Pile Foundations

Overview

Piles transfer structural actions to deeper, more competent strata or develop sufficient capacity by shaft friction and end bearing where shallow foundations are not adequate. They may be designed for compression, tension, lateral load or combinations, and often form groups supporting pile caps or piled rafts.

Design focus: define ground and groundwater conditions, select pile type and construction method, estimate characteristic axial and lateral resistances, assess settlement and group behaviour, model soil–structure interaction (often via springs), and verify design with appropriate factors and load testing.

The choice between driven, bored and screw piles depends on geology, load level, access, vibration tolerance, construction risk and quality control. Piles are part of a system: the foundation, superstructure and soil must be considered together rather than in isolation.

Pile Types & Selection

Driven Piles

Preformed piles (precast concrete, prestressed, steel H-piles, steel tubes, timber) driven into the ground by impact or vibration. Installation displaces soil, often increasing lateral stresses and shaft resistance in granular soils and generating setup effects in clays.

Advantages: factory quality; no spoil; good QA via driving records; rapid installation.
Limitations: noise and vibration; driving damage risk; obstructions; need for headroom and access for plant.
Typical uses: bridges, marine structures, industrial buildings, where displacement and driving effects are acceptable and access is reasonable.

Bored / Cast-in-Place Piles

Piles formed in excavated shafts by drilling and concreting in situ. Includes CFA (continuous flight auger), drilled shafts with temporary casing or drilling fluid support, and rock-socketed piles in competent rock.

Advantages: minimal vibration; flexible diameters and lengths; can form rock sockets; suitable for urban and sensitive environments.
Limitations: spoil disposal; sensitivity to groundwater; construction defects (soft toes, necking, inclusions) if QA is poor.
Typical uses: high-rise building cores, bridge piers, heavily loaded columns and walls in urban environments.

Screw / Helical Piles

Steel shafts with one or more helices installed by torque. Load is carried by bearing of helices and shaft friction, often used in tension and compression for lightweight or modular structures and where rapid installation is required.

Advantages: low noise and vibration; immediate load; removable/replaceable; good where access is restricted and plant is small.
Limitations: sensitive to obstructions and hard layers; limited diameter; corrosion management required.
Typical uses: modular buildings, telecommunication masts, pipelines supports, extensions and retrofits.

Pile Type Selection

Pile type selection should reflect geotechnical conditions, structural demands, environmental constraints and constructability. Comparative assessment of driven, bored and screw piles often considers:

Aspect	Driven	Bored	Screw
Noise / vibration	High	Low	Very low
Ground disturbance	Displacement (densification / heave)	Removal of material	Moderate displacement
QA / QC	Good (driving records)	Variable; relies on supervision & testing	Good (torque logs)
Obstructions	Driving refusal / damage risk	Can drill through with suitable tools	Often not suitable
Typical capacity range	Medium–high	Medium–very high	Low–medium (per pile)

Construction & Quality Control

Driven Piles – Method & QA

Preparation: fabrication of piles, handling and storage procedures to avoid damage; predrilling where hard crust or obstructions are present.
Driving: impact hammers (diesel, hydraulic), vibratory hammers; driveability analysis and hammer selection; monitoring set/blow count versus depth.
Records: driving logs, hammer energy, blows per 0.25 m or per set interval, refusal / hard layer observations.
Effects on soil: densification of loose sands; setup in clays; heave of neighbouring piles and ground; potential damage to adjacent structures due to vibration.
QA/QC: pre-production trials, redriving and restrike tests, dynamic testing, monitoring of pile integrity during driving (e.g. excessive tension/compression stresses).

Bored Piles – Method & QA

Excavation: CFA, rotary drilling, core barrels and rock tools; use of temporary casing or drilling fluids (bentonite, polymers) to support the hole.
Base cleaning: removal of loose material and slurry; verification of clean base and correct founding stratum (e.g. by inspection, sondex, cameras, or simple probing).
Reinforcement & concreting: cage installation, tremie techniques under fluid support, control of concrete volume and return; avoiding segregation, necking, inclusions and cold joints.
Defects: soft toes, reduced diameter zones, contamination layers, poorly formed sockets; recognition via load testing and integrity tests.
QA/QC: construction records, depth and diameter checks, fluid properties, concrete monitoring, integrity testing (PIT, CSL, gamma logging) on a risk-based sample.

Screw / Helical Piles – Method & QA

Installation: hydraulic torque heads mounted on excavators or rigs; continuous monitoring of torque and penetration rate versus depth.
Capacity correlation: use of empirical torque–capacity relationships; calibration where possible against static load tests.
Termination criteria: achievement of specified torque at design depth; management of obstructions and refusal; possible predrilling in hard layers.
Corrosion: protection measures (coatings, sacrificial thickness, cathodic protection) based on environment and design life.
QA/QC: installation logs (torque, depth, inclination), verification testing for representative piles, periodic torque gauge calibration.

Construction Risks & Controls

Unexpected strata, cavities, boulders or obstructions.
Groundwater inflows, loss of drilling fluid, instability of sides of boreholes.
Pile heave in clays for driven and cast-in-situ piles; need for sequence control and level checks.
Variability in workmanship; importance of supervision, records and appropriately scoped testing.
Interface management between geotechnical designer, structural designer and contractor; design assumptions should be explicit and checked during construction.

Axial Capacity & Settlement

Axial pile resistance is derived from shaft friction (skin resistance) and end bearing. Design typically separates characteristic resistance evaluation from application of partial factors or global factors for ultimate limit state checks, followed by settlement assessment at service load levels.

Compression Capacity in Soils

For axial compression, the characteristic ultimate resistance of a single pile can be written as:

R_u = R_s + R_b = ∑ f_s · A_s + q_b · A_b

Clays (α-method): f_s = α · u · s_u, where α depends on consistency and pile type; q_b ≈ N_c s_u for end bearing in clay.
Sands (β-method): f_s = β · σ′_v or CPT-based correlations; q_b derived from bearing capacity factors or CPT tip resistance.
Layered soils: shaft friction integrated by layer; end bearing controlled by the founding stratum and potential punching into underlying soils.
Groups: group efficiency and pile–pile interaction factors may reduce net shaft and base resistance and increase settlement.

Characteristic values are reduced to design resistances via code-specific partial factors or global factors of safety, often with separate factors for shaft and base and for different pile types and test levels.

Rock Sockets & Piles in Rock

For piles socketed into weak or weathered rock, axial resistance is obtained from side resistance in the socket and end bearing on rock. Common approaches include:

Side resistance based on rock strength, RQD, roughness and socket cleanliness; often limited to prevent overstressing of rock and to account for construction variability.
End bearing based on allowable stress fractions of rock strength or code-based limits; generous factors are used to control settlement and brittle failure risk.
Recognition that settlement is often governed by deformability of the rock mass and overlying soils rather than mobilising full rock strength.

Settlement of Single Piles & Groups

Settlement of a pile or pile group at service loads can be estimated using load–transfer methods or simplified analytical approaches:

Elastic methods: approximate the pile as an elastic element in an elastic half-space; use modulus of subgrade reaction or equivalent stiffness to estimate settlement.
Load–transfer (T–Z / Q–Z) methods: integrate non-linear shaft and base load–displacement curves to obtain settlement under service load (see below).
Group settlement: consider overlapping influence zones and reduced stiffness; treat a group as an “equivalent raft” at some depth for settlement estimation.
Consolidation: if compressible layers are present, consolidation settlement due to pile loads must be included, especially for bored piles in soft clays.

Tension Capacity & Negative Skin Friction

Tension capacity: derived from shaft resistance only (no end bearing), based on drained or undrained parameters depending on soil and loading; bond to pile material and structural capacity must also be checked.
Negative skin friction (downdrag): arises where consolidating or settling soil moves downward relative to the pile. It imposes additional compressive load and may reduce available positive shaft resistance for structural loads.
Design usually introduces an equivalent downdrag load or a reduced neutral plane elevation, with ultimate and serviceability checks ensuring combined effects remain within capacity and settlement limits.

Lateral Response & P–Y Models

Lateral pile behaviour is governed by pile stiffness, soil stiffness and strength, loading magnitude and duration, and boundary conditions at the pile head (fixed, pinned or partially restrained). Analysis commonly uses p–y curves representing non-linear soil reaction per unit length.

Elastic & Simple Methods

Simplified “beam on elastic foundation” with constant subgrade modulus k_h for preliminary estimates.
Closed-form solutions for laterally loaded piles in elastic soil; useful for checking trends and sensitivity.
Limit equilibrium of a rigid pile in a plastic soil as a rough upper bound on ultimate lateral capacity.

P–Y Curve Methods

Non-linear springs (p–y curves) at discrete depths, representing soil resistance per unit length as a function of lateral deflection y and, where appropriate, cyclic loading effects.
Empirical or semi-empirical p–y relationships for sands, clays and weak rock; often adopted from standards or design guidelines.
Analysis performed using dedicated p–y software or general structural/FE packages with non-linear spring elements; outputs include deflection profiles, bending moments and soil reaction diagrams.

Lateral capacity and stiffness may degrade under cyclic loading, especially in soft clays and loose sands. Where significant cyclic or seismic loads are present, p–y curves and soil parameters should reflect cyclic behaviour and liquefaction potential as appropriate.

T–Z & Q–Z Load–Transfer Methods

Load–transfer methods represent the pile as a series of non-linear springs along its shaft and at its base, relating local displacement to mobilised shaft and base resistance. They are widely used for settlement prediction and to reconcile design with load test results.

Shaft (T–Z) Curves

T–Z curves relate local pile–soil relative displacement z to mobilised shaft resistance τ (or T per unit length), typically increasing to a limiting value and then flattening or softening.
Curves differ between clays and sands and may be calibrated from instrumented load tests or derived from correlations with strength and stiffness parameters.
Discretising the pile into segments and applying T–Z curves allows prediction of load distribution with depth and pile head settlement for a given load.

Base (Q–Z) Curves

Q–Z curves relate pile base settlement to mobilised end bearing. In many soils, relatively small base movements may mobilise a high proportion of ultimate end resistance.
For piles on rock, base stiffness can be high and settlement small; deformation of overlying soils and shaft resistance may dominate settlement.
Q–Z curves are often more uncertain than T–Z curves and benefit from calibration against static load tests in representative conditions.

Load–transfer models form the basis of many pile analysis tools. They provide a practical way to incorporate non-linear behaviour, soil layering and group effects into settlement prediction while maintaining a relatively simple one-dimensional framework.

Soil–Pile–Structure Interaction

Pile foundations form part of a coupled system of soil, piles, raft or pile caps, and superstructure. Soil–pile– structure interaction (SSI) controls load sharing, stiffness and force distribution, especially for piled rafts, pile groups and laterally loaded frames or bridges.

Load sharing: pile groups and piled rafts distribute load between raft contact and pile shafts and bases in a non-uniform, load-dependent manner.
Group effects: piles influence each other through overlapping stress and displacement fields; interaction factors or numerical analysis are used to quantify these effects.
Stiffness compatibility: bending stiffness of the superstructure influences how foundation movements manifest as differential settlements or rotations.
Dynamic / seismic response: inertia and kinematic interaction, rocking, and radiation damping are relevant for seismic design and vibration-sensitive structures.

Soil Springs in Structural Models

Structural analysis models often represent soil support using springs. For piled foundations, these are usually defined at pile heads (vertical, lateral and rotational springs) or distributed along piles using p–y, t–z and q–z elements in specialist software.

Vertical Springs (K_v)

Derived from load–settlement behaviour of piles or pile groups at serviceability loads.
Single pile K_v can be obtained from load–transfer analysis or from static load test curves (ΔP/Δs around the working load level).
Group or raft K_v should reflect group interaction and raft contribution; simple “spring per column” models can be unconservative if they ignore load sharing and interaction.

Lateral & Rotational Springs

Lateral springs at pile heads may be obtained from p–y analysis by applying a lateral displacement and extracting the corresponding reaction.
Head fixity conditions can be represented by rotational springs; these may be derived from structural stiffness of pile caps and connecting beams or from 3D FE models.
For dynamic problems, frequency-dependent stiffness and damping may be required; simple static springs may be insufficient where vibration is critical.

When using soil springs, their derivation and limitations should be clearly documented. Springs should be consistent with geotechnical assumptions, pile spacing and group effects, and should be updated if design changes significantly.

Pile–Raft Interaction

Piled rafts use both raft bearing and piles to support loads. Properly designed, they can reduce settlements and provide redundancy with fewer piles than a conventional “fully piled” solution. Design requires explicit treatment of load sharing and interaction between piles and raft.

Concept & Behaviour

At low loads, the raft often carries a significant share of load; piles progressively attract more load as settlement increases.
Piles may be arranged under heavily loaded regions to control differential settlement while allowing raft contact elsewhere.
Load sharing and stiffness distribution depend on pile stiffness, raft stiffness, pile arrangement and soil properties.

Design Approaches

Equivalent raft: approximate the piled raft as an “equivalent” raft at some depth with enhanced stiffness; suitable for preliminary settlement estimates.
Simplified interaction methods: use superposition of raft and pile group solutions with interaction factors between piles and the raft.
Numerical analysis: 3D FE or FD models with non-linear soil behaviour, enabling explicit simulation of load sharing, interaction and staged construction; often used for critical or complex projects.

Design checks must consider both ULS (capacity of piles, raft and soil) and SLS (settlement and rotation). Pile design for a piled raft often focuses on settlement control rather than full mobilisation of pile ultimate capacity.

Structural Design of Piles

Structural design ensures that piles can safely carry the factored axial, lateral and bending actions implied by geotechnical design, construction loads and accidental or seismic actions, considering durability and buckling in weak soil zones.

Concrete & Steel Reinforcement

Axial compression design according to concrete and reinforcement standards, with cover and confinement appropriate to exposure and durability requirements.
Minimum longitudinal reinforcement to control cracking, provide ductility and allow temporary bending during construction and service.
Shear and bending design for lateral loads and eccentricity; check combined axial and bending interaction using code interaction diagrams.
For bored piles, reinforcement detailing must consider cage fabrication, lifting and placement tolerances, and potential cage movement during concreting.

Steel & Composite Piles

Steel H-piles and tubular piles designed for combined axial and bending; local buckling checks and reduction factors for thin-walled sections as relevant.
Corrosion allowances (sacrificial steel) or protection systems; reduction of section over time if corrosion is significant.
Composite sections (concrete-filled steel tubes, encased piles) may be used to combine geotechnical and structural advantages.
Stability in soft soils may require buckling checks over unsupported lengths and consideration of group lateral stiffness.

Pile heads and caps must be detailed to transfer actions effectively: anchorage of reinforcement, bearing and shear checks at pile–cap interfaces, and accommodation of tolerances in pile location and inclination.

Groundwater, Durability & Corrosion

Durability is a critical aspect of pile design, particularly for aggressive environments (marine, industrial, contaminated fill). It influences material selection, cover requirements, corrosion allowances and protection systems.

Concrete piles: exposure-based cover and concrete quality; sulfate and chloride resistance; control of crack widths; consideration of carbonation and chemical attack in the ground and groundwater.
Steel piles: corrosion rates dependent on soil and water chemistry, oxygen availability and stray currents; measures include sacrificial thickness, coatings, sleeves and cathodic protection where justified.
Timber piles: decay and borer resistance; need for appropriate treatment and consideration of groundwater fluctuation zones where oxygen and biological activity are highest.
Screw piles: typically steel; similar corrosion issues; design often includes sacrificial thickness and, in severe environments, additional protection systems.

Durability requirements may govern pile dimensions, reinforcement, detailing and protection measures. Design life and maintenance strategy should be clearly defined and agreed at an early stage.

Load Testing & Verification

Pile testing provides direct evidence of capacity, stiffness and integrity, and can be used to calibrate design methods and parameters. Testing strategy should be risk-based, reflecting variability, consequence of failure and construction method.

Static Load Tests

Compression tests: maintained-load or quick tests to determine load–settlement behaviour, ultimate resistance and serviceability performance.
Tension tests: verify uplift resistance and load–displacement; important for tension piles, anchors and screw piles.
Lateral tests: quantify stiffness and ultimate lateral resistance; used to calibrate p–y curves in critical projects.
Bi-directional tests: use of embedded jacks (e.g. Osterberg cells) to separate shaft and base contributions, particularly for large-diameter bored piles.

Dynamic & Integrity Testing

Dynamic load tests (PDA): assess capacity during driving or restrike; back-analysis with signal-matching software; useful for driven piles.
Pile integrity tests (PIT): low-strain impact tests to identify potential defects such as necking or major inclusions; screening rather than definitive.
Cross-hole sonic logging (CSL): evaluates concrete quality between tubes in large bored piles; sensitive to necking, inclusions and major defects.
Other methods: gamma logging, thermal integrity profiling, or other specialised techniques where risk and budget justify them.

Test results should be interpreted cautiously, taking account of test duration, loading history, soil conditions and construction records. Design parameters and resistance factors may be updated where justified by consistent test data.

Worked Examples (Sketches)

Example 1 – Driven Pile in Sand (Axial Compression)

Given: 0.4 m diameter driven concrete pile, length 18 m, founded in medium dense sand. γ = 19 kN/m³, φ = 34°. Water table at 5 m depth. Use a β-method for shaft resistance and a simplified end bearing estimate for illustration.

Compute effective overburden: estimate σ′_v with depth, adjusting for groundwater; adopt an average σ′_v,avg over the shaft length for preliminary β-method calculation.
Shaft resistance: assume f_s = β · σ′_v,avg, with β selected from correlations for driven piles in sand; compute R_s = f_s · π D L.
Base resistance: estimate q_b = N_q σ′_v,base with an appropriate N_q for φ = 34°, then R_b = q_b · A_b. For preliminary design, it may be appropriate to cap q_b to limit settlement.
Characteristic & design resistance: sum shaft and base to obtain R_u, then apply code-specific partial factors or a global factor to obtain design compression resistance.
Settlement check: use load–transfer or simplified elastic methods to estimate settlement at serviceability loads; adjust shaft/base contributions if settlements exceed project limits.

This example is schematic only. Actual design requires code-specific factors, local correlations and, ideally, calibration against load tests.

Example 2 – Bored Pile in Clay (Axial Compression & Settlement)

Given: 1.0 m diameter bored pile, length 20 m, in normally consolidated clay with undrained shear strength increasing with depth. s_u profile from lab/field tests is available. Pile supports a column load of 4 MN (serviceability). Illustrative α-method and settlement assessment:

Undrained capacity: divide the pile into layers with different s_u; adopt α-values for a bored pile in the clay profile. Compute layer-by-layer shaft resistance and add base resistance using q_b = N_c s_u,base with an appropriate N_c.
Selection of characteristic values: apply cautious characteristic s_u for each layer, reflecting variability and investigation quality.
Design resistance: apply partial or global factors to obtain design compression resistance, checking that the required factored load is ≤ design resistance.
Settlement estimation: apply a T–Z / Q–Z load–transfer method or an equivalent elastic method using soil modulus values with depth to estimate settlement under the 4 MN service load.
Consolidation: if the clay layer is significantly compressible and loaded by the pile group, estimate consolidation settlement over time and include this in the total settlement assessment.

Example 3 – Screw Pile in Clay (Uplift)

Given: Steel screw pile with three helices at depths 5 m, 7 m and 9 m in stiff clay. Target characteristic uplift resistance 400 kN. Illustrative design steps:

Helix bearing: for each helix, compute uplift bearing resistance based on undrained shear strength and helix area, with suitable factors for spacing and interaction.
Shaft contribution: include shaft uplift resistance where appropriate, based on drained or undrained parameters, noting that helices often dominate uplift capacity.
Torque correlation: correlate target capacity with installation torque using calibrated empirical relationships; ensure that required torque is achievable without overstressing pile or equipment.
Design check: apply partial or global factors to obtain design uplift resistance; verify that the factored uplift load is ≤ design resistance.
Serviceability: estimate uplift displacement under service loads and check against project criteria, considering cyclic behaviour where relevant.

Example 4 – Pile–Raft Interaction (Conceptual)

Given: Building raft 20 m × 30 m on a group of 64 piles; piles arranged more closely under heavily loaded core. Illustrative steps for a simplified piled raft assessment:

Unpiled raft response: estimate raft settlements and rotations on soil without piles using an elastic or simplified method to understand the baseline behaviour.
Pile group response: estimate stiffness and capacity of pile group assuming no raft contact (pile cap only) using interaction factors or group methods.
Combined model: apply an interaction-based method or 3D FE model in which raft and piles act together; derive load sharing between raft and piles and the resulting settlement profile.
Design adjustment: modify pile layout and pile lengths to reduce differential settlements while keeping the number of piles reasonable; consider locations where piles may be omitted without compromising acceptable movement.
Verification: assess ULS and SLS for piles, raft and soil, and consider construction staging and long-term consolidation in the design.