Calculator Input
Formula Used
γsat = [(Gs + e) / (1 + e)] × γw
γsat is saturated soil unit weight. Gs is soil solid specific gravity. e is void ratio. γw is water unit weight.
For porosity, first convert it with e = n / (1 - n). For dry unit weight, use e = (Gs × γw / γd) - 1.
Submerged unit weight is γ′ = γsat - γw. Saturated water content is w = e / Gs.
How to Use This Calculator
- Select the calculation method matching your available soil data.
- Choose SI units or US customary units.
- Enter specific gravity and water unit weight.
- Enter void ratio, porosity, or dry unit weight.
- Choose the required decimal places.
- Press the calculation button to view results above the form.
Example Data Table
| Soil type | Gs | Void ratio | γw | Approximate γsat |
|---|---|---|---|---|
| Dense sand | 2.65 | 0.45 | 9.81 kN/m³ | 20.97 kN/m³ |
| Silty clay | 2.70 | 0.90 | 9.81 kN/m³ | 18.57 kN/m³ |
| Loose sand | 2.65 | 0.80 | 9.81 kN/m³ | 18.80 kN/m³ |
Understanding Saturated Soil Unit Weight
Saturated soil unit weight shows the total weight of soil solids and water inside a saturated volume. It is important in geotechnical design because groundwater changes the force carried by soil layers. A saturated sample has all voids filled with water. Air is removed or ignored. The result helps estimate stress under foundations, retaining walls, slopes, embankments, and buried structures.
Why Saturation Matters
Soil below the groundwater table behaves differently from dry soil. Water adds weight, but it also creates buoyancy. Engineers therefore compare saturated unit weight with submerged unit weight. Saturated unit weight describes total vertical load. Submerged unit weight describes effective weight when water pressure supports part of the soil skeleton.
Key Inputs
The most common inputs are specific gravity, void ratio, and water unit weight. Specific gravity describes the density of soil solids compared with water. Void ratio describes void volume compared with solid volume. Water unit weight usually equals 9.81 kN/m³ in metric work. In US customary work, it is often 62.4 pcf.
Using Porosity Data
Some reports list porosity instead of void ratio. Porosity is void volume divided by total volume. The calculator converts porosity into void ratio before solving the main equation. This avoids manual conversion errors. It also lets field teams use laboratory data directly.
Using Dry Unit Weight Data
Dry unit weight can also support the calculation. The tool first estimates void ratio from dry unit weight, specific gravity, and water unit weight. Then it calculates the saturated value. This option is useful when compaction records provide dry density but not void ratio.
Interpreting Results
Most mineral soils have saturated unit weights near common engineering ranges. Very low or very high answers deserve review. Wrong units often cause unusual values. A porosity entered as decimal instead of percent can also distort the result. Always compare the output with project soil descriptions and laboratory reports.
Design Use
The calculated value can support overburden stress, bearing checks, slope stability, seepage studies, and lateral earth pressure estimates. It should not replace detailed site investigation. Soil layers vary across depth and location. Use representative parameters for each layer. Confirm critical projects with measured laboratory or field data.
Layered Profiles
Real sites rarely contain one uniform soil. Split the profile into layers. Calculate saturated unit weight for each layer separately. Then apply each value over its matching thickness. This method improves stress estimates. It also helps compare lenses of sand, silt, clay, and fill. Update values whenever the groundwater table rises or falls during seasonal monitoring on active project sites.
Practical Notes
Keep units consistent through the whole calculation. Do not mix pcf with kN/m³ values. Use fresh groundwater assumptions when salinity or temperature changes matter. Enter realistic specific gravity values for the soil mineral type. Review the result before applying it to design loads. Careful checks prevent costly foundation and excavation mistakes.
FAQs
What is saturated soil unit weight?
It is the total unit weight of soil when every void is filled with water. It includes soil solids and pore water within the same total volume.
What formula does this calculator use?
It uses γsat = [(Gs + e) / (1 + e)] × γw. It can also convert porosity or dry unit weight into void ratio first.
What is void ratio?
Void ratio is the volume of voids divided by the volume of soil solids. Higher void ratio usually means a lighter and looser soil structure.
Can I use porosity instead of void ratio?
Yes. Enter porosity as a percentage. The calculator converts it into void ratio using e = n / (1 - n).
What is specific gravity of soil solids?
It compares soil solid density with water density. Many mineral soils use values near 2.65, but laboratory values are better.
What water unit weight should I enter?
Use 9.81 kN/m³ for common metric calculations. Use 62.4 pcf for common US customary calculations.
What is submerged unit weight?
Submerged unit weight equals saturated unit weight minus water unit weight. It is often used for effective stress below groundwater.
Why is my result unusually high?
Check unit consistency first. A wrong water unit weight, dry unit weight, or porosity format can create unrealistic results quickly.
Can this tool handle dry unit weight?
Yes. It estimates void ratio from dry unit weight, specific gravity, and water unit weight. Then it solves saturated unit weight.
Is this suitable for foundation design?
It supports preliminary checks and calculation review. Final design should use project-specific geotechnical testing and professional judgment.
Does salinity change the result?
Yes. Saline water has a different unit weight. Enter the correct water unit weight when saltwater or dense fluids are present.