6 - Piled Foundations

Question 1

What is the difference between displacement and non-displacement piles?

Answer 1

Displacement piles displace the soil around the pile during installation (e.g., driven steel piles, screw piles), whereas non-displacement piles do not disturb the soil (e.g., CFA piles, bored piles).

Question 2

What types of foundation situations may require piled foundations?

Answer 2

Piled foundations may be used when near-surface soils have low strength or stiffness, when large loads are applied, when settlement control is needed, or in marine environments to prevent soil scour.

Question 3

How do displacement piles affect soil in sand and clay?

Answer 3

Displacement piles in sand densify the soil, potentially increasing strength (though not relied upon in design). In clays, they cause heave at the surface, which can damage services.

Question 4

How do non-displacement piles work during installation?

Answer 4

Non-displacement piles, such as CFA piles, do not alter the radial stress significantly. They rely on negative pore pressures to maintain the borehole, and casings are typically used to prevent collapse during installation.

Question 5

Why are displacement piles generally avoided in built-up areas?

Answer 5

Displacement piles, especially hammer-driven ones, are noisy and disruptive, which makes them unsuitable for use in areas with existing infrastructure or residential buildings.

Question 6

How do ground conditions affect the selection of pile type?

Answer 6

If contaminated ground or rocky deposits are encountered, the pile selection may be influenced. Non-displacement methods like CFA remove a large amount of soil, which may create disposal issues, while hollow driven piles can deform in rocky soil.

Question 7

What are micropiles, and in what applications are they used?

Answer 7

Micropiles are small-diameter piles (typically less than 250 mm) used for additional support, especially in tight or low-headroom conditions. They are suitable for small loads, limited depth (typically 10 m), and handling obstructions.

Question 8

What are the key advantages of micropiles?

Answer 8

Micropiles are advantageous because they can be installed in constrained spaces, are able to deal with obstructions, and require smaller diameters, making them ideal for isolated equipment or supporting retaining walls.

Question 9

What are some of the challenges associated with using Continuous Flight Auger (CFA) piles?

Answer 9

CFA piles can create large quantities of soil to be disposed of, which introduces financial and safety risks, especially if contaminated ground is encountered.

Question 10

What is a typical application for micropiles in construction?

Answer 10

Micropiles are often used to support isolated pieces of heavy plant equipment in a factory or to provide additional support to retaining walls, especially when space is limited.

Question 11

What are the two main components of axial resistance for piles?

Answer 11

The two main components of axial resistance for piles are shaft resistance (Qs) and base resistance (Qb).

Question 12

How is the total pile capacity (Q) calculated?

Answer 12

The total pile capacity (Q) is the sum of shaft resistance and base resistance:
Q = Qb + Qs

Question 13

What is the formula for base resistance (Qb)?

Answer 13

The base resistance (Qb) is given by

Qb = pi D ^2 / 4 * QB ,where qb is the bearing capacity

Question 14

What does the shaft resistance depend on?

Answer 14

The average shear stress along the pile length

Question 15

What is the simplified approach to calculating the end-bearing stress in clays?

Answer 15

A simplified approach calculates the end-bearing stress as
qb = Nc ~* Cu
, which is widely used, especially in the databook. However, this does not include the overburden pressure at the foundation depth.

Question 16

When can the overburden pressure
𝜎
0
σ
0
​
be omitted in the calculation of pile resistance?

Answer 16

The overburden pressure increases the resistance available beneath the pile due to the weight of the soil above the pile. Including

𝜎
0
σ
0
​
provides a more accurate estimate of the pile’s resistance.

Question 17

What is the simplified approach to calculating the end-bearing stress in clays?

Answer 17

A simplified approach calculates the end-bearing stress as qb = Nc Cu, which is widely used, especially in the databook. However, this does not include the overburden pressure at the foundation depth.

Question 18

Why should the overburden pressure, σ0, be included in the calculation of end-bearing stress?

Answer 18

The overburden pressure increases the resistance available beneath the pile due to the weight of the soil above the pile. Including σ0 provides a more accurate estimate of the pile’s resistance.

Question 19

When can the overburden pressure σ0 be omitted in the calculation of pile resistance?

Answer 19

If the weight of the pile is not known, or if the pile is in clay with a similar unit weight to the surrounding soil, omitting σ0 is a reasonable simplification.

Question 20

What is the effect of omitting the overburden and self-weight in sand?

Answer 20

Omitting the overburden and self-weight leads to assuming that the surrounding soil has the same unit weight, which introduces error, especially when considering sands with different unit weights from the pile material.

Question 21

What is the typical unit weight of concrete and sand used for pile calculations?

Answer 21

The unit weight of concrete is approximately γconc = 25 kN/m³, and the unit weight of sand is approximately γsand = 16 kN/m³.

Question 22

How does the unit weight of the sand affect the pressure calculations around a pile?

Answer 22

The sand’s unit weight influences the overburden pressure, which affects the resistance at the pile tip. For example, if a pile is 20 m long, the surrounding overburden pressure would be 20 × 16 = 320 kPa.

Question 23

How is the end-bearing stress calculated in rock for piles driven to refusal?

Answer 23

For piles driven to refusal in rock, the end-bearing stress is typically calculated as qb = 3qu, where qu is the unconfined compressive strength of the rock.

Question 24

What is the recommended approach to designing piles in rock?

Answer 24

It is generally recommended to design piles in rock using only end-bearing resistance, avoiding shaft resistance, as the displacement required to mobilize full shaft resistance would already mobilize end-bearing resistance.

Question 25

Why should shaft resistance not be included in pile design in rock?

Answer 25

Shaft resistance should not be included because once a pile displaces enough to mobilize full shaft resistance, the end-bearing resistance is also fully mobilized, making shaft resistance unnecessary in the design.

Question 26

How do studies show the mobilization of shaft resistance and end-bearing resistance for piles in rock?

Answer 26

Studies show that for an equivalent settlement, a greater percentage of maximum shaft resistance is mobilized compared to end-bearing resistance, meaning less displacement is required to fully mobilize shaft resistance.

Question 27

What is negative skin friction (downdrag) in piled foundations?

Answer 27

Negative skin friction occurs when a pile is installed in a soil strata with a compressible layer above a bearing layer. New fill material is placed on top of the compressible layer, causing consolidation. The downward movement of the compressible layer reverses the frictional force on the pile, leading to additional forces that can potentially cause pile failure.

Question 28

How does negative skin friction affect the design of piles?

Answer 28

In design, negative skin friction is accounted for by assuming the entire consolidating layer contributes to downdrag. Shear stress is mobilized along the length of the pile corresponding to the consolidating layer, which can be factored as a temporary action force in addition to the applied load.

Question 29

What value of β is used to estimate the downdrag shear stress for normally consolidated clay?

Answer 29

For a normally consolidated clay, a value of β = 0.25 is considered a reasonable upper bound for the downdrag shear stress.

Question 30

What is the lateral capacity of piles?

Answer 30

The lateral capacity of piles is the ability of the pile to resist horizontal loading. Although vertical loading typically dominates the design of piled foundations, lateral loading may become a critical factor, particularly in applications like piled retaining walls.

Question 31

What are the two main categories of lateral loading on piles?

Answer 31

The two main categories of lateral loading are active loading, where loads are applied to the pile and the soil resists the movement, and passive loading, where the soil moves and the pile resists the movement of the soil.

Question 32

What factors influence the failure mechanism of a pile under lateral loading?

Answer 32

The failure mechanism of a pile under lateral loading depends on the pile's slenderness ratio (L/d), the pile's bending resistance (Mp), and the boundary conditions at the pile head.

Question 33

What is the typical approach for resisting lateral loading in piled foundations?

Answer 33

In most cases, piles are installed vertically to resist lateral loading. However, in some cases, piles may be installed at an angle to the vertical, which is considered a conservative approach compared to the lateral capacity offered by a vertical pile.

Question 34

What factors determine the failure mode of a laterally loaded pile?

Answer 34

The failure mode depends on the pile's length and whether or not it is restrained by a pile cap.

Question 35

How does a short pile with low slenderness ratio fail under lateral loading?

Answer 35

It fails by rotation as a rigid body near the pile tip, with two active pressure zones mobilized, one above and one below the point of rotation.

Question 36

How does a longer pile with a higher slenderness ratio fail under lateral loading?

Answer 36

It develops a plastic hinge at some mid-height, and there will be significant displacement of the pile and soil above the plastic hinge.

Question 37

What are the equations for calculating the horizontal load on a pile under lateral loading?

Answer 37

Horizontal load: H = PA,1 – PA,2 (Equation 6.13) Moment equilibrium: H(e + h) = PA,1(h – lA,1) + PA,2(l – lA,2) (Equation 6.14)

Question 38

What is the moment equilibrium equation for a pile with a plastic hinge under lateral loading?

Answer 38

H(e + h) = PA(h – lA) + Mp (Equation 6.15)

Question 39

How does the failure mode change for a pile restrained by a pile cap?

Answer 39

For short, strong piles, the pile translates horizontally. For short piles with one plastic hinge, the analysis remains the same as for a pile without a cap. For long piles with two plastic hinges, another moment term is added.

Question 40

What failure modes are seen for laterally loaded piles with a rigid pile cap?

Answer 40

1) No hinge (pile translates horizontally), 2) One hinge at the pile head, 3) Two hinges at the pile head and mid-height.

Question 41

What is the lateral pressure distribution on a pile in sands?

Answer 41

The lateral pressure is approximated by: p = Kpσ′v/2 (Equation 6.16), where Kp is the lateral pressure coefficient.

Question 42

What is the lateral force per unit length of a pile in sands?

Answer 42

The lateral force per unit length of the pile is given by: P = Kpσ′vd/2 (Equation 6.17)

Question 43

How is the lateral pressure distribution in clays approximated?

Answer 43

For z ≤ 3d: p = (2 + 7z/3d)cu, and for z > 3d: p = 9cu (Equation 6.19)

Question 44

What is the lateral force per unit length of a pile in clays?

Answer 44

For z ≤ 3d: P = (2 + 7z/3d)cud, and for z > 3d: P = 9cud (Equation 6.20)

Question 45

What is the primary design challenge in pile group design?

Answer 45

The efficiency of a pile group is generally less than the sum of individual pile capacities due to interference between the piles and the reinforcing effect on the soil.

Question 46

What is the efficiency of a pile group, ηg?

Answer 46

ηg = Qg / (nQp), where Qg is the load supported by the pile group, n is the number of piles, and Qp is the load supported by a single pile (Equation 6.21).

Question 47

How does the efficiency of a pile group compare for fine-grained soils in undrained conditions?

Answer 47

The efficiency of a pile group is generally close to 1 for fine-grained soils in undrained conditions.

Question 48

How do displacement piles affect the efficiency of a pile group?

Answer 48

Displacement piles tend to have efficiencies higher than 1 due to the compacting effect, which increases the shaft capacity of adjacent piles.

Question 49

What are the two failure modes of a pile group under ultimate limit state (ULS)?

Answer 49

1) Individual pile failure, 2) Block failure of the soil block enclosed by the piles.

Question 50

How is the group failure load calculated in a pile group design?

Answer 50

The capacity of the soil block in block failure is calculated using the methods outlined in Section 6.2 with α = 1 in undrained conditions and δ′ = φ′ in drained conditions.

Question 51

What is the formula for block failure load in undrained conditions?

Answer 51

Qblock = (2 × LpLb + 2 × LpBb)cu + BbLb × Ncsccu (Equation for block failure load)

Question 52

How does a pile group fail by individual pile failure?

Answer 52

The failure occurs if each pile can no longer support the load Qg/n, and a ULS check must be performed for each pile.

Question 53

What does the block failure mode involve?

Answer 53

The failure occurs when the soil block between the piles fails as a rigid body. The capacity of the block is treated as a pier with length Lp and area Ablock.

Question 54

How is the efficiency of a pile group impacted by the use of finite element methods in analysis?

Answer 54

Finite element methods allow for more evenly spread piles with a greater reinforcing effect, which can improve the efficiency of the pile group while maintaining safe design against bending/shear forces.

Question 55

What is the relationship between a raft foundation and a pile group?

Answer 55

A pile group can be used beneath a raft foundation to reduce settlement to an acceptable level, allowing for a smaller raft and reducing the overall foundation cost.

Question 56

How is the lateral pressure on a pile in sands approximated?

Answer 56

The pressure is given by: p = Kpσ′v/2, where Kp is the lateral pressure coefficient and σ′v is the vertical effective stress (Equation 6.16).

Question 57

What are the design charts for failure modes in sands and clays used for?

Answer 57

Design charts help determine the lateral and axial capacities of piles in both sands and clays for various failure modes, providing guidance for pile group design.

Question 58

What is the key assumption for the lateral pressure distribution on piles in clays?

Answer 58

The lateral pressure increases from 2cu at the surface to 9cu at a depth of 3 pile diameters, with a linear increase in between (Equation 6.19).

Question 59

How does the length of the pile impact the failure mode under lateral loading?

Answer 59

Short piles with low slenderness ratios fail by rigid body rotation near the pile tip, while longer piles develop plastic hinges along their length, leading to displacement.

Question 60

How is the group capacity of a pile group influenced by the individual pile capacities?

Answer 60

The group capacity is typically less than the sum of the individual pile capacities due to interference between the piles, which reduces efficiency.

Question 61

Why is it important to include the soil reinforcement effect in pile group design?

Answer 61

The soil reinforcement effect, caused by the piles compacting the soil, helps distribute the load and can improve the efficiency and load-bearing capacity of the pile group.

Question 62

How does the use of a pile cap affect the failure modes of laterally loaded piles?

Answer 62

A pile cap changes the failure mode, potentially allowing for horizontal translation of the pile or mobilizing one or two plastic hinges in longer piles.

Question 63

How is the moment equilibrium used to solve for the failure load of a laterally loaded pile?

Answer 63

Moment equilibrium involves balancing the applied loads with the resisting moments generated by the lateral pressure, with additional terms for the eccentricity and plastic hinge contributions.

Question 64

What is the purpose of performing a ULS check on a pile group?

Answer 64

A ULS check ensures that each pile in the group can support its portion of the load without failing, either by individual pile failure or block failure.

Question 65

How does lateral loading affect the capacity of a pile?

Answer 65

Lateral loading induces bending and shear stresses along the pile, and the pile's capacity depends on its ability to resist these forces, often influenced by the pile's slenderness ratio and boundary conditions.