Accurately calculate weight of saturated soil, unit weights, and phase relationships.
Soil Properties Input
SI Units (kN, m³)
Imperial Units (lb, ft³)
Select your preferred unit system for calculation.
The total volume of the soil sample (m³ or ft³).
Please enter a positive volume.
Typically between 2.60 and 2.80 for most soils.
Please enter a valid Specific Gravity (> 1.0).
Ratio of volume of voids to volume of solids.
Please enter a valid Void Ratio (> 0).
Total Saturated Weight
0.00
kN
Saturated Unit Weight (γsat)
0.00
Dry Unit Weight (γd)
0.00
Water Content (w%)
0.00 %
Weight of Water
0.00
Formula Used:Wtotal = V × [(Gs + e) / (1 + e)] × γw
Phase Relationship Breakdown
Phase Component
Weight (kN)
Volume (m³)
Solids
0.00
0.00
Water (Voids)
0.00
0.00
Total (Saturated)
0.00
0.00
Table 1: Detailed breakdown of mass and volume for soil phases.
Fig 1: Phase Diagram representing proportional volumes of Solids vs. Water.
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Complete Guide: How to Calculate Weight of Saturated Soil
Geotechnical engineering relies heavily on understanding the physical properties of soil, particularly when water is involved. To calculate weight of saturated soil is to determine the total load a soil mass exerts when its void spaces are completely filled with water. This calculation is critical for foundation design, slope stability analysis, and earthwork estimation.
What is the Weight of Saturated Soil?
The weight of saturated soil refers to the combined weight of the soil solids (minerals, organic matter) and the water filling the pore spaces. In a saturated state, the degree of saturation (S) is 100%, meaning there is no air left in the soil matrix. This state represents the maximum unit weight for a given soil structure before submersion adjustments.
Engineers, construction managers, and hydrologists use this metric to:
Assess Overburden Pressure: Determine the stress on deep soil layers.
Analyze Slope Stability: Saturated soil is heavier and often weaker, leading to landslides.
Note: A common misconception is confusing "saturated unit weight" with "submerged unit weight." Saturated weight is the total weight in air, while submerged weight accounts for buoyancy effects when the soil is below the water table.
Formula to Calculate Weight of Saturated Soil
The calculation is based on the fundamental phase relationships of soil mechanics. The total weight is derived from the specific gravity of the soil solids, the void ratio, and the unit weight of water.
The Core Formula
The Saturated Unit Weight ($\gamma_{sat}$) is calculated as:
γsat = [ (Gs + e) / (1 + e) ] × γw
Once you have the unit weight, the Total Weight ($W_{sat}$) is simply:
W_{sat} = γsat × Total Volume (V)
Variable Definitions
Variable
Meaning
Unit (SI / Imp)
Typical Range
γsat
Saturated Unit Weight
kN/m³ / pcf
18 – 22 kN/m³
Gs
Specific Gravity of Solids
Dimensionless
2.60 – 2.80
e
Void Ratio
Dimensionless
0.3 – 1.2
γw
Unit Weight of Water
9.81 kN/m³ / 62.4 pcf
Constant
Table 2: Key variables used in soil weight calculations.
Practical Examples
Example 1: Foundation Clay Layer (SI Units)
An engineer needs to calculate the weight of a saturated clay block under a building foundation.
Select Unit System: Choose between SI (Metric) or Imperial based on your project requirements.
Input Volume: Enter the total volume of the soil mass you are analyzing.
Input Soil Properties:
Enter the Specific Gravity (Gs). If unknown, 2.65 is a standard default for sand/silt, and 2.70 for clay.
Enter the Void Ratio (e). This can be derived from porosity or lab tests.
Review Results: The tool instantly computes the total weight, unit weights, and water content.
Analyze Phase Diagram: Use the chart to visualize how much of the soil volume is solid matter versus water-filled voids.
Key Factors That Affect Saturated Soil Weight
Several physical properties influence the final calculation. Understanding these allows for more accurate geotechnical modeling.
1. Specific Gravity (Gs)
The density of the mineral particles directly increases the soil's weight. Soils rich in iron minerals will have a higher Gs (> 3.0) and thus be heavier than organic soils (Gs < 2.4).
2. Void Ratio and Porosity
A higher void ratio means more space for water. Since water (G=1.0) is lighter than soil solids (G≈2.7), a higher void ratio typically decreases the overall saturated unit weight, even though the water content increases.
3. Compaction Levels
Compaction reduces the void ratio. As 'e' decreases, more solids are packed into the same volume, significantly increasing the weight of saturated soil per unit volume.
4. Water Unit Weight
While usually treated as constant, the unit weight of water can vary slightly with temperature and salinity (e.g., seawater is roughly 64.0 pcf vs fresh water 62.4 pcf).
5. Soil Mineralogy
Clay minerals (like montmorillonite) absorb more water within their structure compared to inert sands, affecting the effective specific gravity and void behavior.
6. Organic Content
Peat and organic soils have very high void ratios and low specific gravity, often resulting in a saturated weight barely higher than water itself.
Frequently Asked Questions (FAQ)
What happens if the soil is not fully saturated?
If S < 100%, you must use the bulk unit weight formula: γbulk = [(Gs + S×e) / (1 + e)] × γw. This calculator assumes S=100%.
How do I convert Porosity (n) to Void Ratio (e)?
You can use the formula: e = n / (1 – n). For example, if porosity is 0.33 (33%), e = 0.33 / 0.67 ≈ 0.5.
Why is Saturated Unit Weight important for retaining walls?
Water pressure adds hydrostatic force. Saturated soil is heavier than dry soil, increasing the active earth pressure the wall must resist to prevent failure.
Is saturated weight the same as wet weight?
"Wet weight" or "Moist weight" usually implies partial saturation. Saturated weight is a specific case of wet weight where the degree of saturation is exactly 100%.
What is a typical void ratio for sand vs clay?
Sands typically have void ratios between 0.4 and 0.8. Clays can range widely from 0.6 to over 1.5 for soft, highly compressible clays.
Does temperature affect the calculation?
Technically, yes, as it changes the density of water ($\gamma_w$), but for standard engineering calculations, this effect is negligible.
Can this calculator be used for rock?
Yes, provided the rock is porous and saturated. However, rock mechanics often use porosity more frequently than void ratio.
What is the difference between saturated and submerged weight?
Submerged (buoyant) weight is Saturated Weight minus the Weight of Water displaced ($\gamma_{sat} – \gamma_{w}$). It is used for calculations below the water table.
Related Tools and Internal Resources
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