Pile Foundations: Types of Piles for Foundations

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A pile foundation is a deep foundation system made up of long, slender structural members that transfer building or structural loads into the ground. Piles are commonly used when the near-surface soil has inadequate bearing capacity, when settlement must be controlled, or when the foundation must resist uplift or lateral forces, or extend below scour-prone or erodible soil.

A pile transfers load mainly through end-bearing resistance at its toe, shaft (skin) friction along its surface, or a combination of both. Piles can be classified by load-transfer mechanism (end-bearing and friction piles), function, material (timber, concrete, steel and composite), method of installation (driven, bored and cast-in-situ) and soil displacement.

This article explains the main types of piles, their uses, materials, installation methods, advantages and limitations, and the factors that guide selection. The right pile type depends on the soil profile, groundwater, structural loads, settlement limits, site access, equipment, noise and vibration restrictions, and the applicable design standards. Final selection should always be confirmed by a proper geotechnical investigation and engineering design.

How Do Pile Foundations Work?

Pile foundations work by transferring structural loads from a building or other structure through weak or compressible soil to stronger soil or rock below. A pile foundation usually consists of several piles connected at their tops by a reinforced-concrete pile cap. The pile cap receives loads from columns or walls and distributes them among the individual piles.

Load transfer mechanisms

A pile can support a load through two main mechanisms:

End-Bearing Resistance

An end-bearing pile transfers most of its load through its toe or base. The pile extends through weak soil until it reaches a strong layer of dense soil, hard stratum or rock. The load is then transferred vertically through the base of the pile to the supporting layer.

Shaft or Skin Friction

A friction pile transfers load through frictional resistance developed between the pile surface and the surrounding soil. As the pile tends to move downward under load, the soil along the pile shaft resists this movement. This resistance is known as shaft friction or skin friction.

Combined Load Transfer

In practice, most piles develop both end-bearing resistance and shaft friction. The total axial resistance of a pile can be expressed in simplified form as:

Qu = Qb + Qs

where:

• Qu = ultimate axial resistance of the pile;
• Qb = resistance developed at the pile base; and
• Qs = shaft or skin-friction resistance along the pile.

Pile foundations may also be designed to resist uplift and lateral forces. Uplift resistance is important for structures such as basements, towers, tanks and offshore structures, while lateral resistance is required for bridges, retaining structures, marine facilities and buildings subjected to wind or earthquake forces.

The performance of a pile foundation depends on the soil profile, pile material, diameter, length, spacing, installation method, groundwater conditions and structural loading. Therefore, pile capacity and settlement must be evaluated using a geotechnical investigation and appropriate engineering design.

When Is a Pile Foundation Required?

A pile foundation is considered when a shallow foundation, such as a pad footing, strip footing or raft foundation, cannot safely or economically transfer structural loads to the ground. The decision depends on the soil, the structure and the site conditions, and it should be confirmed by a geotechnical investigation.

Common situations where piles are used

1. Weak or compressible soil near the surface
Pile foundations may be required when the upper soil has low bearing capacity or is too soft, loose or compressible to support the structure safely. Examples include soft clay, loose sand, peat, uncontrolled fill and weak alluvial deposits. Piles can transfer loads to deeper, stronger soil or rock, or develop resistance through shaft friction.

2. Heavy or concentrated structural loads
Buildings and structures carrying heavy or concentrated loads may require piles when shallow footings would become excessively large, closely spaced or uneconomical. Piles are commonly used for high-rise buildings, bridges, industrial structures, towers, tanks and heavily loaded columns.

3. Control of settlement
Even where the soil is strong enough, total or differential settlement may exceed what the structure can tolerate. Piles taken down to a stiffer layer can reduce settlement, especially under uneven loading.

4. High or fluctuating groundwater
Where the water table is high, excavating deep footings requires dewatering and temporary support, which can be difficult and costly. Piles can often be installed with less excavation, though the construction method still needs care in water-bearing soils.

5. Uplift and overturning forces
Structures such as basements below the water table, buried tanks, towers and tall masts may be subject to uplift or overturning. Tension piles anchor them against these forces.

6. Significant lateral loads
Bridge abutments, retaining structures, quay walls and wind- or wave-loaded structures need resistance to horizontal forces. Larger-diameter piles or raking (batter) piles are used for this.

7. Scour or erosion
Where bridges, jetties or river structures are exposed to scour, piles are extended below the expected scour depth. They do not prevent scour; they keep the foundation load-bearing below it.

8. Expansive or collapsible soils
Seasonal swelling and shrinkage of expansive clays can damage shallow foundations. Piles (such as under-reamed piles) can be anchored below the active zone, where moisture changes are small. (Link to your expansive soil article here.)

9. Liquefiable ground
In earthquake-prone areas, piles may be used to carry loads through liquefiable layers into stable soil, with design that accounts for the loss of soil support during shaking.

10. Deep excavations or services near the foundation
Where canals, deep drains or neighbouring excavations make shallow foundations unsafe or unstable, piles can transfer loads to a deeper, more stable level.

Resisting different types of load

• Compression: the pile pushes down on the soil through shaft friction and end bearing.
• Uplift (tension): only shaft friction and the pile’s own weight resist upward forces, so tension piles are usually longer or larger.
• Lateral loads: the upper part of the pile bends and mobilises the soil around it. Larger diameters, stiffer piles, or batter (raking) piles improve lateral resistance.

Pile groups and pile caps

Piles are usually installed in groups connected by a pile cap, which spreads the column load evenly among them. Piles in a group must be spaced far enough apart that their stress zones don’t overlap too much. The group can have lower capacity per pile than a single pile, and settlement of the group is often greater than that of a single pile.

Why this matters for pile selection

How a pile transfers load depends on the soil profile, which is why geotechnical investigation determines the pile type, length and diameter.

Also, read: What is expansive soil?

Types of Pile Foundations

Pile foundations can be classified according to their load-transfer mechanism, function, material and method of installation. These classification systems often overlap. For example, a single pile may be a steel driven end-bearing pile or a bored cast-in-situ friction pile.

1. Types of piles based on load transfer

End-bearing piles carry most of the load through the pile toe, which rests on a firm layer such as dense sand, gravel or rock. They are used when strong strata lie at an economical depth below weak soil.

Friction piles (also called floating piles) carry most of the load through shaft resistance along the pile surface. They are used where no firm layer exists within a practical depth, as in deep soft clay or loose sand.

Combined (friction and end-bearing) piles develop significant resistance from both the shaft and the toe. In practice, most piles fall into this group, and the classification depends on which component dominates.

2. Types of piles based on function

Compaction piles densify loose granular soil to increase its bearing capacity. The pile itself is not meant to carry structural load, so it can be made of sand or other weaker material.

Tension (uplift) piles anchor structures against uplift from groundwater pressure or overturning, as under basements below the water table, buried tanks and towers. They rely on shaft resistance.

Laterally loaded piles resist horizontal forces from wind, waves, earth pressure or vehicle impact, as at bridge abutments, retaining walls and quays.

Batter (raking) piles are installed at an angle to resist large horizontal or inclined forces more efficiently than vertical piles.

Anchor piles provide anchorage against horizontal pull, for example for sheet pile walls.

Fender piles and dolphins protect waterfront structures from the impact of ships and floating objects.

Sheet piles form continuous walls for retaining soil or water, cut-offs and cofferdams. They mainly resist lateral loads and control seepage rather than carrying vertical building loads.

3. Types of piles based on material

Timber pile: It consists of the trunk of a tree, stripped of branches. Typically, timber piles are either square or circular in cross-section. The circular timber pile typically ranges in diameter from 300 to 500 mm (millimetres), while the square pile falls within the same size range. It’s recommended that the length of the timber pile does not exceed 20 times its top width. To improve penetration into the ground and safeguard against breakage, one end (bottom) of the timber pile is often fitted with either a cast-iron shoe or a steel strip. These piles are driven into the ground using light pile-driving equipment. To prevent damage to the head of the pile during driving, an iron ring is secured at its top. The diameter of the ring is typically about 25 mm less than the diameter or size of the pile head. Timber piles have a small bearing capacity, and they are prone to damage by hard strata or boulders. They can take loads up to 20 tonnes.

Precast concrete piles are cast and cured in a yard, then driven. They are strong, durable and suitable for many soils, including aggressive ground. The pre-cast pile is used for a maximum design load of about 80 tonnes (800kN), except for the large pre-stressed piles. Pre-cast piles are reinforced to withstand stress. They need space for casting and storage, curing time, and heavy equipment for handling and driving. Prestressed piles handle lifting and driving stresses better. Cutting to length and splicing are more difficult than with steel.

Cast-in-situ concrete piles are formed in the ground, either by boring a hole or by driving a casing, and then filling it with concrete, with reinforcement where needed. Cast-in-situ piles are generally used for a maximum design load of 75 tonnes (750kN) except for compacted pedestal piles. They are installed by pre-excavation, thus eliminating vibration due to driving and handling stresses. They can be tailored to site depth and can reach very high capacity, especially when large-diameter. Quality control during concreting is critical. Cast-in-situ piles may be of two types, namely driven piles and bored piles.

Steel piles may have been used since 1838, in the form of cast iron pipes or solid wrought iron shafts with disk or screw flanges penetrating only short distances. Steel piles include H-piles, steel pipe (tube) piles and box piles. They handle hard driving well, are easy to cut and splice, can reach great depths and carry high loads. The main concerns are corrosion, which must be considered for the design life, and cost. Pipe piles may be driven open- or closed-ended and are often filled with concrete.

Composite piles are suitable where the upper part of a pile is projected above the water table. Such a pile consists of a lower portion of untreated timber and an upper portion of concrete. In other types of composite piles, steel piles are attached to the lower end of cast-in-situ concrete piles. The two most common types of composite piles are:

• Concrete and Timber
• Concrete and Steel

Also, read: Soil Nail and Soil Nailing: Reinforcing Slope Stability

4. Types of piles based on method of installation

Driven piles are installed with a hammer or vibratory driver. Timber, steel and precast concrete piles are commonly driven. They are quick to install in suitable ground and allow capacity to be checked from driving records. Noise, vibration and ground heave can be limiting factors, especially near existing buildings.

Bored piles (drilled shafts) are formed by drilling a hole, supporting it where needed with casing or drilling fluid, placing a reinforcement cage and concreting. They allow large diameters and depths, cause little vibration, and suit sites close to existing structures. Care is needed to avoid collapse of the borehole, base contamination and concrete defects such as necking.

Driven cast-in-situ piles are formed by driving a casing or shell into the ground and filling it with concrete, sometimes withdrawing the casing. The Franki pile, which forms an enlarged base, is a well-known example.

Under-reamed piles are bored cast-in-situ piles with one or more enlarged bulbs. They are widely used in expansive clays and for uplift resistance, because the bulbs anchor the pile below the active zone.

Continuous flight auger (CFA) piles are formed by drilling with a hollow-stem auger and pumping concrete through the stem as the auger is withdrawn. They are fast and low-vibration, and suit many soils above the water table or in water-bearing soils where open boreholes would be unstable. Reinforcement is placed after concreting, which limits depth and cage length.

Screw (helical) piles are steel shafts with helical plates that are screwed into the ground. They produce little noise or vibration, can be loaded quickly, and suit lighter loads, restricted access and tension applications.

Micropiles are small-diameter, high-capacity grouted piles, often with steel reinforcement. They are used where access is limited, for underpinning, for strengthening existing foundations, and in difficult ground.

Also, read: Different Types of Foundation Used in Construction

5. Types of piles based on soil displacement

Displacement matters because installation changes the soil. Driving in loose sand tends to densify it and can increase capacity. Driving in soft clay can cause heave, build up pore pressure and disturb nearby structures. Non-displacement piles avoid these effects but may loosen the soil around the borehole.

Also, read: Bearing Capacity of Soil: The Foundation of Geotechnical Excellence

Installation of Pile Foundations

Piles are installed by driving, boring (drilling), or a combination of both, and the method affects capacity, noise and vibration, and the surrounding soil. Driven piles are hammered or vibrated into place, bored piles are cast in a drilled hole, and methods such as CFA, screw piles and micropiles suit particular sites. For detailed procedures, read our guide to Installation Methods of Piles.

Uses of Pile Foundations

Piles are used wherever loads must be carried to deeper or stronger ground, or where a structure must resist forces that shallow foundations cannot. The main uses are grouped below by the type of load or structure.

1. Carrying vertical (compressive) loads
Bearing piles support the vertical loads of buildings, bridges, industrial plants and storage tanks. They transfer the load through weak soil to a firmer layer (end-bearing) or spread it through shaft friction (friction piles).

2. Resisting uplift (tension) loads
Tension piles hold down structures subject to upward forces, such as basements below the water table, buried tanks, transmission towers, masts and dam aprons.

3. Resisting lateral loads
Laterally loaded and batter piles support structures exposed to horizontal forces, such as retaining walls, bridge abutments and piers, wharves, jetties, and offshore and harbour structures.

4. Marine and waterfront structures
Piles carry jetties, quays and piers, and fender piles and dolphins protect them from ship impact. Piles are also used where erosion or scour could undermine shallow foundations.

5. Retaining and excavation support
Sheet piles and bored pile walls (contiguous or secant) retain soil and water around basements and deep excavations, and form cofferdams and cut-offs.

6. Ground improvement
Compaction piles densify loose granular soil to raise its bearing capacity.

7. Underpinning and strengthening existing foundations
Micropiles, jacked piles and screw piles are used to transfer load to deeper strata when existing foundations settle or when loads increase.

8. Anchorage
Anchor piles resist horizontal pull on sheet pile walls and similar structures.

Advantages of Pile Foundations

The main advantages of pile foundations include:

• They can carry large vertical, uplift and lateral loads.
• They are suitable for weak, variable or compressible ground conditions.
• They can reduce the risk of excessive and differential settlement when properly designed.
• They can transfer loads to deeper, more competent soil or rock.
• They require less surface area than large shallow footings for heavily loaded structures.
• Different materials and installation methods are available for different site conditions.
• Some pile systems can be installed with low noise, low vibration and minimal excavation.
• Certain driven pile systems can be installed rapidly and produce little or no excavated spoil.
• Piles can be arranged in groups and connected with pile caps to support heavily loaded columns and structural frames.

Limitations of Pile Foundations

Despite their advantages, pile foundations have several limitations:

• They are usually more expensive than shallow foundations because they require specialised equipment, materials, testing and skilled labour.
• Large piling rigs may require considerable working space and suitable access roads.
• Driven piles can produce noise, vibration and ground movement that may affect nearby buildings or services.
• Bored piles can produce excavated spoil and may require casing, drilling fluid or groundwater control.
• Hard strata, boulders, underground obstructions and variable soil layers can make installation difficult.
• The quality of bored and cast-in-situ piles depends heavily on borehole stability, base cleaning, reinforcement placement and concrete placement.
• Pile groups may experience group interaction, block failure or settlement, even when the capacity of individual piles is adequate.
• Negative skin friction or downdrag may develop when surrounding soil settles relative to the pile.
• Steel and timber piles may require protection against corrosion, decay, biological attack or other deterioration.
• Piles are difficult to inspect and repair after construction, so installation records and quality-control testing are important.
• A pile foundation does not automatically eliminate settlement or foundation problems. Its performance depends on the soil investigation, pile design, installation quality, pile-cap design and structural loading.

Frequently Asked Questions (FAQs)