What is Soil Mechanics?
Soil mechanics studies how soil behaves under load, water pressure and stress — the foundation of geotechnical engineering. It explains why saturated ground settles, slides or supports a building's foundation.
Soil mechanics is governed by Terzaghi's effective stress principle: σ' = σ − u, where the effective stress (carried by soil grains) equals total stress minus pore water pressure.
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Step-by-step worked examples
At 5 m depth, total stress is σ=90 kPa and pore water pressure is u=49 kPa. Find the effective stress.
σ' = σ − u = 90 − 49 σ' = 41 kPa
A soil layer has unit weight γ=19 kN/m³. Find the total stress at 4 m depth (dry, no water table).
σ = γ×z = 19×4 σ = 76 kPa (u=0, so σ'=76 kPa too)
Water table is at the surface. At 6 m depth, γ_sat=20 kN/m³ and γ_w=9.81 kN/m³. Find the effective stress.
σ = γ_sat×z = 20×6 = 120 kPa u = γ_w×z = 9.81×6 = 58.86 kPa σ' = σ−u = 120−58.86 = 61.14 kPa
Flashcards
Quick quiz
Q1.Terzaghi's effective stress formula is…
Q2.σ=120 kPa, u=50 kPa. What is the effective stress?
Q3.A rising water table…
Q4.Effective stress is carried by…
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Common mistakes
Confusing total stress with effective stress. — Correct: Total stress includes water pressure; effective stress is what the soil skeleton actually feels.
Assuming pore pressure is always zero. — Correct: Below a water table, u=γw×depth of water — it's rarely zero in saturated soil.
Using dry unit weight below the water table. — Correct: Use saturated unit weight γsat below the water table when computing total stress.
Thinking effective stress can be negative in normal conditions. — Correct: In typical static groundwater, σ' ≥ 0; negative values signal special cases like artesian pressure.
FAQ
What is soil mechanics?
The study of how soil behaves under stress, water pressure and load — the basis of geotechnical engineering.
What is the soil mechanics effective stress formula?
σ' = σ − u, Terzaghi's principle: effective stress equals total stress minus pore water pressure.
How do you calculate effective stress?
Subtract the pore water pressure (u) from the total stress (σ) at that depth.
What are real-world examples of soil mechanics?
Foundation settlement, slope stability, retaining wall design and liquefaction during earthquakes.