Pile foundations transfer building loads through weak surface soils down to hard soil layers or rock. A pile is driven or bored into the ground, and the settlement of a pile foundation is much smaller than that of a shallow foundation carrying the same load. Quality control is harder than for shallow foundations because the finished member is buried and cannot be inspected visually. Understanding how driven pile foundations behave in groups and how their capacity is tested provides a practical starting point for deep foundation work.
This article explains when piles become necessary, how piles are classified by load transfer mechanism and by construction method, the loads they must resist, and how soil conditions and cost settle the final choice between piles and other foundation systems.
When Pile Foundations Become Necessary
Shallow foundations are the first choice because they are usually the cheapest solution, but ground conditions and structural loads often rule them out. The decision follows a geotechnical investigation that compares the bearing capacity of the near surface soil with the loads the structure will impose. Once a deep foundation is selected, the pile foundation design methodology proceeds through soil investigation, capacity estimation, structural design, and load testing.
Weak Ground Conditions
When the ground is weak, the allowable bearing capacity is low, so a shallow foundation grows in area and its settlement increases. Compressible soils such as peat make a shallow foundation unworkable altogether because the structure would keep settling for years. In these situations a deep foundation carries the loads down to a firmer stratum, and the most cost effective pile type is selected from the options available.
High Column Loads
High column loads also push designers toward piles. The shallow foundation area grows until a raft becomes the only spread option, and a raft is limited by the bearing capacity of the soil. As a rough guide, buildings up to about ten storeys can be built on raft foundations where the soil is adequate. Above that level, most high rise structures use piles because of their very high axial loads. The construction cost of a pile foundation and a raft foundation can end up at a similar level, with the raft usually slightly cheaper, so the comparison is economic as well as technical.
The common triggers for selecting piles are:
- Low bearing capacity in the upper soil layers
- Compressible deposits such as peat or soft clay near the surface
- Very high axial column loads from tall buildings
- Uplift or lateral loads that a shallow foundation cannot resist
- Scour risk where flowing water can remove supporting soil
The decision sequence in practice:
- Complete a geotechnical investigation and identify the bearing strata
- Estimate shallow foundation sizes and check settlement
- Compare shallow, raft, and pile options on cost and programme
- Estimate pile capacity from soil parameters or field tests
- Detail the chosen system and verify capacity with load tests
Classification by Load Bearing Mechanism
The way a pile transfers load separates pile foundations into two main families. The classification matters because the practical usages of pile foundations follow directly from the load transfer mechanism that the ground conditions allow. An end bearing pile and a friction pile behave very differently under the same structure, so the pile type is fixed early in the design.
End Bearing Piles
Most piles constructed worldwide are end bearing piles. The toe of the pile resists vertical movement, and the penetration resistance at the toe is the end bearing component. The pile rests on soil, weathered rock, or fresh rock, and the end bearing capacity changes with the strength of that material and the toe area. Rock produces the highest end bearing pressure, while soil produces the lowest.
Calculating End Bearing Capacity
The end bearing component is calculated as Fb = fb x Ab, where Fb is the pile end bearing force, fb is the end bearing pressure of the founding material, and Ab is the toe area. A settlement check is still required, because a pile founded on stiff clay may have adequate toe pressure yet still settle under load.
Friction Piles
When a hard stratum is not available at a reasonable depth, the pile carries load through skin friction along the shaft. The shaft surface area multiplied by the average friction between the pile and the soil gives the frictional capacity. Long piles in soft clay or loose sand often work mainly in friction, and their capacity grows with length rather than toe area.
| Property | End Bearing Pile | Friction Pile |
|---|---|---|
| Load transfer | Toe resistance on a hard stratum | Skin friction along the shaft |
| Suitable ground | Rock or dense soil near the toe | Deep soft or medium deposits |
| Settlement | Small and load dependent | Larger and time dependent |
| Capacity driver | Toe area and end bearing pressure | Shaft area and soil adhesion |
| Design check | Toe bearing plus settlement | Shaft capacity plus group effects |
Loads on Pile Foundations
A pile must resist more than the gravity load of the column above it. The design accounts for every action the structure transfers, and engineers check the different types of loads on pile foundations and their calculations before the pile section is finalised. Underestimating any one of them shows up as excessive settlement or cracking.
Axial Compression and Uplift
Axial compression comes from the weight of the structure and its contents. Uplift forces develop in light roofs under wind suction, in water retaining structures where buoyancy acts, and in tall slender towers. Piles resist uplift through skin friction and, where needed, through a widened toe or mechanical anchorage.
Lateral Loads
Wind, seismic action, earth pressure, and berthing forces on jetties impose lateral loads on piles. Lateral capacity depends on the stiffness of the pile and the modulus of the surrounding soil. Piles in a group resist lateral load better than a single pile, because the group mobilises a larger soil mass.
Negative Skin Friction
When the soil around a pile settles more than the pile itself, it drags the pile downward and adds load. This downdrag occurs beside recent fill, after dewatering, or where soft clay consolidates under a new embankment. The design adds the negative skin friction to the structural load, and coatings or sleeving can reduce it.
Pile Types by Construction Method
The construction method controls speed, cost, noise, and the quality checks that are possible on site. Engineers compare pile types based on construction method against the ground conditions and the neighbouring environment before choosing the installation system.
Driven Piles
Driven piles are prefabricated concrete, steel, or timber members that are hammered or vibrated into the ground. They displace the soil as they advance, which densifies granular soils and improves their capacity. Driven piles suit sands and gravels, but the noise and vibration of driving limit their use near existing buildings.
Bored Cast In Situ Piles
Bored piles are formed by drilling a hole, placing a reinforcement cage, and filling the hole with concrete. They produce no driving vibration and can be built to large diameters. The quality of a bored pile depends heavily on concreting, because the concrete is placed below ground level, often under water or bentonite slurry.
Construction Sequence for Bored Piles
The sequence on a typical bored pile contract runs in six steps:
- Position the rig over the pile position and set the guide
- Drill the hole to the design depth with the correct diameter
- Clean the base and remove loose material
- Place the reinforcement cage
- Pour concrete by tremie from the bottom upward
- Withdraw the temporary casing and trim the top
Screw and Continuous Flight Auger Piles
Screw displacement piles are screwed into the ground without removing soil, and continuous flight auger piles place concrete through a hollow stem as the auger is withdrawn. Both methods are fast and produce little vibration, which makes them popular in urban work.
| Property | Driven Pile | Bored Cast In Situ Pile |
|---|---|---|
| Installation | Hammered or vibrated into the ground | Hole drilled, then concrete placed |
| Ground suitability | Granular soils, densified by driving | Clays, silts, and congested sites |
| Noise and vibration | High during driving | Low, no driving impact |
| Quality control | Records of driving resistance | Concreting under slurry or water |
| Typical diameter | Small to medium, precast units | Medium to very large |
| Programme | Fast once units are ready | Slower, dependent on drilling rate |
Selecting Piles Based on Soil Conditions
The geotechnical profile usually decides the pile family. The selection of foundations based on different types of soil starts with the strength and compressibility of the layers between the surface and the bearing stratum, and it ends with the cheapest pile that reaches the required capacity.
Granular Soils and Rock
Driven piles perform well in sands and gravels because driving densifies the soil as it advances. Where rock is close to the surface, end bearing piles socket into the rock to carry very high loads with small settlement.
Soft Clays and Organic Soils
Deep soft clays call for long friction piles, and organic soils such as peat should be avoided as founding material altogether. Bored piles reduce the disturbance to sensitive clays compared with driving, and negative skin friction must be checked where soft layers will consolidate under the new fill or structure.
Groundwater and Corrosion
A high water table affects the way the pile is installed and concreted, and it can make dewatering necessary for open excavations. Aggressive groundwater attacks steel piles and concrete, so designers respond with adequate cover, coatings, sacrificial thickness, or cathodic protection.
When a Raft Foundation Competes with Piles
The final choice between piles and other systems is economic as much as technical. For moderate loads on adequate soil, a raft foundation avoids pile costs completely, and engineers weigh the raft foundation types and advantages against the pile option before committing to deep foundations.
Comparing Cost and Programme
A raft spreads the load over the whole footprint and needs no pile caps, no driving rigs, and no load testing programme. Piles add cost through the piles themselves, the pile caps, the testing, and the longer programme. When bearing capacity is adequate, the raft usually wins on cost; when it is not, piles are the only reliable option.
Quality Control in Pile Construction
Because a completed pile is invisible, quality control depends on records: driving logs, torque readings, concrete placement records, and the results of static and dynamic load tests and integrity testing. These records are checked against the design assumptions before the foundation is accepted.