Week 8 Soil Strength 1 Flashcards
Why is soil strength difficult to predict and understand compared to materials like concrete or steel?
Soil strength is not a constant property and varies depending on various factors such as soil type, formation, grading, previous stresses, and confinement.
What factors influence the strength properties of soil?
Soil type, formation, particle size (grading), previous applied stresses, confinement, stress conditions, and drainage conditions.
What are the three factors contributing to soil shear strength?
1) Cohesion (c) of the soil. 2) Friction (Φ) between soil particles. 3) Normal stress (σn) acting across the shear surface.
What is meant by the ‘angle of friction’ or ‘angle of shearing resistance’?
It refers to the friction (Φ) between soil particles.
What are ‘Shear Strength Parameters’?
The cohesion (c) and friction (Φ) values of a soil, which can be determined through tests.
Why is normal stress (σn) not fixed in a given drainage condition?
It depends on the stress within the soil due to unit weight, depth, and external loads.
What happens when soil is subjected to increased stress?
It undergoes strain or displacement of particles, resulting in mobilized shear resistance.
What is ‘yield stress’ in soil?
The stress level causing strains so large that it may form a ‘serviceability’ limit state, not necessarily the maximum shear strength.
What is ‘peak stress’ in soil?
The maximum shear stress the soil can withstand, typically used to determine shear strength parameters.
What is ‘ultimate stress’ in soil?
A constant value of stress achieved during very large strains, common in loose sands or soft clays that undergo work hardening.
What is ‘residual strength’ in soil?
The lower shear strength value achieved after a peak stress, especially in soils that have previously failed and undergone large displacements.
Why is residual strength important in geotechnical engineering?
It determines the required applied shear stress to re-activate old failure surfaces, such as those within landslides.
What is the Mohr-Coulomb equation for shear strength?
τ = c + σn tan Φ, where: τ = Shear strength, c = Cohesion, σn = Normal stress, Φ = Angle of friction.
What is the effective stress version of the Mohr-Coulomb equation?
τ = c’ + σ’n tan Φ’, where: c’ = Effective cohesion, σ’n = Effective normal stress, Φ’ = Effective angle of friction.
Why is soil strength often determined in effective stress terms?
Because shear movement is resisted by the soil skeleton, not the pore water, so effective stress reflects the actual stress passing through the soil particles.
How is effective stress calculated during soil strength tests?
By subtracting pore water pressure from the total stress.
