Bored Pier Retaining Walls: Engineering for Steep Sites
For taller retaining walls, steep sites or walls carrying significant surcharge load, bored piers remain the go-to foundation method. Here’s why they perform where lighter systems can’t.
Why Bored Piers Suit Demanding Retaining Walls
Bored piers can be engineered to significant depths and diameters, giving them the capacity to resist the substantial soil pressures generated by taller retaining walls, or walls carrying a surcharge load like a driveway or structure on the high side. This depth and load capacity is exactly what makes them the standard choice once a retaining wall moves beyond what lighter foundation systems like screw piles can reliably support.
How Depth and Spacing Are Determined
An engineer calculates pier depth, diameter and spacing along the wall line based on the retained height, any surcharge load, and the soil conditions from a site-specific geotechnical assessment. Taller walls or heavier loads generally require deeper, larger-diameter, or more closely spaced piers — there’s no standard specification that applies universally, since every wall’s engineering is a function of its own specific height, load and ground conditions.
Steep Site Considerations
On steep sites, retaining walls often carry more significant loads over a shorter horizontal distance, and site access for drilling equipment can itself be a real construction challenge. Bored piers are frequently the practical solution here specifically because they can be engineered for the higher loads steep sites often generate, even where access and staging require careful planning around the site’s slope and constraints.
Construction Sequence
Bored pier retaining wall construction typically follows: drilling piers to engineered depth along the wall line, placing reinforcement cages, pouring concrete and allowing it to cure, then installing sleepers or panels between the cured piers to form the finished wall face. This sequence takes longer than a screw pile equivalent due to the concrete curing wait, but delivers the higher capacity these more demanding applications require.
Combining With Proper Drainage
Regardless of foundation method, drainage behind the wall remains critical — see our guide to retaining wall drainage for why this matters so much to long-term performance. Bored pier walls on steep or heavily loaded sites arguably need this even more, since these applications are already carrying near their engineered capacity, leaving less margin for the added pressure poor drainage introduces.
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Frequently Asked Questions
When is a bored pier retaining wall needed instead of screw piles?
Typically for taller walls, walls under significant surcharge load, or sites where engineering assessment determines screw piles don’t provide adequate capacity.
How deep do bored piers go for a retaining wall?
It’s calculated by an engineer based on the specific wall height, load and soil conditions — there’s no standard universal depth.
Does a steep site always need bored piers for retaining walls?
Often, given the higher loads steep sites typically generate, though the specific engineering assessment for your site determines the actual requirement.
How long does a bored pier retaining wall take to build?
Longer than a screw pile equivalent, due to the concrete curing time required before sleeper installation can proceed — timeline varies with wall size and site conditions.
Is drainage still necessary with bored pier walls?
Yes — drainage matters for every retaining wall regardless of foundation method, and arguably even more on heavily loaded bored pier walls with less engineering margin to spare.
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