Piles transfer building loads through weak surface soils into stronger layers below. Among deep foundation options, cast-in-situ bored piles dominate because they can be drilled to any required depth, sized to match the applied load, and installed with minimal vibration around existing structures. The design of a single pile comes down to two numbers: the capacity the ground can supply and the capacity the pile section can resist. Both are calculated in every pile design, and the smaller of the two governs. Coordinating the foundation with the layout above is part of the wider [architectural design and building envelope design process], where envelope systems, acoustics, and sustainable site design are resolved together with the structural scheme.
Why Bored Piles Are the Default Deep Foundation Choice
Bored piles are installed by drilling a shaft, placing a reinforcement cage, and filling the hole with concrete. The engineer can inspect the founding stratum and adjust the socket length before the pile is poured. Driven piles are forced into the ground instead and cannot be inspected once installed. When the axial capacity of a shallow foundation is insufficient, or when settlement limits control the design, bored piles take over. Typical diameters range from 450 mm to 1,500 mm, and working capacities commonly fall between 500 kN and several meganewtons depending on the ground.
When Shallow Foundations Stop Working
Shallow foundations fail when the ground is too weak, too compressible, or too wet. Common triggers include:
- Allowable bearing pressure below about 100 kPa under heavily loaded columns
- Thick layers of soft clay or peat at shallow depth
- Large point loads from tall or heavily loaded structures
- Settlement predictions that exceed the project limits
Where any of these conditions appear, a deep foundation is usually the answer: piles bypass the weak layers and deliver the load to a capable stratum.
Bored versus driven installation
Bored piles avoid the impact energy of driving, which makes them the preferred option near occupied buildings, and the toe can be inspected before concrete is placed. Driven piles are faster in some ground conditions but require heavy driving equipment and can suffer refusal before reaching design depth. The comparison between concrete bored piles and [steel piles in foundation design] comes down to cost, ground conditions, and construction access.
Starting with the Geotechnical Investigation
Every pile design begins with the geotechnical investigation report, which supplies the soil and rock parameters that drive the capacity equations. The report normally provides either net allowable end bearing and allowable skin friction, or ultimate values that the engineer converts to allowable figures before comparing them with service loads.
What the Report Must Deliver
A usable geotechnical report for pile design includes:
- Soil stratification with depths and thicknesses of each layer
- Net allowable end bearing pressure at the founding level
- Allowable skin friction along the shaft, split by soil type
- Rock quality data, including RQD, when socketing into rock is planned
- Recommendations for pile diameter and socket length
The values must be in the same units used in the design; allowable values, not ultimate values, are compared with working loads. Where the report is silent, the values should come from laboratory or field tests rather than assumption.
Allowable versus ultimate values
Ultimate capacities are divided by a factor of safety before comparison with service loads, commonly between 2 and 3 for end bearing and skin friction alike. Mixing ultimate capacities with working loads is one of the most common errors in pile design, and it can overstate the available capacity by a factor of two or more.
The foundation scheme also interacts with the layout above. A [single-story home on a wide lot with an open concept design], for example, may concentrate its heaviest loads on a few interior points rather than distributing them across load-bearing walls, which changes the pile layout and socket depths under each column.
Calculating Geotechnical Capacity
The geotechnical capacity of a pile is the sum of the load carried at the pile base, from end bearing, and the load carried along the shaft, from skin friction.
End Bearing Capacity
End bearing capacity = net allowable end bearing x cross-sectional area of the pile base. The base area comes from the pile diameter. A 600 mm pile has a base area of 0.283 m2, and with a net allowable end bearing of 2,000 kPa, the end bearing capacity is 566 kN.
Skin Friction Capacity
Skin friction capacity = allowable skin friction x surface area of the pile in socket length. The surface area is the pile perimeter times the socket length, the portion of the shaft embedded in the load-bearing stratum. For a 600 mm pile the perimeter is 1.885 m, so a 600 mm socket with an allowable skin friction of 300 kPa gives a skin friction capacity of 339 kN. Typical allowable skin friction values vary widely with the ground:
| Ground condition | Allowable skin friction (kPa) |
|---|---|
| Soft clay | 20 to 40 |
| Stiff clay | 60 to 120 |
| Loose sand | 30 to 60 |
| Dense sand | 80 to 150 |
| Weathered rock | 150 to 300 |
| Fresh rock | 300 to 600 |
These ranges are illustrative. The governing values must come from the geotechnical investigation for the specific site.
Socket length rules
Unless the geotechnical report specifies otherwise, piles are socketed to a depth of about one pile diameter into fresh rock. Longer sockets increase the skin friction capacity and are specified when end bearing alone is insufficient or the rock surface is weathered.
Geotechnical capacity = end bearing capacity + skin friction capacity, or 566 + 339 = 905 kN for the example above. The same terms appear when piles work in groups; the [pile load capacity calculation for single piles and group piles] article steps through group effects and factor selection in more detail.
Structural Capacity of the Pile
The pile is a structural member designed as a column carrying axial load through soil and rock. Its structural capacity depends on the concrete grade and the cross-section, and it is evaluated independently of the ground capacity.
Pile as a Column
The surrounding soil braces the pile laterally along most of its length, so it behaves as a short column. A common rule of thumb is:
Structural capacity = 0.25 x fcu x Ac
where fcu is the characteristic cube strength of concrete and Ac is the cross-sectional area of the pile. The 0.25 factor covers the material partial factor and a conservative allowance for load transfer into the ground.
Applying the rule of thumb
For a 600 mm pile in C30 concrete, fcu is 30 MPa and Ac is 0.283 m2, giving a structural capacity of 0.25 x 30,000 x 0.283 = 2,123 kN. This is well above the 905 kN geotechnical capacity, so the ground governs the design. When the geotechnical capacity controls, a higher concrete grade buys nothing; when the structural capacity controls, raising the grade or the diameter both work.
Buckling in Very Soft Soils
In very soft soils such as peat, lateral support is weak and buckling must be checked. The pile is analyzed as a slender column with an effective length related to the depth of soft material. Unsupported lengths, low soil stiffness, and high axial loads can reduce the structural capacity below the geotechnical value, so the buckling effect should be checked explicitly rather than relying on the short-column rule.
Once the single pile capacity is known, the connection to the superstructure is made through the pile cap. The [pile cap design for a group of piles] spreads the column load across the pile heads and resists punching shear, so its depth and reinforcement follow from the pile capacities and column load.
Setting the Design Capacity
The design capacity is the lesser of the structural capacity and the geotechnical capacity. In most soil-founded piles the geotechnical value controls; in heavily loaded piles on rock, the structural value can.
| Capacity check | Example value | Governs? |
|---|---|---|
| Geotechnical capacity | 905 kN | Yes, lesser of the two |
| Structural capacity | 2,123 kN | No |
| Design capacity | 905 kN | Compared with the service load |
Worked Example Sequence
The steps in the design are:
- Obtain net allowable end bearing and allowable skin friction from the geotechnical report
- Convert ultimate values to allowable values if the report gives ultimate figures
- Calculate the base area and shaft surface area from the trial diameter
- Compute the end bearing capacity and the skin friction capacity
- Sum the two to get the geotechnical capacity
- Calculate the structural capacity with the 0.25 fcu Ac rule
- Take the lesser of the two as the design capacity
- Compare the design capacity with the applied service load, including negative skin friction drag
- Adjust the diameter or socket length and repeat until the capacity is adequate
Adjusting the Design
If the applied load exceeds the design capacity, the engineer has several levers:
- Increase the pile diameter, raising both base area and shaft surface area
- Extend the socket length into better rock to add skin friction
- Raise the concrete grade to increase the structural capacity
- Add piles or redistribute column loads to reduce the demand on each pile
Each option changes the cost and the construction program, so the alternatives are compared before the final section is fixed. The same logic of member capacity and connection verification appears in [structural steel design principles] for steel framing and connection design.
Checks and Precautions Before Construction
Two precautions deserve attention before the pile design is finalized: negative skin friction and construction quality.
Negative Skin Friction
Skin friction usually helps the pile by transferring load into the ground. Negative skin friction does the opposite: when the soil around the pile settles more than the pile itself, the soil drags the pile downward and adds load. This happens with recently placed fill, soft compressible clays, or dewatering of nearby sites. The drag load is added to the applied load in the capacity check.
Construction Quality Checks
The calculated capacity assumes the pile is built as designed. Field controls include:
- Drilling records that confirm the socket length and the founding level
- Reinforcement cage positioning so that concrete cover is maintained
- Concrete placement without segregation, using tremie methods below water
- Integrity testing after casting to detect necking, voids, or inclusions
- Load testing on selected piles to verify the assumed capacity
Pile load tests give the most direct confirmation that the design assumptions hold, and codes typically require one test for every 50 to 100 piles, loaded to 1.5 to 2.0 times the working load to verify both capacity and settlement behavior.
The same discipline of checking capacity against demand runs through civil engineering. The [pavement design principles] for flexible and rigid pavements apply the same logic of layer capacity, load repetition, and material properties that governs pile design.