Soil Effective Stress Calculator

Understanding the soil effective stress calculator — concept and principle

The strength and settlement of soil are controlled not by the total weight pressing on it but by the part carried through grain-to-grain contacts, the effective stress. This calculator finds the total vertical stress, the pore water pressure and the effective stress at a depth below a level ground surface with a water table, for a two-layer unit weight profile.

Of the 116 kPa at 6 m, water holds 39.24 and the soil skeleton 76.76.

How it is calculated

Total stress: σ = γ × min(z, z_w) + γ_sat × max(0, z − z_w), in kPa when unit weights are in kN/m³ and depths in m.
Pore pressure (hydrostatic): u = γ_w × max(0, z − z_w), with γ_w = 9.81 kN/m³.
Effective stress: σ′ = σ − u.

The principle

Karl Terzaghi's principle of effective stress states that the water in the pores pushes equally in all directions and cannot carry shear, so only σ′ = σ − u changes the soil's strength and volume. The total stress is just the weight of everything above the point: soil above the water table at its moist unit weight, and saturated soil below it at its saturated unit weight. Below the water table, part of that weight is balanced by water pressure, which grows by 9.81 kPa for each metre of depth. Equivalently, below the water table you can use the submerged unit weight γ′ = γ_sat − γ_w directly.

Worked example

Inputs: depth 6 m, water table 2 m, γ = 18 kN/m³ above, γ_sat = 20 kN/m³ below.
Total stress = 2 × 18 + 4 × 20 = 36 + 80 = 116 kPa.
Pore pressure = 9.81 × 4 = 39.24 kPa.
Effective stress = 116 − 39.24 = 76.76 kPa.

Check with γ′: 36 + 4 × (20 − 9.81) = 36 + 40.76 = 76.76 kPa.

Each point is the calculator re-run with only “Depth (m)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Depth (m)	Total stress (kPa)
1.5	27
3	56
4.5	86
6	116
7.5	146
9	176
12	236

How to read the result

Effective stress is the starting point for settlement, shear strength (τ = c′ + σ′ tan φ′) and pile friction calculations. If the water table rises, u increases and σ′ falls, which lowers strength; that is why rising groundwater can trigger slope failures. If the water table is lowered by pumping, σ′ rises and soft clays consolidate, causing ground settlement. Typical unit weights are about 16–20 kN/m³ above the water table and 18–22 kN/m³ when saturated.

Common mistakes

Using the saturated unit weight above the water table or the dry one below it.
Subtracting pore pressure twice by using γ′ and then also subtracting u.
Assuming hydrostatic pressure where groundwater is flowing or artesian.
Ignoring layer boundaries; real profiles need each layer added separately.

Keep in mind: Educational estimate for a simple level-ground profile. Real foundations and earthworks must be designed to your local code using site investigation data and checked by a qualified engineer.

Frequently asked questions

What happens to effective stress in capillary soil above the water table?

Capillary suction makes pore pressure negative, raising effective stress; this simple model ignores it, which is conservative for strength.

Why is effective stress zero in quicksand?

Upward seepage raises pore pressure until it equals total stress, so the grains lose contact and the soil cannot carry load.

Does a surface load change pore pressure?

In saturated clay, a quick load is first carried by excess pore pressure, which dissipates over time as consolidation.

Sources: Terzaghi, K., Theoretical Soil Mechanics, Wiley (1943) · Das, B. M., Principles of Geotechnical Engineering, Cengage

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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