Soil Mechanics
What is Soil Mechanics?
The total force per unit area acting at a point within the soil mass, due to the weight of overlying soil, water, and any external loads.
Key formula / rule: Effective Stress
Key points
- Define effective stress and pore water pressure.
- Calculate effective stress in various saturation conditions.
- Explain the significance of effective stress in soil mechanics.
- Relate effective stress to soil strength and compressibility.
Common exam trap
Confusing total stress with effective stress.
Definitions
- Term
Total Stress (σ)
- Meaning
The total force per unit area acting at a point within the soil mass, due to the weight of overlying soil, water, and any external loads.
- Term
Pore Water Pressure (u)
- Meaning
The hydrostatic pressure exerted by the water occupying the voids (pores) within the soil mass.
- Term
Effective Stress (σ')
- Meaning
The stress transmitted through the soil skeleton (particle-to-particle contact), calculated as the difference between total stress and pore water pressure.
Learning objectives
Define effective stress and pore water pressure.
Calculate effective stress in various saturation conditions.
Explain the significance of effective stress in soil mechanics.
Relate effective stress to soil strength and compressibility.
Formulae
- Name
Effective Stress
- Note
σ' = Effective Stress, σ = Total Stress, u = Pore Water Pressure
- Expression
\σ' = \σ - u
- Name
Total Stress (Overburden)
- Note
γi = unit weight of layer i, hi = thickness of layer i, q = surcharge load
- Expression
\σ = \sumi=1^{n} \γi hi + q
- Name
Pore Water Pressure (Saturated)
- Note
γw = unit weight of water, hw = height of water column above the point
- Expression
u = \γw hw
Prerequisites
Basic concepts of stress and strain.
Understanding of hydrostatic pressure.
Soil composition (solids, voids, water).
Common mistakes
Confusing total stress with effective stress.
Incorrectly calculating pore water pressure, especially in stratified or layered soils.
Ignoring the effect of capillary rise in partially saturated soils.
Assuming pore water pressure is always zero in the presence of water.
Keywords
Effective Stress
Total Stress
Pore Water Pressure
Terzaghi's Principle
Soil Mechanics
Geotechnical Engineering
Shear Strength
Consolidation
Practice preview
Which of the following relationships correctly defines the porosity (n) in terms of void ratio (e) for a soil mass?…
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The Plasticity Index (PI) of a soil is defined as the numerical difference between:…
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According to Darcy's Law, the rate of flow through a saturated soil is directly proportional to the:…
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