Pile Foundations and Deep Foundation Systems

Introduction and Context

Pile foundations transfer structural loads through weak or compressible near-surface soils to deeper soil or rock capable of sustaining load. They are used when shallow foundations would experience excessive settlement, inadequate bearing capacity, unacceptable differential movement, or instability under uplift and lateral actions. Piles are also useful where construction loads must be carried through water, loose fills, expansive soils, or highly variable ground.

A pile is not merely a long column embedded in soil. Its behaviour depends on interaction among the pile material, installation process, surrounding soil, groundwater, loading rate, group geometry, and structural connection to the superstructure. Consequently, pile design requires both geotechnical and structural checks. The governing condition may be axial compression, tension, lateral resistance, settlement, structural strength, durability, vibration, or constructability.

This chapter addresses driven and bored piles, load-transfer mechanisms, pile groups, downdrag, lateral response, dynamic installation effects, load testing, and construction decision-making.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain the mechanisms by which piles transfer axial and lateral loads.

2. Estimate single-pile compression, tension, and group capacity using appropriate assumptions.

3. Distinguish the behaviour of driven and bored piles.

4. Evaluate negative skin friction and neutral-plane effects.

5. Interpret basic lateral pile response and dynamic installation observations.

6. Select and interpret static, dynamic, integrity, and lateral load tests.

7. Identify constructability, environmental, durability, and risk issues in deep-foundation projects.

1. Pile Types and Selection

Piles may be classified by material, installation method, geometry, or load-transfer mechanism.

Driven piles are installed by impact hammer, vibratory hammer, or hydraulic pressing. Common types include steel H-piles, closed-ended steel pipe piles, precast concrete piles, and timber piles. Installation displaces soil and may increase lateral stresses and, in suitable sands, improve local density. Driven piles provide useful installation records, but they can generate noise, vibration, heave, and damage risk.

Bored piles, also called drilled shafts when large diameter, are formed by excavating a hole and placing reinforcement and concrete. Temporary casing, drilling fluid, or an open-hole method may be used to maintain stability. Bored piles cause relatively little vibration, but their capacity is sensitive to excavation quality, groundwater control, base cleanliness, concrete placement, and possible softening or smearing of the shaft wall.

Other systems include continuous-flight-auger piles, CFA piles, micropiles, helical piles, suction piles, and barrettes. The selected system should reflect ground conditions, load demands, access, headroom, environmental constraints, quality assurance requirements, and available equipment.

A useful early distinction is between:

• End-bearing piles, in which a substantial part of the load reaches a strong stratum or rock through the base.
• Friction or shaft-resistance piles, in which load is transferred mainly through shear along the shaft.
• Combined piles, which mobilize both base resistance and shaft resistance.

The distinction is conceptual rather than absolute. Most piles develop both components.

2. Axial Load Transfer and Capacity

For a single pile under static axial compression, the ultimate geotechnical resistance may be represented as

\[

Q_u = Q_b + Q_s

\]

where \(Q_u\) is ultimate compressive resistance, \(Q_b\) is ultimate base resistance, and \(Q_s\) is ultimate shaft resistance. This expression assumes that base and shaft components can be defined consistently and that interaction effects are incorporated in the adopted design method.

The nominal design resistance is obtained by applying the relevant factor of safety or resistance factors. In a simple allowable-stress format,

\[

Q_{allow} = \frac{Q_u}{F}\n\]

where \(F\) is a factor of safety. Modern limit-state codes generally use separate partial factors for actions, soil resistance, and model uncertainty; the code method must govern over this simplified expression.

2.1 Shaft resistance in effective-stress analysis

For a pile segment in drained or long-term conditions, a widely used effective-stress relationship is

\[

\tau_s = \beta \sigma'_v

\]

and therefore

\[

Q_s = \sum_i \tau_{s,i} A_{s,i}

= \sum_i \beta_i \sigma'_{v,i} A_{s,i}

\]

where \(\tau_s\) is unit shaft resistance, \(\beta\) is an empirical coefficient, \(\sigma'_v\) is effective vertical stress at the representative depth, and \(A_s\) is pile surface area within a soil layer. The coefficient reflects soil type, pile roughness, installation method, stress history, and interface behaviour. It must not be transferred indiscriminately between driven and bored piles.

For a uniform circular pile of diameter \(D\) and embedded length \(L\), the surface area is \(\pi DL\). For layered soil, the shaft should be divided into intervals so that changes in effective stress and soil properties are represented.

In undrained clay, a total-stress method is commonly expressed as

\[

\tau_s = \alpha s_u

\]

where \(s_u\) is undrained shear strength and \(\alpha\) is an adhesion factor. Values of \(\alpha\) depend strongly on the ratio of pile-soil strength, stress level, pile type, and installation disturbance. The method is generally intended for short-term undrained loading and should not be used as a substitute for an appropriate long-term effective-stress analysis.

2.2 Base resistance

A general bearing-capacity form for a pile base is

\[

Q_b = A_b q_b

\]

where \(A_b\) is the pile base area and \(q_b\) is unit base resistance. In an undrained clay analysis, a simplified expression may be written

\[

q_b = N_c s_u

\]

where \(N_c\) is an appropriate deep-foundation bearing factor, often taken near 9 for idealized, deep, circular conditions. The value is not universal: pile geometry, soil layering, plugging, base roughness, embedment, and construction effects may alter the result.

For a circular pile, \(A_b = \pi D^2/4\). In sand, base resistance is usually related to effective stress and friction angle, but correlations are sensitive to density, stress level, compressibility, and the possibility of crushing or punching into a weaker layer. High calculated capacities in dense sand should therefore be checked against settlement and pile structural resistance.

2.3 Structural and geotechnical limit states

The pile must be checked as a structural member for compression, tension, bending, shear, buckling where relevant, fatigue, corrosion, concrete cracking, splice strength, and connection capacity. The allowable or factored pile load is governed by the lesser of geotechnical resistance and structural resistance, with code-specific treatment of factors.

Serviceability is equally important. A pile may possess adequate ultimate resistance but produce unacceptable settlement because shaft resistance is mobilized progressively and base resistance may require relatively large displacement. Differential settlement in a pile group may control the design even when total group capacity appears satisfactory.

3. Driven and Bored Piles: Behavioural Differences

Driving changes the soil state. In normally consolidated or loose sands, displacement may increase horizontal stress and density, potentially increasing shaft resistance. In sensitive or saturated clays, driving can generate excess pore-water pressure and temporarily reduce effective stress. Capacity may increase later through set-up as pore pressures dissipate and the soil reconsolidates. Conversely, relaxation may occur in some dense sands or weakly cemented materials.

Bored-pile construction removes soil and may reduce lateral confinement near the shaft. Slurry, casing, groundwater inflow, overbreak, sediment at the base, and inadequate concrete continuity can reduce performance. In a drilled shaft, the base resistance is particularly dependent on a clean, competent base and satisfactory concrete placement. Inspection and construction records are therefore central to design reliability.

The same nominal geometry and soil profile do not imply the same capacity for a driven and bored pile. Installation method is a primary design variable, not a secondary construction detail.

4. Pile Groups and Interaction

Piles are commonly connected by a pile cap. A group does not behave as a collection of independent piles because stress zones overlap and the cap may transfer load directly to the soil. Group analysis should consider:

• pile spacing and arrangement;
• pile stiffness and relative stiffness of the cap;
• load eccentricity and cap contact with soil;
• installation sequence;
• overlapping shaft and base stress zones;
• group settlement and consolidation;
• load redistribution among piles; and
• lateral and seismic interaction.

For a preliminary compression check, the sum of individual capacities is

\[

Q_{sum} = n Q_{single}

\]

where \(n\) is the number of piles. If group interaction reduces resistance, a group efficiency factor \(\eta_g\) may be used only when supported by an appropriate method:

\[

Q_{g,shaft} = \eta_g n Q_{single,shaft}

\]

This is not a universal law; \(\eta_g\) depends on soil, spacing, pile type, failure mechanism, and loading. In cohesive soils, an alternative check treats the group as a block, with perimeter shear resistance and base resistance beneath the overall group envelope. The lower credible resistance from individual-pile and block mechanisms is commonly considered in design.

A rigid pile cap distributes vertical load approximately according to pile stiffness and position. Under a centered vertical load, idealized equal sharing gives \(P_i = P/n\). Under a moment, outer piles may experience increased compression while opposite piles may unload or enter tension. In practice, cap flexibility, pile-soil nonlinearity, and construction tolerances make exact equal sharing unlikely.

5. Downdrag and Negative Skin Friction

If surrounding soil settles relative to a pile, shear is mobilized downward along the shaft. This negative skin friction, or downdrag, adds load to the pile and does not constitute useful resistance. Causes include consolidation of soft clay, placement of new fill, lowering of groundwater, and densification or settlement of loose deposits.

The depth at which pile and soil have equal settlement is the neutral plane. Above it, the soil may drag the pile downward; below it, the pile transfers structural load to the soil through positive resistance. The maximum compressive force in the pile may therefore occur near the neutral plane rather than at the pile head.

Design must distinguish:

1. structural load from the superstructure;

2. dragload transferred from settling soil; and

3. available positive resistance below the neutral plane.

Mitigation can include preloading or surcharge, sleeving, bitumen or low-friction coatings, compressible coatings, delaying pile installation until fill-induced settlement occurs, and selecting a pile terminating in a competent stratum. Coatings require durability and constructability verification; they should not be assumed effective without evidence.

6. Lateral Pile Response

Lateral pile behaviour is governed by pile flexural stiffness, soil reaction, embedment, head restraint, group effects, load eccentricity, cyclic loading, and stratigraphy. A laterally loaded pile transfers load through soil reaction along its embedded length and through pile-head shear and moment.

For small deflections, an elastic beam-on-foundation model may be used:

\[

EI \frac{d^4 y}{dz^4} + p(y,z) = 0

\]

where \(E I\) is pile flexural rigidity, \(y\) is lateral deflection, \(z\) is depth, and \(p\) is soil reaction per unit length. If the soil is idealized as linear, \(p = k_h y\), where \(k_h\) is a subgrade modulus. In real soils, \(p\)-\(y\) relationships are nonlinear, depth-dependent, and often limited by ultimate soil resistance. Thus, a constant \(k_h\) is a modelling approximation, not a fundamental soil property.

Analysis methods include Broms-type limit-equilibrium solutions, elastic subgrade models, nonlinear \(p\)-\(y\) curves, finite-element analysis, and physical load testing. Broms solutions are useful for preliminary estimates but involve idealized soil profiles and failure mechanisms. Nonlinear methods are preferable for important structures, cyclic loading, layered ground, or large deformation.

Pile groups experience shadowing: the leading pile may attract different soil reaction from trailing piles. Group reduction factors are influenced by spacing, direction of loading, cap restraint, and cyclic effects. Lateral pile design must also check pile-head rotation, serviceability deflection, structural bending, shear, connection forces, scour, and possible kinematic seismic demand.

7. Dynamic Installation Effects and Wave Analysis

Impact driving generates stress waves in the pile. A simplified one-dimensional wave relationship is based on pile impedance,

\[

Z = \frac{EA}{c}

\]

where \(Z\) is impedance, \(E

enote{\(E\)} is pile modulus, \(A\) is cross-sectional area, and \(c\) is wave speed in the pile. For an elastic homogeneous pile, \(c = \sqrt{E/\rho}\), where \(\rho\) is mass density. Real piles have splices, variable sections, soil interaction, damping, and nonlinear impacts, so field interpretation requires calibrated wave-equation analysis.

Dynamic monitoring may measure force and velocity near the pile head. Signal matching or wave-equation methods can estimate transferred energy, stresses, and mobilized resistance. Dynamic capacity is not automatically equal to static capacity. It depends on damping, quake, soil type, setup or relaxation, hammer performance, and the validity of the analysis model.

Driving criteria should control both capacity and structural integrity. Excessive tensile or compressive stresses can crack concrete or buckle steel; refusal may indicate a hard layer, obstruction, plugging, or damage rather than successful capacity. Driving records should include blow count, penetration, hammer energy, cushion condition, pile set, verticality, and unusual events.

8. Load Testing and Quality Assurance

8.1 Static axial tests

A maintained-load compression test applies load in increments and measures pile-head movement. The test can evaluate load-settlement behaviour, construction quality, and design assumptions. Interpretation of ultimate resistance should follow the governing standard because different criteria define failure differently.

Tension tests assess shaft resistance and pile-head connection. They do not directly validate base compression resistance. If uplift is cyclic, the test programme should reproduce representative loading where practicable.

8.2 Static lateral tests

Lateral tests measure horizontal load, displacement, rotation, and sometimes bending-moment distribution. Reaction piles, frames, or deadmen must be designed so that the reaction system does not control the result. Testing should distinguish pile response from movement of the reaction apparatus.

8.3 Dynamic and integrity tests

High-strain dynamic testing can assess driving stresses and mobilized resistance. Low-strain integrity testing can identify major changes in impedance associated with necking, inclusions, voids, or discontinuities, but it cannot prove full structural adequacy or geotechnical capacity. Crosshole sonic logging is useful for large concrete shafts with access tubes and can identify anomalies between tubes.

Testing is most valuable when planned before construction. A test pile should represent the production system, soil variability, installation sequence, curing period, and loading condition. Test results should be integrated with geological interpretation rather than treated as isolated numbers.

9. Constructability, Durability, and Governance

A technically adequate design may fail if it cannot be built consistently. Constructability review should address working platforms, access, casing or slurry management, reinforcement cages, concrete supply, tremie continuity, pile cut-off, spoil disposal, obstructions, tolerances, adjacent structures, and emergency procedures.

Driven-pile projects require assessment of vibration, noise, ground heave, and effects on nearby utilities. Bored-pile projects require control of bore stability, slurry properties, base cleanliness, cage flotation, concrete segregation, and pile continuity. Micropiles may solve restricted-access problems but involve many grout-quality and load-transfer uncertainties.

Durability design considers groundwater chemistry, sulfate and chloride attack, corrosion allowance, abrasion, freeze-thaw exposure, concrete quality, steel protection, and design life. Quality records should be traceable from materials and drilling logs to concrete placement, testing, and as-built survey.

Ethically, engineers must not conceal refusal, poor concrete placement, excessive deviation, failed tests, or unexplained changes in ground conditions. A defensible design documents uncertainty, uses appropriate factors, and specifies hold points for decisions that affect public safety. Regulatory requirements for noise, vibration, spoil handling, groundwater discharge, worker safety, and contaminated soil must be incorporated into the method statement.

10. Worked Example: Preliminary Axial Capacity of a Bored Pile

Consider a circular bored pile of diameter \(D=0.60\,m\) and length \(L=15\,m\) in uniform undrained clay with \(s_u=75\,kPa\). For a preliminary illustration, take \(\alpha=0.55\) and \(N_c=9\). These assumed parameters are illustrative and would require site-specific justification.

Pile shaft area:

\[

A_s = \pi DL = \pi(0.60)(15)=28.27\,m^2

\]

Unit shaft resistance:

\[

\tau_s=\alpha s_u=0.55(75)=41.25\,kPa

\]

Ultimate shaft resistance:

\[

Q_s=41.25(28.27)=1166\,kN

\]

Base area:

\[

A_b=\frac{\pi D^2}{4}=\frac{\pi(0.60)^2}{4}=0.2827\,m^2

\]

Unit base resistance:

\[

q_b=N_c s_u=9(75)=675\,kPa

\]

Ultimate base resistance:

\[

Q_b=675(0.2827)=190.8\,kN

\]

Thus,

\[

Q_u=Q_s+Q_b=1166+190.8=1356.8\,kN

\]

Using an illustrative global factor of safety of 2.5,

\[

Q_{allow}=\frac{1356.8}{2.5}=543\,kN

\]

This is not a final design value. The calculation ignores layering, groundwater variation, construction disturbance, settlement, structural resistance, downdrag, group effects, and code-specific resistance factors. It demonstrates the calculation sequence and the relative contribution of shaft and base resistance.

11. Illustrative Case: Pile Foundation for an Urban Bridge Pier

Background. An urban bridge pier is founded in 8 m of recent fill and soft clay over dense sand. The pier carries large compression, moderate uplift from overturning, and substantial transverse lateral load. A hospital is located 35 m from the pier.

Problem. Driven piles would provide useful installation control and may densify the sand, but vibration and noise could disturb the hospital. Bored piles would reduce vibration, but the fill and groundwater create a risk of unstable excavation and contaminated spoil.

Analysis. The design team compares steel pipe piles, CFA piles, and bored reinforced-concrete piles. Geotechnical investigations include cone penetration tests, laboratory strength testing, groundwater monitoring, and seismic assessment. Axial capacity is evaluated using both shaft and base mechanisms. Lateral response is assessed with nonlinear soil-reaction curves, and group effects are included. Potential settlement of the fill is analysed for downdrag.

Approach. The selected solution is a cased bored-pile group terminating in dense sand. Temporary casing is extended through the fill and soft upper soils. A sacrificial working platform and spoil-management plan are specified. A preliminary test pile is subjected to compression, tension, and lateral tests after representative curing and installation conditions. Crosshole sonic logging and concrete volume records are required for production piles.

Outcome. Testing confirms the predicted axial resistance but identifies greater lateral deflection than anticipated because the upper fill provides limited reliable restraint. The pile cap is enlarged, pile-head fixity is improved, and the lateral model is recalibrated. Downdrag is included in the structural load envelope, and construction monitoring confirms acceptable vibration at the hospital.

Learning points. The case demonstrates that pile selection is a system decision. Low vibration alone does not guarantee performance; excavation stability, concrete quality, lateral stiffness, testing, and long-term settlement must be considered together. A test programme can improve design decisions before production construction is complete.

12. Contemporary Developments and Critical Limitations

Current practice increasingly combines in-situ testing, automated drilling records, digital pile logs, instrumented hammers, three-dimensional numerical models, and probabilistic reliability analysis. Machine-learning tools can classify drilling signatures or detect anomalous concrete-placement records, but they require representative, quality-controlled data and engineering review. AI should support—not replace—geotechnical interpretation, responsibility, and code compliance.

Sustainability considerations include embodied carbon in concrete and steel, reuse of existing piles, optimized pile numbers, low-carbon binders, reduced spoil, and construction-energy assessment. However, reducing pile quantity without accounting for uncertainty, durability, testing, and consequences of failure is not sustainable engineering.

Important limitations remain. Empirical capacity correlations may be site-specific. Static tests sample only limited locations and loading histories. Dynamic methods require calibration. Lateral models are particularly sensitive to soil stiffness assumptions and cyclic degradation. Numerical sophistication does not remove uncertainty in stratigraphy or construction quality. Robust design therefore combines sound ground investigation, conservative mechanisms, observational data, testing, and documented decision rules.

Best-Practice Design Workflow

1. Define structural actions, performance criteria, design life, and consequences of failure.

2. Develop a ground model including stratigraphy, groundwater, variability, and construction hazards.

3. Select candidate pile systems based on both geotechnical performance and constructability.

4. Evaluate axial compression, tension, settlement, downdrag, lateral response, seismic effects, and structural resistance.

5. Analyse group interaction, cap stiffness, load redistribution, and tolerances.

6. Specify installation controls, inspection, testing, acceptance criteria, and contingency measures.

7. Use representative preliminary tests to calibrate design assumptions.

8. Monitor production installation and update the model when field evidence differs from predictions.

9. Record as-built geometry, materials, defects, repairs, and departures from design.

10. Review long-term durability, maintenance, monitoring, and residual risks.

Chapter Summary

Pile foundations transfer load through shaft resistance, base resistance, or both. Driven and bored piles behave differently because installation changes stress state, pore pressure, confinement, and construction quality. Axial design must consider ultimate resistance, settlement, structural capacity, downdrag, tension, and group interaction. Pile groups may experience overlapping stress zones, block failure, load redistribution, and lateral shadowing.

Negative skin friction develops when surrounding soil settles relative to a pile and can increase the maximum compressive force near the neutral plane. Lateral behaviour depends on pile stiffness, soil reaction, head restraint, group effects, and cyclic loading; nonlinear soil-reaction models are often needed for important projects. Dynamic driving records and wave analysis provide valuable information but require calibration to static behaviour.

Static axial and lateral tests, dynamic monitoring, and integrity testing serve different purposes and must not be treated as interchangeable. Successful deep-foundation engineering integrates ground investigation, analytical models, testing, construction control, durability, environmental management, and ethical reporting. The most reliable design is one that remains safe when soil variability and construction imperfections are explicitly considered.

Key Terms

Pile: A deep foundation element that transfers load to soil or rock.

Driven pile: A pile installed by impact, vibration, or pressing.

Bored pile: A cast-in-place pile formed by drilling or excavating a hole.

Shaft resistance: Shear resistance mobilized along the pile-soil interface.

Base resistance: Soil or rock resistance mobilized beneath the pile base.

End-bearing pile: A pile deriving a major component of resistance at its base.

Pile group: Several piles connected by a common pile cap.

Group efficiency: The ratio describing group resistance relative to the sum of isolated-pile resistances.

Downdrag: Downward shear transferred to a pile by settling surrounding soil.

Negative skin friction: Shaft shear acting downward on a pile.

Neutral plane: Depth at which pile and soil settlements are approximately equal.

Pile set-up: Increase in pile capacity after installation, often due to pore-pressure dissipation and reconsolidation.

Pile relaxation: Reduction in resistance after installation in some soils.

Pile impedance: Dynamic pile property relating force and wave velocity.

\(p\)-\(y\) curve: Relationship between soil reaction and lateral pile deflection.

Pile cap: Structural member connecting a group of piles.

Static load test: Controlled loading test measuring pile displacement and resistance.

Integrity test: Test intended to identify possible defects or discontinuities in a pile.

Tremie concrete: Concrete placed underwater or beneath drilling fluid through a pipe.

Constructability: Degree to which a design can be built safely, reliably, and economically.

Review Questions

Short Answer Questions

1. Distinguish shaft resistance from base resistance.

2. Why may driven piles gain capacity after installation in saturated clay?

3. Define the neutral plane in a pile subjected to downdrag.

4. State two limitations of low-strain pile-integrity testing.

5. Why does a pile group not necessarily have capacity equal to the sum of isolated-pile capacities?

Long Answer Questions

1. Compare driven and bored piles with respect to soil disturbance, capacity, quality control, environmental effects, and constructability.

2. Explain the calculation of axial pile capacity using shaft and base components, including the assumptions of effective-stress and undrained methods.

3. Discuss the mechanisms producing negative skin friction and methods for reducing downdrag.

4. Explain the main factors governing lateral pile response and compare preliminary and nonlinear analytical methods.

5. Describe a comprehensive pile-testing and quality-assurance programme for a major bridge foundation.

Analytical / Application Questions

1. A proposed pile passes through recently placed fill over soft clay and terminates in dense sand. Develop a design checklist addressing axial capacity, downdrag, settlement, group effects, testing, and construction monitoring.

2. A driven-pile project near a hospital records high vibration and unusually high driving stresses before reaching the specified penetration. Analyse possible causes and recommend an evidence-based decision process.

3. A bored-pile load test confirms axial resistance but records excessive lateral displacement. Explain how the design, pile cap, ground model, and construction method should be reviewed and modified.

Multiple Choice Questions with Answers

Question 1

Which expression represents the basic decomposition of ultimate axial compression resistance of a single pile?

A. Qu = Qb + Qs

B. Qu = Qb − Qs

C. Qu = QbQs

D. Qu = Qb/Qs

Correct Answer: A. Qu = Qb + Qs

Question 2

Negative skin friction develops when:

A. The pile moves downward faster than the soil

B. The surrounding soil settles relative to the pile

C. The pile base reaches rock

D. The groundwater table rises around a rigid pile

Correct Answer: B. The surrounding soil settles relative to the pile

Question 3

In the illustrative worked example, the calculated ultimate pile resistance is approximately:

A. 191 kN

B. 543 kN

C. 1,167 kN

D. 1,357 kN

Correct Answer: D. 1,357 kN

Question 4

Which installation method generally produces greater vibration and noise?

A. Driven piling

B. Bored piling with casing

C. Continuous-flight-auger piling

D. Hand-installed micropiling

Correct Answer: A. Driven piling

Question 5

The neutral plane is the depth at which:

A. Base resistance becomes zero

B. Pile and soil settlements are approximately equal

C. Groundwater pressure is maximum

D. The pile changes from steel to concrete

Correct Answer: B. Pile and soil settlements are approximately equal

Question 6

In a lateral pile model, a p-y curve relates:

A. Pile load to concrete strength

B. Pile diameter to embedment

C. Soil reaction to lateral pile deflection

D. Driving energy to blow count only

Correct Answer: C. Soil reaction to lateral pile deflection

Question 7

Which test is primarily intended to identify possible discontinuities in a concrete pile?

A. Low-strain integrity test

B. Compression load test

C. Tension load test

D. Plate-load test

Correct Answer: A. Low-strain integrity test

Question 8

For a circular pile of diameter D, the base area is:

A. πD

B. 2πD

C. πD²

D. πD²/4

Correct Answer: D. πD²/4

Question 9

A major concern specific to bored-pile construction is:

A. Hammer cushion wear

B. Excavation instability and base cleanliness

C. Pile-driving refusal

D. Impact-wave tensile stress only

Correct Answer: B. Excavation instability and base cleanliness

Question 10

Dynamic pile capacity interpretation should be calibrated because it depends on:

A. Only pile diameter

B. Only concrete age

C. Damping, hammer performance, soil response, and setup or relaxation

D. Pile-cap thickness alone

Correct Answer: C. Damping, hammer performance, soil response, and setup or relaxation