Pile Foundations Flashcards
What is the ultimate axial resistance of a pile to failure?
Ultimate shaft resistance + ultimate base resistance
What is the ultimate shaft resistance of a pile?
Qs = pi D int(τult) over L
What is τult?
Interface friction between soil and pile
What is the base resistance of a pile?
Qb = pi r2 qbu
What is qbu?
Ultimate base pressure
How can τult be found from a β value?
τult = β σ’vo
When can τult be found from a β value?
With the assumption that soil is drained fine grained soil where δ’= φ’
How can qbu be approximated?
Nq σ’vo
What is the value of Nq?
(1+sin φ’)/(1-sin φ’) * e^(pi tan φ’)
What is the basic inequality of the ultimate limit state?
ΣL <= R(X)
L is actions
R is resistance
X is material properties
How do safety factors affect the ultimate limit state?
Actions are increased
Resistances and properties are decreased
What is the base stiffness of a pile with diameter D?
2(DG/1-ν)
What is the shaft stiffness of a soil element? The shaft has diameter D, length L and an average shear modulus of Gbar over its length.
2 pi L Gbar/ln(2rmax/D)
What is the settlement of a pile?
Applied load/(base stiffness + shaft stiffness)
How is ultimate pile capacity defined?
The capacity at a displacement equal to 10% of the pile diameter
What must be remembered about the failure of a pile group block as opposed to an individual pile?
The friction angle is the internal soil friction angle, not the interface friction angle
What is the limiting lateral pressure Pult?
Pult = Pu/D = NpSu + σ’vo
How can τult be found from an α value?
τult = αSu
How can qbu be found from an Nc value?
qbu = NcSu
If means are not given to calculate qbu for a pile base resistance, what can you assume?
qbu = Nc Su where Nc is 9
What is the settlement at the top of a pile in terms of the base stiffness and the shaft stiffness?
s = Q / (Kb + Ks)
Describe the operation of a static load test?
Hydraulic pump drives a hydraulic ram which pushes the test pile into the ground
A loadcell records the stress that is being used to drive the pile into the ground
Describe a dynamic pile test
Hammer blows sends stress waves down the length of the pile
Fibre optics along the outside of the pile record this stress wave
Approximately, how does Pult vary with depth?
From 2SuD at the surface to 9SuD at a depth of 3D
What is e/D?
The distance of the point if application of a horizontal load above the ground divided by the pile width
How is a piles resistance to tensile loading different to compression loading?
Qb is not used
The EC7 factors are different
