Week 7 Bearing Capacity and Foundations Flashcards
What is meant by ‘Applied pressure’, ‘Bearing pressure’, and ‘Contact stress’ in foundation engineering?
They refer to the amount of pressure imposed on the ground at the base of a foundation, often represented by the symbol ‘q’.
What is the formula for total pressure (q) applied by a foundation?
Total q = Total pressure from loads on the foundation + Pressure from the self-weight of the foundation.
What is overburden pressure (po)?
The existing vertical stress in the ground at a certain depth caused by the self-weight of the soil.
What happens to soil stress when soil is removed for foundation construction?
The net applied pressure is the gross applied pressure minus the overburden pressure removed.
What is bearing capacity?
The value of applied pressure causing shear failure or excessive settlement of supporting soil.
What soil types are associated with General Shear failure?
Low compressibility soils, dense sand, saturated clay, and undrained clays.
What soil types are associated with Local Shear failure?
Medium compressibility soils, medium-dense sands.
What soil types are associated with Punching Shear failure?
High compressibility soils, loose sands, and drained clays.
What is Terzaghi’s general bearing capacity equation?
qult = cNc + poNq + 0.5γBNγ.
Under what conditions is Terzaghi’s bearing capacity equation valid?
For shallow, long strip foundations where D ≤ B and shear failure is assumed to be in 2D without groundwater considerations.
What modifications are required for Terzaghi’s equation in other scenarios?
Modifications are needed for shape factors, inclined foundations, ground surface inclination, depth of foundation, and presence/variation of groundwater.
What are the typical problems addressed by bearing capacity calculations?
1) Required foundation size for a given load. 2) Safe load capacity of a foundation. 3) Factor of safety of a foundation under a given load.
What factors affect stress distribution below a foundation?
Foundation stiffness, soil compressibility and stiffness, loading conditions, and soil property variations.
What is meant by ‘Safe Bearing Capacity’ (qs)?
qs = qult / F, where F is the factor of safety.
What is meant by ‘Allowable Bearing Pressure’ (qa)?
The bearing pressure typically related to allowable settlements, ensuring the structure’s safety.
What other ground movements are considered in foundation design?
Mining, earthquakes, shrinkage/swelling of soils, frost heave, and other ground movements.
What defines a ‘Shallow’ foundation?
Depth (D) of foundation is less than width (B) or D is less than 3m (within normal excavation reach).
What defines a ‘Deep’ foundation?
Foundations that apply pressure at deeper, more competent strata, often to bypass poor soil conditions near the surface.
