Shear Strength Flashcards
(17 cards)
What is shear strength dependant on?
The stress state in the soil. Stress is force per unit area. The largest normal stress is called “major principle stress” and the smallest normal stress is called the “minor principle stress”.
Symbol for shear stress
τ, tau
Symbol for normal stress
sigma (σ)
Equation for effective stresses with known pore pressure, u
σ’ = σ - u (x/y/z)
τ’ = τ (xy/yz/zx)
remember x,y,z
What is strain in the y direction for a long structure
εy=0, therefore the stresses only need to be considered in the x-z plane, and can be reduced to 2 dimensions.
Why is geotechnical analysis often carried out in two-dimensions?
Because 3 dimensions makes it difficult to find analytical solutions. Certain geotechnical structures can be approximated by two-dimensional models, such as long structures.
What is the Mohr circle of stress
If the stresses acting in two perpendicular directions at a point are unknown, the normal stress and shear stress in any direction at that point can be found using the Mohr Circle, a graphical method, instead of using the transformation equations.
Mohr-coulomb failure theory
When the major (σ1) and the minor (σ3) principle acting on a point in a material have a specific functional relation, the material will fail:
(σ1-σ3)=f(σ1+ σ3)
f=specific function, varies with material
Mohr-coulomb failure theory, shear strength of soils equation
τf= c + σtanθ
Effective stress principle (Terzaghi 1925)
It is effective stress (σ’= σ-u) that determines the shear strength.
τf = c’ + σ’tanθ’=c’+(σ-u) tanθ’
How do you control soil density in the direct shear test?
For loose sand, it should be rapidly poured into the shear box from a small height. Compaction and vibration should be avoided.
For dense sand, it should be slowly poured into the shear box from a relatively high drop. The sand should be placed in 3 layers, each layer subjected to tamping.
Symbol for max shear load
S
What is max shear load used to calculate, in the direct shear test?
Shear strength τ
τ = S / A
Disadvantages of direct shear test
Fixed breadth, difficult to control drainage
Advantages of triaxial test
Can control drainage, can have varied consolidation, doesn’t have a plane
What test is used when a graph shows the effective stress as the total stress
Consolidated-drained triaxial test
What test is used when a graph shows the pore water pressure measurements
Consolidated-undrained triaxial test
