Week 9 Soil Strength 2 Flashcards
Why is soil strength not a constant value?
Several factors, including cohesion, friction, normal stress, strain, and drainage conditions, cause variability in soil strength.
What is the Coulomb equation for maximum shear stress (τ)?
τ = c + σn tan Φ, where: c = Cohesion, σn = Normal stress, Φ = Friction angle.
What factors remain constant for a soil in a given drainage condition?
Cohesion (c) and Friction (Φ).
What factor in the Coulomb equation depends on how much stress is acting on the soil?
Normal stress (σn).
What is meant by ‘ultimate strength’ in soil?
The lower value of strength observed at large strains, especially common in clay soils.
What is ‘residual strength’ in soil?
An even lower strength value that may occur at very large strains in clay soils.
What is effective stress (σ’)?
The stress passing through the framework of inter-connected soil particles (soil skeleton).
How is total stress in soil calculated?
Total stress = Effective stress (σ’) + Pore water pressure (u).
What happens to effective stress (σ’) if pore water pressure (u) is high?
Effective stress (σ’) will be relatively low.
Why is low effective stress (σ’) significant for soil strength?
Low effective stress (σ’) results in low normal stress (σn) and therefore lower overall soil strength.
What happens when an increased load is applied to saturated soil?
It causes an increase in total stress and pore water pressure, resulting in seepage if water can move.
What is a ‘drained condition’ in soil?
When pore water pressure can dissipate quickly, allowing effective stress to increase.
What is an ‘undrained condition’ in soil?
When water movement and pore pressure dissipation are slow, causing excess pore water pressure to remain high.
What are ‘undrained’ soil strength parameters?
cu and Φu, referring to cohesion and friction in terms of total stress.
What are ‘drained’ soil strength parameters?
c’ and Φ’, referring to cohesion and friction in terms of effective stress.
What are the soil strength parameters?
Cohesion (c) and Friction (Φ).
How can soil strength parameters be determined graphically?
By plotting shear stress against normal stress using Mohr circles and drawing a failure envelope.
What does the intersection of the failure envelope with the vertical shear stress axis represent?
The value of cohesion (c) in kN/m².
What does the angle of the failure envelope line represent?
The value of friction (Φ) in degrees.
