Bored Piers Explained: Deep Foundations for Difficult Sites
When a site’s surface soil simply can’t support a standard slab or footing, bored piers offer a reliable way to found a structure deeper, on more stable ground. Here’s how they work.
What Are Bored Piers?
Bored piers are deep, cylindrical concrete footings formed by drilling a hole to a specified depth, placing a steel reinforcement cage inside it, and filling it with concrete. They transfer a structure’s loads down past problematic surface soil to more stable, higher-bearing-capacity material or bedrock further below, effectively bypassing the ground conditions that would make a standard slab or shallow footing inadequate.
Installation Process
A drill rig bores each pier hole to the depth and diameter specified in the engineering design, with spoil removed as drilling progresses. Once the hole reaches target depth, a fabricated reinforcement cage is lowered in, and concrete is poured to fill the hole, typically from the bottom up to avoid segregation. The concrete then cures before the pier can be loaded, and multiple piers are usually installed across a site according to the structural layout.
When Bored Piers Are Needed
Bored piers are typically specified where a site’s near-surface soil has inadequate bearing capacity or excessive reactivity for a standard slab or shallow footing, on sloping or unstable sites, for retaining wall foundations under significant load, or wherever an engineer needs the structure founded at a specific depth to reach reliable bearing material. They’re a proven, reliable solution across a wide range of otherwise difficult sites.
How Depth and Diameter Are Determined
Both pier depth and diameter are calculated by the engineer based on the loads the pier needs to carry and the bearing capacity of the ground at various depths (informed by the soil report). Larger, deeper piers carry more load; the specific combination for your project depends entirely on what the structure requires and what the ground actually offers at depth — not a generic standard size.
Bored Piers vs Alternatives
For lighter structures or where minimising ground disturbance and installation time matters, screw piles are often a faster, lower-disruption alternative — though not always a substitute where higher loads or specific depths are required. See our bored piers vs screw piles comparison for how that decision is made.
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Frequently Asked Questions
How deep do bored piers typically go?
It varies entirely by site and engineering requirements — depth is calculated based on where adequate bearing material is found for your specific site and structure.
Do bored piers create a lot of mess?
There is spoil generated during drilling that needs to be managed and removed, more disruptive than some alternatives like screw piles, but well-managed as a standard part of the process.
Are bored piers stronger than screw piles?
They can typically carry higher loads and reach greater depths, which is why they’re often specified for larger structures or more difficult sites — though the right choice depends on your specific engineering requirements.
How long does concrete in a bored pier take to cure?
Similar to other concrete elements, strength develops progressively over about 28 days, though the pier may be loadable sooner depending on the engineering requirements.
Can bored piers be used for retaining walls?
Yes — they’re commonly used as the post foundation for concrete sleeper and other retaining wall systems, particularly for taller walls or those under significant load.
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