QB3 Pile Design

Question 1

How are the partial factors determined for part a)?

Answer 1

Depending on if the question states ‘favourable’ or ‘unfavourable’ the partial factors can be selected from Table 6-8

Question 2

How is Wgk (weight of the concrete pile) calculated?

Answer 2

Yconc x cross sectional area of the pile (Pi x dia.^2/4) x length of the pile

Question 3

How is Fcd calculated in set 1 and set 2, and what are the final units?

Answer 3

Fcd is the ultimate design load calculated using the partial factors separately in set 1 and set 2

Units are kN

Question 4

How are partial factors determined for part b) (design resistance) ?

What factors are required?

Answer 4

The partial factors are determined by the type of pile (driven, bored, cfa) and by whether SLS is used or not.

Ys (shaft resistance)
Yb (base resistance)
Yr;d (model factor)

Question 5

What value is model factor typically?

When is the model factor different?

Answer 5

Typically 1.4

Model factor becomes 1.2 when a maintained load test has been taken to failure

Question 6

How is the design resistance of the base calculated?

Rc;b;k

Answer 6

The strength of material at the base of the pile (kN/m^2)

x

The cross-sectional area of the base (m^2)

x

Nc (bearing capacity factor)

Question 7

What is Nc (bearing capacity factor) normally equivalent to?

Answer 7

9

Question 8

How is Rc;s;k calculated?

Answer 8

Strength of the soil (c) at the centre of the pile (average cohesion)

x

Shaft area (perimeter of shaft x length)

x

Alpha (adhesion factor)

Question 9

What is alpha (the adhesion factor) equal to for a bored pile in clay?

Answer 9

0.45

Question 10

How is the design resistance calculated for set 1 and set 2?

Answer 10

Set 1 = (shaft resistance/partial factors) + (base resistance/partial factors)

Set 2 is the same but with set 2 partial factors

Question 11

How is degree of utilisation calculated?

Answer 11

The design action is divided by the ground resistance and multiplied by 100 to obtain a % of utilisation

Question 12

How are the actions determined to be suitable?

Answer 12

Design actions should be less than or equal to the actual ground resistance

Question 13

What information should be included when commenting on utilisation?

Answer 13

Percentage of utilisation, utilisation gap (%), how the utilisation could be increased/decreased

Question 14

How can utilisation % be increased?

Answer 14

Reducing the pile length and/or diameter

Question 15

What is the avg cost per metre saving for a reduction in length of a pile?

Answer 15

~ £100

Question 16

How is Rcsk (resistance of the shaft) calculated? (Formula)

Answer 16

Alpha x Cave x As

Question 17

What is alpha equal to in bored piles?

Answer 17

0.45

Question 18

What is Cave?

Answer 18

The average value for cohesion along a pile

Question 19

What is As?

What is it equal to?

Answer 19

Perimeter of the shaft

Pi x dia. x L

Question 20

What is Rcbk (base resistance) equal to? (Formula)

Answer 20

Nc x Cb x Ab

Question 21

What is Nc and what is it equal to?

Answer 21

Bearing capacity factor

9.0

Question 22

What is Cb?

Answer 22

The cohesion at the base of the pile

Question 23

What is Ab and how is it calculated?

Answer 23

The area of the base of the pile

Pi x d^2 / 4 for circles and L^2 for square piles