Deep Foundations: Driven and Bored Piles and Capacity Mechanisms

Piles, groups and load transfer

Deep foundations transfer loads through soil or rock along a distributed shaft and at an embedded tip. This changes not only the location of force transfer but also the observations needed to justify a design. Pile type, installation method and load direction all matter. Driven piles can alter local density and pore pressure; drilled shafts can be sensitive to wall stability, base cleanliness and concrete placement. A resistance equation without a method statement is incomplete because the construction process creates much of the as-built foundation behavior.

The simple capacity decomposition into shaft plus toe is valuable for teaching. It clarifies the influence of diameter, length and unit interface resistance and provides a transparent sensitivity study. It becomes misleading if the designer assumes that the peak shaft and toe resistances are mobilized at precisely the same head displacement. Load-transfer models and pile tests reveal the coupling between displacement and resistance. Negative skin friction from settling ground introduces another mechanism: the soil may move down relative to the pile and apply drag instead of useful upward resistance in that interval.

Pile groups behave as systems. Caps redistribute loads according to stiffness, eccentricity, cap flexibility and boundary conditions. Closely spaced piles can interact through overlapping stress zones; the group may settle more than an isolated pile under comparable average load. An arbitrary efficiency factor may teach parameter sensitivity but cannot replace group analysis, block stability or field tests. In lateral loading, the same interaction can reduce ground resistance for downstream piles, while structural pile bending and pile-cap demands must remain within limits.

Verification is not an optional final exercise. Test programs should be planned before production, with criteria chosen for the specific failure mechanism and type of foundation. Integrity information, static proof tests, dynamic monitoring and construction logs answer overlapping but nonidentical questions. A persuasive design record shows why the tests are representative and what the team will do if observations depart from the assumptions.

Case integration: pile termination in variable sand. A bridge pier is proposed above soft clay over dense sand, and preliminary pile lengths are based on a single interpreted sand-top elevation. Nearby logs show that the sand horizon fluctuates and one sounding encountered a hard gravel layer that could be mistaken for rock during drilling. A pile driven to refusal might stop prematurely; a bored pile might penetrate deeper but require rigorous base cleaning and side support. Prepare competing termination criteria and explain how they will be verified. For an axial model, plot shaft and toe contributions with changing length, and then evaluate a group under eccentric cap loading. Do not equate the total of nominal shaft and toe maxima with acceptable service movement. A construction quality plan needs pile identification, installation sequence, geotechnical logs, concrete or driving records, and interpretation of representative proof tests. Specify what happens if a production element records abnormal depth, unexpected concrete volume or low test response. The as-built pile system should be compared with the geological model that justified the initial design, and substantial discrepancies should trigger engineered reassessment rather than automatic acceptance.

Four lessons and their applications

Lesson 4.1 — Pile mechanisms and selection

Why this mechanism matters

A pile may mobilize shaft resistance, base resistance, structural capacity, and lateral restraint in varying proportions. Driven and bored construction can alter surrounding soil differently, so capacities inferred from a single installation method cannot be transferred without qualification. Negative skin friction from consolidating surrounding ground adds drag load and changes neutral-plane behavior. Installation tolerances, integrity, noise and spoil management affect choice.

Evidence, scales and the engineering model

The choice of model changes the interpretation of pile mechanisms and selection: a single observation establishes a condition at a place and time, while an engineering prediction requires a link from that observation to a repeatable physical mechanism. Establish the relevant load or environmental scenario, then identify whether the response is governed by strength, deformation, pore pressure, geometry, or construction sequence. The question to ask before entering numerical values is not simply which equation applies? but which field evidence could make the equation inapplicable? The answer often determines whether the apparent precision of an output is justified.

Resistance mobilization. Maximum resistance depends on compatibility of movement and mechanism. Different components of an embedded foundation may require different displacements to mobilize. For this lesson, make the assumed boundary conditions visible in a section drawing and distinguish measured inputs from correlations. A parameter that is merely convenient should not be presented as if a project investigation had confirmed it.

A useful field-to-model workflow is to identify an observed quantity, the geological or structural mechanism, the chosen constitutive assumption, the calculation, and a practical verification measurement. If the observation cannot distinguish two mechanisms, carry both possibilities into the sensitivity analysis. The reported result should preserve units, stress convention, drainage condition, stage and uncertainty bounds. It must be possible to trace a final design recommendation back to the underlying evidence.

Worked case 13 — method and interpretation

Given. A 0.60 m diameter pile is 18 m deep with uniform illustrative shaft resistance fs=55 kPa and toe resistance qb=1,500 kPa.

Solution. Qs=π×0.6×18×55=1,866 kN; Qb=π×0.6²/4×1500=424 kN; nominal sum≈2,290 kN.

Interpretation. The shaft and toe components may need different movements to mobilize; do not treat the sum as a working load.

The numerical answer above uses deliberately narrow assumptions to make the dependency transparent. If any assumption is relaxed, the expression may need a different correction or an altogether different mechanical model. Before applying the result to another footprint, soil layer or construction state, ask whether the test procedure and the selected parameters represent the new condition. Check dimensional consistency and confirm that the direction of a predicted change agrees with the underlying physics.

Deeper technical application

Diagnostic study — pile resistance and construction change. Shaft resistance develops because relative displacement mobilizes soil-pile interface shear; base resistance develops under the pile toe with a different displacement requirement. A drilled pile may obtain a different shaft interface from a driven pile in the same nominal soil. The simple perimeter-times-length calculation assumes that unit resistance is uniform over the included length, while an actual pile passes through strata with different stress conditions and installation effects. Toe resistance also depends on base cleaning, ground integrity and deformation. The shaft-plus-toe formula is a bookkeeping tool for a selected failure mechanism, not evidence that two peak values mobilize together.

Decision exercise. Take the 0.6 m diameter pile at 18 m embedment and separate the 1,866 kN shaft estimate from the 424 kN toe estimate. Divide the shaft into six 3 m segments. If the upper 6 m are soft clay with only 20 kPa mobilized unit shaft resistance, replace the original uniform estimate there and recompute the total before considering the remaining segments. Show how the toe contribution stays mathematically constant under the assumed geometry even though the real bearing layer might change. Plot Qs with length and identify the kinks caused by changing strata. Finally, describe which construction records and load test observations would test the assumed interface values. A high computed ultimate resistance does not establish a safe service settlement or adequate pile structural capacity.

Counterfactual test

In this lesson, begin by writing the input assumption in a calculation sheet and introduce one perturbation at a time. Consider a 20% increase in the most uncertain load, a plausible change in groundwater, and a different depth or stiffness of a critical layer. Do not automatically multiply every output by the same factor: soil response is often nonlinear and the mechanism itself can change. Record which sensitivity is purely numerical and which requires more field data. If the decision to use shallow versus deep foundations changes, that is evidence that additional investigation may have a higher value than further decimal places.

Field practice and failure-first review

Practical action. Separate geotechnical and structural checks, and classify compression, tension and lateral actions. Prioritize direct observations whenever a correlation could conceal a critical stratum or groundwater path. Document who reviews anomalies, whether repeat tests are required, and which construction stage is exposed. Failure trap: Pile labels such as end-bearing and friction pile describe tendencies, not exclusive behavior.

Make a brief alternative hypothesis for the same observations. Ask whether a weak pocket, high water level, unrecognized load eccentricity, or installation defect might explain an unexpectedly large deformation. The right response is not always more conservative material strength; sometimes it is a different foundation system, a modified construction sequence, or a targeted test that resolves the uncertainty. Protect neighboring structures and utilities when the mechanism can propagate outside the footprint.

Study questions

1. Explain the difference between a measured site observation and an inferred parameter in the context of pile mechanisms and selection. Which information would you insist on before approving a preliminary calculation?
2. Rework the example after increasing the most influential input by 20%. Show units and explain whether a linear response is physically justified.
3. Sketch resistance and displacement, separating load-transfer components.
4. Give one mode of failure or serviceability loss that the worked equation cannot detect, and propose a corresponding observation or laboratory test.
5. With the two unit laboratories, Differential Settlement and Tilt and Pile Shaft and Toe Resistance, describe one experiment that would reveal the limitations of an oversimplified model.

Lesson 4.2 — Axial resistance and settlement

Why this mechanism matters

Simple shaft resistance may be estimated by integrating unit resistance along pile circumference, while toe resistance uses area times unit toe resistance. These values do not necessarily mobilize at the same displacement. Pile load-settlement response can be nonlinear, and long-term drag effects can exceed initial intuition. Design uses appropriate factors or resistance procedures from a governing standard and local experience.

Evidence, scales and the engineering model

The choice of model changes the interpretation of axial resistance and settlement: a single observation establishes a condition at a place and time, while an engineering prediction requires a link from that observation to a repeatable physical mechanism. Establish the relevant load or environmental scenario, then identify whether the response is governed by strength, deformation, pore pressure, geometry, or construction sequence. The question to ask before entering numerical values is not simply which equation applies? but which field evidence could make the equation inapplicable? The answer often determines whether the apparent precision of an output is justified.

Testing and response. Define how field observations will validate model assumptions and what a failing test will trigger. Inspection without decision thresholds is documentation, not control. For this lesson, make the assumed boundary conditions visible in a section drawing and distinguish measured inputs from correlations. A parameter that is merely convenient should not be presented as if a project investigation had confirmed it.

A useful field-to-model workflow is to identify an observed quantity, the geological or structural mechanism, the chosen constitutive assumption, the calculation, and a practical verification measurement. If the observation cannot distinguish two mechanisms, carry both possibilities into the sensitivity analysis. The reported result should preserve units, stress convention, drainage condition, stage and uncertainty bounds. It must be possible to trace a final design recommendation back to the underlying evidence.

Worked case 14 — method and interpretation

Given. Nine identical piles each have an illustrative nominal 800 kN resistance and the assumed group efficiency is η=0.80.

Solution. Group nominal=9×800×0.80=5,760 kN, rather than the unmodified 7,200 kN.

Interpretation. Neither the assumed efficiency nor the summation verifies block failure, lateral effects or group settlement.

The numerical answer above uses deliberately narrow assumptions to make the dependency transparent. If any assumption is relaxed, the expression may need a different correction or an altogether different mechanical model. Before applying the result to another footprint, soil layer or construction state, ask whether the test procedure and the selected parameters represent the new condition. Check dimensional consistency and confirm that the direction of a predicted change agrees with the underlying physics.

Deeper technical application

Diagnostic study — pile group interaction. A group of piles is not simply several independent resistances placed beneath one cap. Their stress bulbs overlap and their movements may interact through intervening soil. A rigid cap distributes axial loads according to its force and moment requirements; piles along the heavily loaded side may receive higher forces. Spacing, diameter, soil stiffness and pile installation order affect whether group settlement or block failure becomes important. The illustrative coefficient η compresses these mechanisms into one uncertain number, which is useful for sensitivity but not a substitute for the governing theory. It also has no reason to be constant as loading changes or as piles enter different strata.

Decision exercise. Compare nine piles each assigned 800 kN with η=0.80 and η=0.65. The nominal group totals become 5,760 and 4,680 kN respectively, a 1,080 kN spread generated entirely by the efficiency assumption. If total vertical action is 4,500 kN, the second case leaves a much smaller apparent nominal margin even before factors and group settlement. Ask which additional observations could constrain group interaction: pile spacing, toe levels, load transfer tests, cap geometry and site modulus data. Sketch a four-corner cap under axial force plus moment, allocate pile head forces using a consistent structural model, and state when load reversal or uplift must be checked. Report group resistance, group movement and pile-cap design as distinct tasks.

Counterfactual test

In this lesson, begin by writing the input assumption in a calculation sheet and introduce one perturbation at a time. Consider a 20% increase in the most uncertain load, a plausible change in groundwater, and a different depth or stiffness of a critical layer. Do not automatically multiply every output by the same factor: soil response is often nonlinear and the mechanism itself can change. Record which sensitivity is purely numerical and which requires more field data. If the decision to use shallow versus deep foundations changes, that is evidence that additional investigation may have a higher value than further decimal places.

Field practice and failure-first review

Practical action. Build a resistance-versus-depth diagram and a deformation-compatible load transfer picture. Prioritize direct observations whenever a correlation could conceal a critical stratum or groundwater path. Document who reviews anomalies, whether repeat tests are required, and which construction stage is exposed. Failure trap: Adding independent maximum shaft and toe resistances can overstate working behavior.

Make a brief alternative hypothesis for the same observations. Ask whether a weak pocket, high water level, unrecognized load eccentricity, or installation defect might explain an unexpectedly large deformation. The right response is not always more conservative material strength; sometimes it is a different foundation system, a modified construction sequence, or a targeted test that resolves the uncertainty. Protect neighboring structures and utilities when the mechanism can propagate outside the footprint.

Study questions

1. Explain the difference between a measured site observation and an inferred parameter in the context of axial resistance and settlement. Which information would you insist on before approving a preliminary calculation?
2. Rework the example after increasing the most influential input by 20%. Show units and explain whether a linear response is physically justified.
3. Write a hold point and a concrete action for a field result outside the assumed range.
4. Give one mode of failure or serviceability loss that the worked equation cannot detect, and propose a corresponding observation or laboratory test.
5. With the two unit laboratories, Differential Settlement and Tilt and Pile Shaft and Toe Resistance, describe one experiment that would reveal the limitations of an oversimplified model.

Lesson 4.3 — Pile groups and interaction

Why this mechanism matters

Closely spaced piles influence overlapping soil zones. A group may have a block failure mechanism or enhanced total settlement even when individual pile capacities appear adequate. Distribution of cap loads depends on pile stiffness, cap rigidity and moment. Pile alignment, construction damage, ground improvement and load reversal require explicit attention.

Evidence, scales and the engineering model

The choice of model changes the interpretation of pile groups and interaction: a single observation establishes a condition at a place and time, while an engineering prediction requires a link from that observation to a repeatable physical mechanism. Establish the relevant load or environmental scenario, then identify whether the response is governed by strength, deformation, pore pressure, geometry, or construction sequence. The question to ask before entering numerical values is not simply which equation applies? but which field evidence could make the equation inapplicable? The answer often determines whether the apparent precision of an output is justified.

Mechanism of improvement. Treat water, soil fabric and load paths as separate levers. A technique that accelerates drainage may not reduce ultimate compression. For this lesson, make the assumed boundary conditions visible in a section drawing and distinguish measured inputs from correlations. A parameter that is merely convenient should not be presented as if a project investigation had confirmed it.

A useful field-to-model workflow is to identify an observed quantity, the geological or structural mechanism, the chosen constitutive assumption, the calculation, and a practical verification measurement. If the observation cannot distinguish two mechanisms, carry both possibilities into the sensitivity analysis. The reported result should preserve units, stress convention, drainage condition, stage and uncertainty bounds. It must be possible to trace a final design recommendation back to the underlying evidence.

Worked case 15 — method and interpretation

Given. A soil layer consolidates downward relative to a pile and is assumed to impose 15 kPa average downward drag along a 10 m length of a 0.5 m diameter pile.

Solution. Drag force in this simplified mobilized segment is πDLf = π×0.5×10×15 ≈236 kN.

Interpretation. Neutral-plane location and changes in shaft resistance with depth must be considered before treating drag as a separate applied force.

The numerical answer above uses deliberately narrow assumptions to make the dependency transparent. If any assumption is relaxed, the expression may need a different correction or an altogether different mechanical model. Before applying the result to another footprint, soil layer or construction state, ask whether the test procedure and the selected parameters represent the new condition. Check dimensional consistency and confirm that the direction of a predicted change agrees with the underlying physics.

Deeper technical application

Diagnostic study — down-drag and the neutral plane. Negative skin friction occurs when surrounding soil settles downward relative to the pile. Interface shear then acts downward on the pile over part of its length instead of contributing useful upward resistance. The relative displacement changes sign at a neutral plane; its depth depends on pile stiffness, soil consolidation and toe resistance. Treating a drag load as a fixed force while simultaneously claiming full upward positive shaft resistance in the same layer can double-count incompatible mechanisms. The effective stress increase from embankment fill or dewatering may cause surrounding clay to settle and change pile axial force over time, even if the original superstructure load remains unchanged.

Decision exercise. For the 0.5 m diameter pile and 10 m affected shaft at 15 kPa downward interface stress, compute roughly 236 kN down-drag. Compare a case where settling clay occupies only 4 m with a case where it occupies 12 m, and explain why actual drag depends on mobilized relative displacement rather than just soil thickness. Draw axial force versus depth: building load enters at the head, drag adds axial compression above the neutral plane, and positive resistance below it removes load. Identify a pile test or settlement observation that could help locate the behavior. A credible design separates pile structural compression checks from geotechnical resistance and uses appropriate code treatment of drag rather than adding arbitrary margins.

Counterfactual test

In this lesson, begin by writing the input assumption in a calculation sheet and introduce one perturbation at a time. Consider a 20% increase in the most uncertain load, a plausible change in groundwater, and a different depth or stiffness of a critical layer. Do not automatically multiply every output by the same factor: soil response is often nonlinear and the mechanism itself can change. Record which sensitivity is purely numerical and which requires more field data. If the decision to use shallow versus deep foundations changes, that is evidence that additional investigation may have a higher value than further decimal places.

Field practice and failure-first review

Practical action. Compare the sum of single-pile resistances against group mechanisms and stiffness distribution. Prioritize direct observations whenever a correlation could conceal a critical stratum or groundwater path. Document who reviews anomalies, whether repeat tests are required, and which construction stage is exposed. Failure trap: A group efficiency coefficient is not a universal material property.

Make a brief alternative hypothesis for the same observations. Ask whether a weak pocket, high water level, unrecognized load eccentricity, or installation defect might explain an unexpectedly large deformation. The right response is not always more conservative material strength; sometimes it is a different foundation system, a modified construction sequence, or a targeted test that resolves the uncertainty. Protect neighboring structures and utilities when the mechanism can propagate outside the footprint.

Study questions

1. Explain the difference between a measured site observation and an inferred parameter in the context of pile groups and interaction. Which information would you insist on before approving a preliminary calculation?
2. Rework the example after increasing the most influential input by 20%. Show units and explain whether a linear response is physically justified.
3. Show which input to the foundation model changes after treatment and which does not.
4. Give one mode of failure or serviceability loss that the worked equation cannot detect, and propose a corresponding observation or laboratory test.
5. With the two unit laboratories, Differential Settlement and Tilt and Pile Shaft and Toe Resistance, describe one experiment that would reveal the limitations of an oversimplified model.

Lesson 4.4 — Testing and installation quality

Why this mechanism matters

Static axial load tests observe load-displacement behavior under a defined procedure, whereas dynamic tests require interpretation and calibration. Integrity screening may flag anomalies without uniquely proving capacity. Construction records should report installation depth, concrete volume, blow counts or torque logs as applicable, interruptions, obstructions and testing dates. Test piles should be representative of actual production.

Evidence, scales and the engineering model

The choice of model changes the interpretation of testing and installation quality: a single observation establishes a condition at a place and time, while an engineering prediction requires a link from that observation to a repeatable physical mechanism. Establish the relevant load or environmental scenario, then identify whether the response is governed by strength, deformation, pore pressure, geometry, or construction sequence. The question to ask before entering numerical values is not simply which equation applies? but which field evidence could make the equation inapplicable? The answer often determines whether the apparent precision of an output is justified.

Limits and communication. Use explicit design basis, sensitivity study and independent check. Charts are decision aids whose input assumptions should be inspectable. For this lesson, make the assumed boundary conditions visible in a section drawing and distinguish measured inputs from correlations. A parameter that is merely convenient should not be presented as if a project investigation had confirmed it.

A useful field-to-model workflow is to identify an observed quantity, the geological or structural mechanism, the chosen constitutive assumption, the calculation, and a practical verification measurement. If the observation cannot distinguish two mechanisms, carry both possibilities into the sensitivity analysis. The reported result should preserve units, stress convention, drainage condition, stage and uncertainty bounds. It must be possible to trace a final design recommendation back to the underlying evidence.

Worked case 16 — method and interpretation

Given. A static load test records settlement of 4, 9, 19 and 38 mm at loads 400, 800, 1,200 and 1,600 kN.

Solution. Incremental movement is 5, 10 and 19 mm per 400 kN, showing progressively lower secant stiffness.

Interpretation. A test interpretation depends on creep, hold time and specified criteria. No universal failure load can be read from these four points.

The numerical answer above uses deliberately narrow assumptions to make the dependency transparent. If any assumption is relaxed, the expression may need a different correction or an altogether different mechanical model. Before applying the result to another footprint, soil layer or construction state, ask whether the test procedure and the selected parameters represent the new condition. Check dimensional consistency and confirm that the direction of a predicted change agrees with the underlying physics.

Deeper technical application

Diagnostic study — what a pile test demonstrates. A static axial load test measures one foundation element's load-movement response under specified increments and holding periods. It does not by itself reveal an independently mobilized ultimate shaft and toe capacity or guarantee that every production pile will behave identically. Test pile installation, elapsed time since driving, groundwater, concrete condition and testing direction can change the curve. Interpreting an apparent yield point without reference to test procedure and criteria can be misleading; extrapolating beyond the measured load is even more uncertain. Integrity screening addresses possible defects and is not an interchangeable substitute for a load test.

Decision exercise. Plot the load-displacement points (400,4), (800,9), (1200,19) and (1600,38) using consistent kN and mm units. Compute the secant stiffness of the last load point, roughly 42 kN/mm, and compare it with the first point, 100 kN/mm. The reduction suggests nonlinear response, but the observation alone does not identify whether shaft slip, toe movement or time-dependent creep dominates. Describe a load-hold schedule, unloading observation and independent survey required to improve interpretation. Next explain what a dynamic test could add and which uncertainty it leaves unresolved. Write a response plan for a production pile whose concrete placement record or installation refusal departs from the test pile history.

Counterfactual test

In this lesson, begin by writing the input assumption in a calculation sheet and introduce one perturbation at a time. Consider a 20% increase in the most uncertain load, a plausible change in groundwater, and a different depth or stiffness of a critical layer. Do not automatically multiply every output by the same factor: soil response is often nonlinear and the mechanism itself can change. Record which sensitivity is purely numerical and which requires more field data. If the decision to use shallow versus deep foundations changes, that is evidence that additional investigation may have a higher value than further decimal places.

Field practice and failure-first review

Practical action. Specify what observation would force a redesign, additional testing or change in construction procedure. Prioritize direct observations whenever a correlation could conceal a critical stratum or groundwater path. Document who reviews anomalies, whether repeat tests are required, and which construction stage is exposed. Failure trap: Passing an integrity screen is not the same as establishing axial load resistance.

Make a brief alternative hypothesis for the same observations. Ask whether a weak pocket, high water level, unrecognized load eccentricity, or installation defect might explain an unexpectedly large deformation. The right response is not always more conservative material strength; sometimes it is a different foundation system, a modified construction sequence, or a targeted test that resolves the uncertainty. Protect neighboring structures and utilities when the mechanism can propagate outside the footprint.

Study questions

1. Explain the difference between a measured site observation and an inferred parameter in the context of testing and installation quality. Which information would you insist on before approving a preliminary calculation?
2. Rework the example after increasing the most influential input by 20%. Show units and explain whether a linear response is physically justified.
3. State the observation that would make the current recommendation unacceptable.
4. Give one mode of failure or serviceability loss that the worked equation cannot detect, and propose a corresponding observation or laboratory test.
5. With the two unit laboratories, Differential Settlement and Tilt and Pile Shaft and Toe Resistance, describe one experiment that would reveal the limitations of an oversimplified model.

Unit 4 synthesis

This unit connects its four lessons through one overarching design decision: the right model follows from the mechanism and evidence, not from a convenient formula. Prepare a one-page design-basis table listing assumptions, parameter origin, uncertainty, sensitivity, and the condition requiring professional escalation. Pair that table with a labeled cross-section. For the investigation or structure in your own scenario, justify whether the controlling concern is ultimate resistance, unacceptable movement, unexpected groundwater, temporary construction loading or interaction with neighbors. Do not confuse a numerical check with an approval; identify the external standard and reviewer required for a real project.

Paired simulations: [Lab 07](../labs/lab-07.html) and [Lab 08](../labs/lab-08.html).

Assessment: [Unit 04 questions](../assessments/unit-04.html).

Project: Pile foundation installation and evidence plan — Choose between driven precast and bored piles for a fictional site with noise-sensitive neighbors and soft clay over sand. Present load-transfer hypotheses, resistance versus depth, group interaction questions, installation risks, acceptance tests and a response to anomalous concrete volume or driving refusal.