. Check out our organic soil calculator for more specific applications.