Pile Foundations

Question 1

What is the ultimate axial resistance of a pile to failure?

Answer 1

Ultimate shaft resistance + ultimate base resistance

Question 2

What is the ultimate shaft resistance of a pile?

Answer 2

Qs = pi D int(τult) over L

Question 3

What is τult?

Answer 3

Interface friction between soil and pile

Question 4

What is the base resistance of a pile?

Answer 4

Qb = pi r2 qbu

Question 5

What is qbu?

Answer 5

Ultimate base pressure

Question 6

How can τult be found from a β value?

Answer 6

τult = β σ’vo

Question 7

When can τult be found from a β value?

Answer 7

With the assumption that soil is drained fine grained soil where δ’= φ’

Question 8

How can qbu be approximated?

Answer 8

Nq σ’vo

Question 9

What is the value of Nq?

Answer 9

(1+sin φ’)/(1-sin φ’) * e^(pi tan φ’)

Question 10

What is the basic inequality of the ultimate limit state?

Answer 10

ΣL <= R(X)
L is actions
R is resistance
X is material properties

Question 11

How do safety factors affect the ultimate limit state?

Answer 11

Actions are increased
Resistances and properties are decreased

Question 12

What is the base stiffness of a pile with diameter D?

Answer 12

2(DG/1-ν)

Question 13

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.

Answer 13

2 pi L Gbar/ln(2rmax/D)

Question 14

What is the settlement of a pile?

Answer 14

Applied load/(base stiffness + shaft stiffness)

Question 15

How is ultimate pile capacity defined?

Answer 15

The capacity at a displacement equal to 10% of the pile diameter

Question 16

What must be remembered about the failure of a pile group block as opposed to an individual pile?

Answer 16

The friction angle is the internal soil friction angle, not the interface friction angle

Question 17

What is the limiting lateral pressure Pult?

Answer 17

Pult = Pu/D = NpSu + σ’vo

Question 18

How can τult be found from an α value?

Answer 18

τult = αSu

Question 19

How can qbu be found from an Nc value?

Answer 19

qbu = NcSu

Question 20

If means are not given to calculate qbu for a pile base resistance, what can you assume?

Answer 20

qbu = Nc Su where Nc is 9

Question 21

What is the settlement at the top of a pile in terms of the base stiffness and the shaft stiffness?

Answer 21

s = Q / (Kb + Ks)

Question 22

Describe the operation of a static load test?

Answer 22

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

Question 23

Describe a dynamic pile test

Answer 23

Hammer blows sends stress waves down the length of the pile
Fibre optics along the outside of the pile record this stress wave

Question 24

Approximately, how does Pult vary with depth?

Answer 24

From 2SuD at the surface to 9SuD at a depth of 3D

Question 25

What is e/D?

Answer 25

The distance of the point if application of a horizontal load above the ground divided by the pile width

Question 26

How is a piles resistance to tensile loading different to compression loading?

Answer 26

Qb is not used
The EC7 factors are different