Piled Foundations

Question 1

What are piles?

Answer 1

Piles are columns of concrete, steel or timber installed into the ground.

Question 2

In what forms can piles come in?

Answer 2

• Piles can be a single pile under a load bearing column
• A large number of piles that support the structure via a pile cap which transfers load from the structure into the piles.

Question 3

How are piles classified by function?

Answer 3

Based on where they derive most of their load-carrying resistance from: the base (end-bearing) or the shaft (friction).

Question 4

What is an end-bearing pile?

Answer 4

A pile that derives most of its resistance from the soil or rock at its base (tip).

Question 5

What is a friction pile?

Answer 5

A pile that derives most of its resistance from the shaft friction along its length.

Question 6

What is a combination pile?

Answer 6

A pile that relies on both tip resistance (end-bearing) and shaft friction to carry the load.

Question 7

When are piles used?

Answer 7

1. Where the upper soils have a low bearing capacity - a pile can transmit loads to a deeper stratum with a higher bearing capacity.
2. Where the loads are so great the spread footings would be too large.
3. If the upper soil is subject to scour or undermining (e.g. bridge foundations).
4. If the foundation needs to be installed under water (e.g. foundations of offshore wind turbine / oil rig).
5. If uplift forces will be large.
6. If the horizontal (lateral forces will be large).
7. If there is likely to be an excavation adjacent to the site at a later date.
8. Beneath tall buildings to increase stability.
9. Compaction of loose sands.

Question 8

When is a foundation a pile?

Answer 8

A foundation is a pile if the foundation depth to breadth ratio exceeds 5, i.e. if L
D >5.

Question 9

What is a driven pile?

Answer 9

• Steel (H-section or tubular), concrete (reinforced prestressed).
• Driven into the soil by hammer blows or pushed in hydraulically
• Classified as displacement piles.
• Can be installed underwater (offshore).

Question 10

What are bored and cast in place piles?

Answer 10

• Hole bored into ground and then filled with concrete
• Large diameter
• May be under-reamed

Question 11

What are continuous flight augered piles?

Answer 11

• Minimal vibration
• Low noise
• No support needed for walls
• Quick installation.

Question 12

At what settlement is the shaft resistance (Rsf) typically fully mobilised?

Answer 12

At very small settlements: 5-10 mm, or about 0.5% of the shaft diameter.

Question 13

At what settlement is the base resistance (Qbf) typically fully mobilised?

Answer 13

At much larger displacements: about 10% of the base diameter.

Question 14

What is the formula for pile capacity?

Answer 14

P + W = Rs + Qb

Question 15

According to Poulos (2000), what is one of the most effective ways of dealing with uncertainties in pile design?

Answer 15

Pile testing

Question 16

What are the two main types of in-situ static pile load tests?

Answer 16

1. Maintained load test (most common in practice)
2. Constant rate of penetration test

Question 17

What are the two categories of pile load tests based on their purpose?

Answer 17

1. Preliminary test: Assists in design before construction.
2. Working test pile (proof test): Forms part of the actual structure to verify performance.

Question 18

What is the primary benefit of conducting pile load tests in terms of design?

Answer 18

They can allow a reduction in the factor of safety (FoS), which can lead to shorter and more economical pile lengths.

Question 19

What are the two common reaction systems used to provide resistance for a pile load test?

Answer 19

1. Jacking against kentledge (heavy weights)
2. Jacking against a beam restrained by ground anchors

Question 20

What is the α method used for in pile design?

Answer 20

It is a simple method used to predict the capacity of piles in undrained materials (clays), particularly driven piles.

Question 20

List three reasons for the limited reliability of the α method predictions.

Answer 20

1. Variations in measured undrained shear strength (su) due to different testing methods, data scatter, and sample disturbance.
2. Differences in pile construction/installation methods compared to those used to develop the original database.
3. The inherent simplicity of the method.

Question 21

What is the key theoretical advantage of the β-method over the α-method?

Answer 21

It is based on more fundamental soil mechanics principles.

Question 22

Why is the β-method considered more reliable than the α-method in practice?

Answer 22

It uses effective vertical stress (σ′ᵥ), which can be estimated more precisely than undrained shear strength (sᵤ).

Question 23

What is the most rigorous way to select β values?

Answer 23

Using site-specific soil test results.

Question 24

What alternative approach can be used to select β values when site data is limited?

Answer 24

Empirical correlations derived from test databases.

Question 24

Whats the earth pressure coefficient for normally consolidated and over consolidated clay?

Answer 24

NC - K0 = Ks OC - (1.5 +/- 0.5)K0

Question 25

What is meant by negative skin friction on a pile?

Answer 25

Downward shear force acting on a pile due to surrounding soil moving down relative to the pile.

Question 26

Why can soil contribute additional load to a pile?

Answer 26

Consolidating soil settles and drags the pile downward, increasing axial load.

Question 27

In which type of soil is negative skin friction most likely to develop?

Answer 27

Soft, compressible clay undergoing consolidation.

Question 28

What is the spacing factor for pile groups, and what minimum value is typically recommended?

Answer 28

The centre-to-centre distance between piles divided by the pile diameter (x/D). It should typically be greater than 2.

Question 29

What are the two distinct modes of failure that must be considered when designing pile groups?

Answer 29

1. Failure of individual piles (e.g., by punching). 2. Block failure - the piles and the soil between them act as a single large block that fails as a unit.

Question 30

How is group efficiency defined for a pile group?

Answer 30

The average load per pile at failure divided by the failure load of a single, isolated pile.

Question 31

What is the typical value of group efficiency, and what does it imply?

Answer 31

It is usually less than 1, meaning the group is less efficient than the sum of its individual piles. Values greater than 1 are rarely used in design.

Question 32

What are the two calculations that must be performed for pile group design, with the lesser value taken as the design capacity?

Answer 32

1. Number of piles × capacity of individual pile × group efficiency (typically ~0.8 in clays) 2. The ultimate load assuming block failure

Question 33

What factors govern choice of piles?

Answer 33

* Location and type of structure * Ground conditions * Durability * Cost * Disposal of spoil

Question 34

When is shaft resistance fully developed?

Answer 34

At about 0.5% of the pile diameter

Question 35

What are the 3 phases of pile driving?

Answer 35

1. Installation 2. Equilibration 3. Loading

Question 36

What happens during installation?

Answer 36

* Displacement of the soil - outward direction. * Excess pore-water pressures are generated in clays, but not in sands. * Remolding of the soil next to the pile due to large displacements caused by the pile driving (change in fabric, residual shear surfaces can form) * The mobilized shear strength is close to strength values obtained in interface tests in the ring shear apparatus at large displacements.

Question 37

What happens during equilibration?

Answer 37

* The porewater pressure generated during installation dissipates (consolida- tion) (u↓) * Therefore the effective radial stress increases (σ′r ↑).

Question 38

What happens during loading?

Answer 38

* During loading the shaft resistance (first) and (later) the base resistance will be mobilized * The peak strength will not be mobilised simultaneously at all points along the pile shaft - it is important to be aware of this when the material is strain-softening.

Question 39

What is ultimate capacity "plunging failure"?

Answer 39

This is defined as the load at which settlement continues indefintiely

Question 40

What is deformation controlled capacity?

Answer 40

10% failure load (i.e. deformation is 10% of pile diameter)

Question 41

What are ⍺ values dependent on?

Answer 41

1. the soil type for which they were developed 2. the pile type and installation method 3. the method used to calculate the undrained strength in the original correlation

Question 42

Explain using sketches as appropriate the difference between a CFA pile and bored pile.

Answer 42

* A bored pile is formed by excavating an open hole using a rotary auger or bucket. * A CFA (Continuous Flight Auger) pile is constructed by drilling to depth with a hollow-stem continuous auger.