Bored Piles: How They Are Built and When to Use Them

Bored piles are cast-in-place concrete piles formed by drilling a hole into the ground, inserting a reinforcement cage and filling the hole with concrete. Because the soil is removed rather than displaced, they can be built to large diameters and great depths with little vibration, which makes them a mainstay of high-rise foundations, bridges and basement walls, particularly in dense urban areas.

This article explains how these piles are constructed, how the bore is kept stable, how load capacity is developed and tested, and when an engineer chooses them over other pile types. For an overview of the full range, see our guide to pile foundation types.

What bored piles are and how they carry load

A bored pile is a replacement pile: soil is excavated and replaced with reinforced concrete. Diameters commonly range from around 600 mm to well over 2 m, and lengths can run to many tens of metres. The pile carries load in two ways:

Shaft friction: resistance mobilised along the sides of the pile as it moves relative to the surrounding soil or rock.
End bearing: resistance at the base, where the pile bears on a stiff stratum or rock.

In many designs, especially piles socketed into rock, shaft friction in the socket carries much of the working load, with end bearing contributing more as settlement increases. The balance depends on ground conditions, the cleanliness of the base and the construction method. A thorough site investigation, with boreholes extending well below the expected pile toe, is the foundation of any reliable design.

How a bored pile is constructed

The typical sequence is:

Set out and install a temporary casing at the top of the bore to guide the drilling and protect against collapse of loose surface soils.
Drill the bore using a rotary rig with augers or drilling buckets in soil, and core barrels or rock augers in rock.
Support the bore as needed with longer casings, bentonite or polymer fluid.
Clean the base to remove loose debris and sediment, which otherwise reduce end bearing and increase settlement.
Lower the reinforcement cage, fitted with spacers to maintain cover.
Concrete by tremie, placing concrete from the bottom up through a pipe kept embedded in the fresh concrete, displacing any fluid upward.
Withdraw temporary casing while the concrete is still workable, and trim the pile head to sound concrete at cut-off level.

Concrete for tremie placement needs high workability and good cohesion so that it flows into place under its own weight without segregating. The pile head is then incorporated into a pile cap or raft, which distributes column loads across the group.

Keeping the bore stable

The key engineering challenge is preventing the sides of the hole from collapsing before concrete is placed. Three approaches are used, often in combination:

Dry boring: possible in stiff, self-supporting clays or rock above the water table, where the hole stands open unsupported.
Cased boring: a steel casing is installed to the full depth of unstable soils, then extracted during concreting or left permanently.
Fluid support: the bore is kept full of bentonite or polymer slurry, whose pressure supports the walls. Fluid properties must be monitored and controlled so that the slurry does not leave a thick filter cake or contaminate the concrete.

In soft clays and loose sands, poor support control can lead to necking (a local reduction in pile diameter), soil inclusions or voids. These defects are why integrity testing is routine.

Bored piles in Singapore ground conditions

Singapore’s geology varies sharply over short distances, from soft marine clay and reclaimed fill to the weathered sedimentary rocks of the Jurong Formation, the dense Old Alluvium in the east, and the granite of the Bukit Timah area. Bored piling suits this variability because the bore can be extended until it reaches suitable bearing material, and the drilled spoil gives direct evidence of the ground encountered.

For tall buildings, large-diameter bored piles socketed into rock or very dense soil are common. Where soft clay overlies the bearing stratum, negative skin friction from consolidating clay may add load to the pile and must be considered. The ability to build them with low noise and vibration also makes them suitable next to existing buildings, railways and utilities. As with all foundation work, the design and supervision are carried out under the building control regime, with the structural Qualified Person responsible for the design.

Testing and quality control

Because the pile is cast unseen below ground, verification is essential. Common methods include:

Static load tests: a maintained load applied to a test pile or working pile using kentledge or reaction piles, measuring settlement directly.
Bi-directional load tests: a sacrificial jack cast into the pile pushes the upper and lower sections apart, measuring shaft and base resistance without large reaction systems.
High-strain dynamic tests: a heavy drop weight strikes the pile head and sensors record the response for analysis.
Integrity tests: low-strain sonic tests at the pile head, cross-hole sonic logging through pre-installed tubes, or thermal integrity profiling, used to detect defects in the shaft.

Records of each pile, including bore depth, strata encountered, base cleaning, fluid properties, concrete volumes and casting times, are as important as the tests themselves. A concrete volume well above the theoretical value can indicate overbreak or a cavity; one well below can indicate necking.

When to choose bored piles

They are generally preferred when:

Loads are high and large diameters or rock sockets are needed.
Noise and vibration must be minimised near existing structures.
Ground conditions are variable, so pile length needs to be adjusted on site.
Obstructions or hard layers would stop a driven pile.
Piles also form part of a retaining wall, as in contiguous or secant pile walls.

They are less attractive for lighter loads, where driven precast or steel piles may be faster and cheaper, or where restricted headroom or access rules out large rigs, in which case smaller systems such as micro piles may be the better answer. The choice should come from the ground, the loads and the site constraints together, assessed against the bearing capacity of the strata available.

Frequently asked questions

What is the difference between bored piles and driven piles?

Bored piles are formed by drilling out soil and casting concrete in place, while driven piles are prefabricated and hammered or pressed into the ground, displacing soil. Bored piles cause less vibration and can be larger, while driven piles are often faster and their capacity can be inferred during driving.

How deep do bored piles go?

Depth depends on the loads and the ground, and is set by the design and confirmed on site by the strata encountered. Piles for tall buildings may extend many tens of metres to reach rock or very dense soil, while lighter structures may need much shorter piles.

Why is bentonite used in bored piling?

Bentonite slurry fills the bore and exerts pressure on the walls, preventing collapse in unstable soils and below the water table. Concrete is then placed from the bottom up by tremie, displacing the slurry. Polymer fluids are an alternative with easier handling and disposal.

How are bored piles tested?

Capacity is verified by static, bi-directional or high-strain dynamic load tests on selected piles, and shaft quality is checked by integrity tests such as low-strain sonic testing, cross-hole sonic logging or thermal integrity profiling, supported by detailed construction records for every pile.

Is bored piling suitable near existing buildings?

Generally, yes. Because they are drilled rather than driven, these piles produce low vibration and noise. Ground movement from boring and casing withdrawal still needs to be controlled and monitored when working close to sensitive structures.

Assessing a foundation scheme or reviewing bored pile records on your project? Talk to our consultancy or get in touch.