3 - Shallow Foundations Flashcards
(26 cards)
What is the ultimate bearing capacity (qult) of a shallow foundation?
The ultimate bearing capacity (qult) is the load at which the soil fails to support the foundation.
What is the equation for shear strength of the soil according to Mohr-Coulomb failure criterion?
τ=c+σtanφ’
What is the general equation for the ultimate bearing capacity of a shallow foundation?
q,ult = cNc + Nqp0 + 1/2 * y * B * Ny
What does the term “Nc” in the bearing capacity equation represent?
Nc is the bearing capacity factor associated with the cohesion (c) of the soil
What are the three broad factors considered in the bearing capacity equation for a shallow foundation?
The constant component of strength in the soil (c),
The surcharge loading around the foundation (𝑝0)
The self-weight of the soil (γ).
What is the significance of the Mohr-Coulomb failure envelope for soil?
The Mohr-Coulomb failure envelope represents the relationship between shear strength (τ) and normal stress (σ) in soil, providing a model for soil failure
What is the purpose of the bearing capacity factors
These factors are empirical and theoretical values that modify the components of the bearing capacity equation based on soil properties and foundation conditions.
How do you find out what the bearing capacity factors are?
By looking in the data book.
What equation is used to calculate the bearing capacity of a foundation under undrained conditions (short-term analysis of stiff clay)?
q = ScNcCu + p0
What does the term Cu represent in undrained conditions for bearing capacity?
is the undrained shear strength of the soil, which provides base resistance to the foundation.
How does the surcharge load p0 affect the bearing capacity under undrained conditions?
The surcharge load p0 increases the stress state of the soil, which in turn increases its bearing capacity.
In ideal conditions, how is the bearing capacity calculated for a strip footing under undrained conditions?
qult = (2 + pi)Cu + p0
What is the value of the bearing capacity factor Nc in undrained conditions for a strip footing?
Nc = 2+ pi
What is the shape factor sc for a strip footing under undrained conditions?
sc = 1 + BL/5
What is the general limitation of the Nc factor in design for deeply embedded foundations?
In design, the Nc factor is typically limited to a value of 9 for deeply embedded square foundations, beyond which the analysis becomes insufficient.
In the drained case, what is the Mohr-Coulomb model?
Shear stress = c’ + sigma’*tan phi’
What is c’?
The effective cohesion?
What is phi’?
The effective angle of shearing resistance
Many experienced engineers will draw a Mohr-Coulomb diagram for clay under an effective stress, giving an effective friction angle through the y-intercept, judging the soil to have an effective cohesion, why is this incorrect and what are they suggesting?
Under very low effective stress
levels, the engineer is suggesting that the soil has a shear strength that exceeds the effective stress level by some margin. As such one would say that this clay has cemented,
as though it were rock, but this is not the case. Imagine a soft clay with no load applied
to it? Its more like a liquid than a solid, and certainly not cemented! Always avoid this
trivial error, and if you’re arguing the point with someone, grab a bucket of clay and a
shear box, and prove it to them!
Beneath a vertically loaded foundation, where there is a rotation of principal stresses, what equations arise?
p0’ = p0 - u, q; = q-u
(qult - u) = Nq(p0 - u)
In practice, it is rare that a foundation will be loaded solely vertically, what other effect to we need to consider on the bearing capacity of the foundation?
Inclined loading.
What is the rotation in principal stresses linked to?
A shift in the Mohr circle for the soil, where the magnitude of the shift, is equal to the diameter of the circle multiplied by the angle of rotation.
What is the change in effective stress, when considering the rotation of principal stresses?
2Cuangle of rotation
If the principal stress has rotates from vertical to horizonatal, What does the change in effective stress equal?
pi * Cu
