Effective Stress
The soil skeleton never carries the full load alone. Here is how water and grains share the weight above them — and why that quiet arithmetic controls strength, settlement and stability.
Why effective stress matters
Picture placing a 100 kN load on the ground. Does the soil skeleton carry all 100 kN of it? No — part of that load is carried by the water sitting in the voids, and the rest is carried by the soil particles themselves. The portion carried by the particle skeleton is what we call effective stress, and it is the part that actually governs how the ground behaves.
Three types of stress in soil
σ
The total load per unit area acting at a point in the soil, carried jointly by water and grains.
σ = γ × zγ = unit weight of soil, z = depth
u
The pressure exerted by water sitting in the voids. Above the water table (ignoring capillary effects), it is approximately zero.
u = γw × hγw ≈ 9.81 kN/m³ (often taken as 10), h = depth below water table
σ′
The stress transmitted through the soil particles. It controls shear strength, settlement, bearing capacity, compressibility and slope stability.
σ′ = σ − uTerzaghi's Principle
Terzaghi's Effective Stress Principle
Karl Terzaghi's relationship, σ′ = σ − u, is one of the most important equations in geotechnical engineering. Drag the sliders below to move the water table and the point of interest, and watch the three stresses respond live.
Interactive soil column
Assumes γdry = 18, γsat = 20, γw = 9.81 kN/m³. Notice: raise the water table and effective stress falls immediately — even though total stress barely changes.
Putting the numbers together
Example 1 — dry layer
A soil layer is 5 m thick, unit weight = 18 kN/m³, and the water table is below the layer. Find the total stress at the bottom.
Example 2 — below the water table
A point is 4 m below the groundwater table. Saturated unit weight = 20 kN/m³, water unit weight = 10 kN/m³.
Effect of the water table
The water table strongly influences effective stress. Tap either state below.
- Effective stress decreases.
- Shear strength decreases.
- Settlement characteristics change.
Water climbing against gravity
Water can rise above the groundwater table through small pores due to surface tension. This height is the capillary rise. Finer soils have higher capillary rise because of smaller pore spaces.
| Soil | Approximate rise |
|---|---|
| Gravel | Negligible |
| Coarse sand | 0.05 – 0.30 m |
| Fine sand | 0.30 – 1.50 m |
| Silt | 1 – 5 m |
| Clay | Up to 10 m or more |
Engineering importance: causes dampness in buildings, increases foundation moisture, influences frost action, and affects pavement performance.
Quicksand (boiling) condition
When upward seepage force equals the submerged weight of the soil, particles lose contact with each other and the soil behaves like a liquid. This is called quicksand, boiling, or hydraulic failure — it is a seepage phenomenon, not "ordinary sand behaving magically."
Gs = specific gravity of solids, e = void ratio. If the actual hydraulic gradient exceeds ic, quicksand conditions may develop.
Consequences: failure of excavation bottoms, uplift of foundation bases, instability near sheet piles, piping beneath dams.
Prevention: dewatering systems, cutoff walls, sheet piles, relief wells, filter layers, reducing the hydraulic gradient.
Common Loksewa MCQs
Interview questions
Memory box
Quicksand occurs when upward seepage reduces effective stress to nearly zero. Effective stress governs strength, settlement and bearing capacity.
From this point onward, almost every major geotechnical topic — consolidation, shear strength, bearing capacity, earth pressure, and slope stability — uses the concept of effective stress. Make sure you can derive and apply σ′ = σ − u without hesitation.
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