Soil Mechanics / Effective stress and stress in the ground
Stress profiles with a water table and a surface load
Build the three-line σ, u, σ′ diagram down a layered ground, and see what a surcharge does.
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Read the lesson register and outstanding review notesWhy this matters
Real ground is layered, with different unit weights, and a water table somewhere in it. Being able to draw the profile of total stress, pore pressure and effective stress with depth is a core geotechnical skill — it is the starting point for almost every settlement and strength calculation.
From first principles
Building the stress profile
We want to show: Compute σ, u and σ′ at any depth in a layered soil with a water table.
Total stress is just the accumulated weight above the plane — add up each layer's unit weight times its thickness. Below the water table, use the saturated unit weight; above it, the bulk. Pore pressure is hydrostatic below the water table. Effective stress is the difference.
Try it
Effective-stress profile explorer
Sand over clay. Move the water table and add a surface load; watch the three lines respond.
- At 8 m — total σ
- 151.0 kPa
- — pore u
- 58.9 kPa
- — effective σ'
- 92.1 kPa
Read the diagram
- Total σ (dark) rises fastest — it carries everything above.
- Pore pressure u (blue) is zero above the water table, hydrostatic below.
- Effective σ′ (orange) = σ − u is what the soil skeleton feels.
At the water table the effective-stress line kinks: below it, its gradient is the submerged unit weight γ′, not the saturated γsat.
Worked example
Effective stress at depth in a two-layer profile
Given
- Sand 0–3 m: γbulk = 18, γsat = 20 kN/m³
- Clay below 3 m: γsat = 19 kN/m³
- Water table at 3 m depth
- γw = 9.81 kN/m³
Find
Total stress, porewater pressure and effective stress at 6 m depth.
Assumptions
- Sand above the water table is moist (bulk); everything below is saturated; hydrostatic pore pressure.
Practice
For the same profile (sand 0–3 m, clay below), the water table rises to the ground surface. What is the effective stress at 6 m depth now? (Answer in kPa.)
Summary
- Total stress accumulates unit weight × thickness (bulk above the water table, saturated below), plus any surcharge.
- Porewater pressure is hydrostatic below the water table; zero above it.
- Below the water table the effective-stress gradient is the submerged unit weight γ′.
- Raising the water table lowers effective stress; a surcharge raises it.