Steel Post Spacing & Embedment Depth for Retaining Walls

Spacing and embedment are the two measurements that finish a post specification once you've already settled on series and profile. Get either one wrong and even the correctly sized steel retaining wall posts can underperform badly over time. This guide covers standard post centres, embedment depth by wall height, how soil type changes the numbers, the difference between a concrete footing and a compacted base, and the mistakes that most commonly show up in walls that lean or move within their first few years.

If you want the deep dive on just one of these two specs, our site also has a dedicated post embedment guide and a dedicated post spacing guide covering each in isolation. This guide is deliberately different: it treats spacing and embedment as one combined decision, since in practice the two are rarely worked out separately on a real wall. A tighter spacing changes the load on each post, which can change the embedment it needs, and the two are worth reading together rather than as two unrelated numbers.

It's worth treating spacing and embedment as equally important to series selection, even though they get less attention in most buying guides. A correctly specified 150 UC post set into a shallow, undersized footing performs no better than an undersized post in a correctly designed footing. Both halves of the specification need to be right together.

Standard Post Spacing (Centres)

Post spacing, measured centre to centre between adjacent posts, is generally set to match standard sleeper lengths so each sleeper spans cleanly between two posts without cutting or joining mid span.

Getting spacing right matters for reasons beyond convenience. Every sleeper acts as a small beam spanning between two posts, and the wider that span, the more bending stress the sleeper has to resist for the same soil load. A sleeper rated for a standard span isn't necessarily rated for a noticeably wider one, which is why post spacing and sleeper specification need to be considered together rather than treating spacing as a purely cosmetic or convenience-driven decision.

• Standard sleepers are commonly supplied at 2000mm lengths
• Post centres are typically set at around 1.8 to 2.0m to suit this standard length, allowing for the post's own width within the span
• Wider spacing increases the bending load each sleeper has to carry across its span, and is generally avoided unless an engineer has specifically designed for it
• Tighter spacing than standard is sometimes used on heavily loaded or taller 150 Series walls, where reducing the span between posts is one way of managing higher soil pressure

It's worth measuring your actual sleeper stock before finalising post centres, since sleeper lengths can vary slightly between suppliers and product lines. Setting posts to a generic 2.0m centre when your specific sleepers are a slightly different length leaves you either trimming sleepers on every course or leaving an awkward gap.

Corners, Ends and Gate Openings

Standard centres apply to a straight run of wall, but corners, wall ends and any gate or step openings need to be planned separately. A corner post needs to be positioned so that sleepers from both directions meet it at a standard length, which sometimes means the last standard bay before a corner needs to be shortened slightly to keep the corner post in the right position relative to the rest of the run. Similarly, a gate or step opening interrupts the standard spacing pattern and needs its own dedicated posts either side of the opening, sized for the specific load that section carries. It's worth mapping out the entire wall run, including every corner, end and opening, before finalising post positions, rather than working out spacing for a straight section and hoping corners and openings will simply fit in afterwards.

Embedment Depth by Wall Height

Embedment depth is the portion of the post that sits below ground level, and it's what actually resists the post rotating forward under the load of the retained soil.

It's helpful to picture the post as a lever. The exposed, above-ground section is being pushed by the retained soil, and the embedded, below-ground section is what resists that push, acting a bit like the buried portion of a fence post resisting wind on the fence panel above it, but with a far more sustained and significant load behind it. The deeper and more securely fixed that embedded section is, the more effectively the post resists rotating forward as the soil pushes against it, which is why embedment depth is never really a figure worth rounding down.

A commonly used starting rule of thumb is that the in-ground portion of the post should be at least one-third of the post's total length. Working this through with a worked example: for a wall retaining 1.0m of soil, using a rule of one-third embedment relative to total post length, the post needs to be roughly 1.5m long overall, giving approximately 500mm embedded below ground and 1.0m exposed above it.

Retained wall height	Approx. total post length	Approx. embedment depth
0.6m	0.9 to 1.0m	Minimum 600mm regardless
1.0m	1.5m	Approx. 500mm
1.2m	1.8m	Approx. 600mm

Note the practical minimum embedment depth, often around 600mm, that applies even to short walls where the one-third rule alone would suggest something shallower. A very low wall still needs enough embedment for the post to resist rotation reliably, so the minimum depth acts as a floor beneath the proportional rule, not a suggestion to ignore it. This floor exists because very short posts embedded to a proportionally shallow depth simply don't have enough surrounding soil or concrete engagement to resist rotation reliably, regardless of how modest the load above ground happens to be.

These figures are a general starting guide for standard residential conditions. For the full detailed breakdown, including how post length and embedment interact more precisely across a wider height range, see our guide on how deep retaining wall posts should be.

Adjusting for Surcharge Load

The embedment figures above assume no significant load sitting above the wall beyond the retained soil itself. Where a driveway, shed, pool or other structure adds surcharge above the wall, the effective load on each post increases beyond what the retained height alone would suggest, and embedment depth needs to increase to match. As a practical approach, treat any wall with meaningful surcharge as though it were a taller wall than its retained height alone suggests when working out embedment, and confirm the specific adjustment with a supplier or engineer rather than assuming the baseline figures still apply unchanged.

Why Spacing and Embedment Aren't Really Two Separate Numbers

It's tempting to look up a spacing figure and an embedment figure independently and treat the job as done, but the two are connected. Each post carries the soil load for the sleeper span on either side of it, so if you space posts wider than standard to save on post count, each post is now resisting more load, which generally means it needs deeper embedment or a larger footing to compensate. Conversely, if you tighten spacing on a heavily loaded 150 Series wall, each post carries less load individually, which can sometimes allow embedment closer to the standard baseline rather than the deeper figure a wider spaced equivalent would need. Working the two out together, rather than picking a spacing for convenience and an embedment depth from a separate lookup, is what actually determines whether the finished wall performs the way it's meant to.

Soil Type Adjustments

The one-third rule and the figures above assume reasonably firm, well drained soil. Several soil conditions push embedment depth beyond this baseline:

• Reactive clay, common across large parts of Melbourne, Adelaide and outer Sydney, expands and contracts with moisture, and generally needs greater embedment than the baseline rule to resist the additional cyclical pressure it places on a wall
• Loose or sandy soils can drain well but may not provide the same passive resistance around the embedded post as firmer ground, sometimes requiring deeper embedment or a larger diameter footing to compensate
• Fill or previously disturbed ground is less predictable than undisturbed natural soil, and generally warrants a more conservative embedment depth, or a proper geotechnical assessment before finalising post length

As a practical approach, treat the standard embedment figures as a minimum baseline for firm, well drained soil, and add depth, or seek engineering advice, whenever your site's soil falls into any of the categories above. Treating these figures as a genuine minimum, rather than a target to trim down wherever the ground seems reasonably firm, is the safer habit to build into every project regardless of scale.

Identifying Your Soil Type

Most residential builders won't commission a full geotechnical report for a low garden wall, and in many cases that's a reasonable call. A simple approach is to dig a test hole to the depth you're planning to embed posts, and look at what comes out. Sticky, plastic soil that holds its shape when rolled into a ball is a sign of clay content, and the more pronounced that behaviour, the more reactive the clay is likely to be. Loose, crumbly soil that falls apart easily suggests sandy or loamy conditions. Local knowledge helps too. Many established suburbs have well known soil reputations among local builders and landscapers, and checking with neighbours or a local supplier who's worked in the area is a reasonable, low cost way to sense check your own observations before finalising embedment depth.

Concrete Footing vs. Compacted Base

Once embedment depth is set, the post still needs to be fixed into the ground using either a concrete footing or a compacted base.

This choice is sometimes treated as a matter of personal preference or convenience on site, but it genuinely affects how reliably the post resists rotation over the wall's lifetime, particularly through wet seasons when soil pressure peaks. Understanding what each method actually provides helps explain why concrete is generally recommended as the default rather than an optional upgrade.

Concrete footing. The post hole is filled with concrete around the base of the post. This is the more common and more predictable method for retaining wall posts, since concrete provides a consistent, known resistance to the post rotating forward under load, largely independent of how well the surrounding soil happens to be compacted.

Compacted base. The post hole is backfilled with layers of road base or similar material, compacted in stages around the post. This can be adequate for very light, low walls in firm soil, but generally needs a deeper hole than an equivalent concrete footing to achieve the same holding strength, and its performance depends heavily on how thoroughly each layer is actually compacted during installation.

For anything beyond the lightest garden wall, and certainly for any 150 Series or taller 100 Series wall, a concrete footing is the safer default and the method most sizing guides and engineers assume when calculating embedment depth.

Drainage also plays a role here that's easy to overlook. Water pooling around a footing, whether concrete or compacted base, adds hydrostatic pressure the post wasn't necessarily sized for. Our retaining wall drainage guide covers ag pipe, backfill and weep holes in detail, and is worth reading alongside this one before finalising your footing plan.

Footing Diameter Matters Too

Embedment depth gets most of the attention, but the width or diameter of the footing hole also affects how much resistance a concrete footing provides against the post rotating forward. A narrow footing around a post relies on a smaller volume of concrete and surrounding soil to resist the same load a wider footing spreads across more material. As a general residential guide, footing holes are commonly dug somewhere in the 300 to 450mm diameter range depending on post series and load, with heavier 150 Series posts generally suiting the wider end of that range. If you're increasing embedment depth to account for reactive clay or surcharge, it's worth checking whether footing width should increase alongside it, rather than assuming a deeper but equally narrow hole automatically solves the problem on its own.

Common Mistakes

• Measuring post length as only the exposed above-ground height, forgetting to add embedment depth on top of that figure, which results in posts ordered too short for the wall once they're actually installed
• Using the one-third rule as an exact minimum without adding the practical depth floor for shorter walls, which needs enough embedment regardless of how modest the exposed height is
• Setting posts on compacted base alone for a wall or soil condition that really calls for a concrete footing, often to save time on a weekend build without fully weighing up the long-term consequence
• Widening post spacing beyond standard centres to save on post costs, without checking that sleepers are actually rated to span the increased distance
• Ignoring soil type entirely and applying the same generic embedment figures across an entire property regardless of local ground conditions, particularly where a block includes both firm and low, damper sections
• Skipping drainage around the footing, which can effectively increase the load a correctly embedded post has to resist over time, undermining an otherwise correctly specified footing

Most of these mistakes share a common thread: treating spacing and embedment as a fixed, one-size-fits-all number rather than a calculation that responds to your specific wall height, soil type and drainage arrangement carefully.

The good news is that none of these mistakes require expensive equipment or specialist skills to avoid, just a bit of care working through wall height, soil type and surcharge before digging the first hole. Confirming these figures on paper before any concrete is mixed is far cheaper than correcting a post that's already set into an undersized footing.

FAQs

Getting spacing and embedment right is genuinely as valuable to long-term wall performance as series and profile selection. A correctly specified post set into a shallow or undersized footing can still fail, while an appropriately embedded post at the right spacing gives the rest of the wall system, sleepers, brackets and any fencing above it, the stable foundation they're all built to rely on.