Test: Effective & Neutral Stress - Civil Engineering (CE) MCQ

Effective stress:

• Effective stress or intergranular stress is defined as the stress caused due to soil particles coming close to each other due to the distribution of the water.
• Its function of shear strength effective stress is an abstract quantity, it cannot be measured directly in the laboratory.

Total stress (σ) = effective stress (σ’) + pore water pressure (u)
Effective stress = Total stress - pore water pressure 
Effective stress = Y - X

Calculation: 
Given: γ_sat = 20kN/m³, γ_w = 10 kN/m³, 
The total stresses, σ = γ_w h₁ + γ_sat h₂
pore water pressure u = γ_w (h₁ + h₂)
effective stress σ̅  = σ - u  
σ̅  = γ_w h₁ + γ_sat h₂ -γ_w (h₁+h₂) 
σ̅  = γw h₁ + γ_sat h₂ - γ_w h₁ - γ_wh₂
σ̅  = γ_sat h₂ - γ_wh₂
σ̅  = 20 × 5 - 10 × 5
σ̅  = 50 kN/m²

Total Stress:
Total stress at a layer present at a certain depth below ground level is the total weight of the soil present above that layer per unit surface area of the soil mass. Effective stress is the part of the total stress that is resisted by soil particles by grain-to-grain interaction
Total Stress = Effective Stress + Pore Water Pressure
Effective Stress = Total Stress – Pore Water Pressure

Effective Stress

• Effective stress can be defined as the stress that keeps particles together.
• In soil, it is the combined effect of pore water pressure and total stress that keeps it together.
• It can also be defined in equation form as the total stress minus the pore pressure.

Pore water pressure

• Pore water pressure refers to the pressure of groundwater held within a soil or rock, in gaps between particles.
• Pore water pressures below the phreatic level of the groundwater are measured with piezometers.
• The vertical pore water pressure distribution in aquifers can generally be assumed to be close to hydrostatic

Decrease in water content of soil results in decrease in the volume of the soil thus causing the shrinkage of the soil.

When saturated soil is dried, a meniscus develops in the voids at the solid surface. Formation of meniscus leads to tension in the soil water that leads to the compression of the soil structure and consequent reduction in the volume.

Cation exchange capacity is factor influencing the swelling of soil. The degree of shrinkage depends upon type and amount of clay minerals, mode and environment of deposition.

Free swell of a soil is the increase in the volume of soil without any constrains on submergence in water and it ceases when the water content reaches the plastic limit.

When the water enters the voids (containing air during drying process), due to immersion, menisci is formed, causing high pressures and subsequent explosion of the voids leading to the disintegration soil structure.

Concept:
For rigid footing:
When a rigid footing rests on a soil then the settlement will be uniform, but the contact pressure will be variable, which depends upon the type of soil underneath it.
For different soil the variation will be:

For flexible footing:
In case of flexible footing contact pressure will be uniform for all type of soil but the settlement will be variable.

Given:
γ_w = 10 kN/m³, γ_sat = 20 kN/m³, γ_b = 18 kN/m³ 

Effective stress at a depth of 10 m below the ground level:
γ_eff = γ_b × 3 + γ_sub × 7
γ_eff = 18 × 3 + (20 - 10) × 7
γ_eff = 124 kN/m³