Leaking retaining walls — core-filled block and concrete

Start with the drainage, not the wall

A properly built retaining wall has three things working behind it: a drainage cell or a layer of free-draining aggregate against the back face, an ag line — a perforated agricultural drain sitting in that gravel — and weep holes through the wall so any water that does get past can escape. All three are invisible. All three fail quietly.

The test takes one wet afternoon. Find where the ag line discharges: a pipe end at a garden bed, a kerb, a pit or a stormwater connection at the low corner of the site. During or immediately after heavy rain, water should be running out of it. If the wall is wet and the drain is not running, the drain is blocked with silt, crushed, root-invaded, or was never connected to anything. That is your leak.

Then look at the face. Weep holes rendered over during a repaint, painted shut, or buried behind a raised garden bed are one of the most common causes we find, and one of the cheapest things on this page to correct. Check the ground surface behind the wall too — a garden bed that stays saturated, a downpipe discharging at the back of the wall, or a paved area falling toward it will each keep loading the wall no matter what is injected into it.

Where drainage exists but is blocked, clearing and reinstating it is the repair. Where drainage was never installed, injection alone is a compromise and it has to be sold as one, in writing, before anyone starts.

• Ag line discharging freely during rain — if not, jet, camera or replace before any injection is quoted
• Weep holes open, unblocked and above the ground line on the exposed face
• Drainage cell or free-draining backfill actually present behind the wall, not clay backfilled straight against it
• No garden bed, paving or downpipe discharging directly onto the retained side
• Surface levels behind the wall falling away from it, not toward it

Stop and get an engineer if you see any of these

Waterproofing a retaining wall assumes the wall is structurally sound and doing its job. Some of the things that look like a leak are actually the wall telling you it is losing the argument with the ground behind it. Sealing those hides a warning sign, and the water was never the dangerous part.

If any of the following are present, we will not quote injection until a structural engineer has looked at it. That is not us being cautious for the sake of it — a retaining wall failure is sudden, and it takes whatever is in front of it.

• Any measurable lean out of plumb, or a lean that has changed since you last looked
• A bulge or belly in the face of the wall
• Step cracking following the mortar joints in a staircase pattern through blockwork
• Cracking that widens toward the top of the wall
• Diagonal cracking in a reinforced concrete wall, as distinct from a straight vertical shrinkage crack
• Ground behind the wall that has dropped, or a slab, path or driveway near it that has cracked or stepped
• A wall that was built without engineering design and is retaining more than about a metre — worth confirming before spending anything on it

Core-filled block and reinforced concrete are two different repairs

Besser block retaining walls dominate the 1970s to 1990s housing stock across Brisbane's inner north and the hillside blocks on the Gold Coast. Reinforced concrete walls are more common on newer builds, commercial sites and anywhere an engineer designed the wall from scratch. They fail differently.

In blockwork, the water path is almost never through the blocks. It is along the mortar joints and up through the cores — the hollow cells inside each block that were supposed to be filled with grout and reinforcement. That path is diffuse: it spreads through the wall rather than running along one line. In reinforced concrete, the path is a defined crack or the construction joint between two pours, and it behaves much more predictably.

Unfilled cores swallow resin without sealing anything

A core-filled block wall is meant to have its vertical cells filled solid with grout around reinforcing bar. In practice, a great many walls in South East Queensland were filled partially, filled with grout that segregated, or filled only at the bars. What you get is a network of open voids running the full height of the wall, connected to each other and to the ground behind.

Inject into that and the resin does exactly what physics says it will: it disappears. It fills a void you cannot see, in a volume you cannot predict, and the face of the wall stays wet because you never sealed the path the water is actually using. This is the single most common way a retaining wall injection quote goes badly wrong for both parties — the contractor loses money on resin and the client still has a leak.

The answer is to establish what is behind the face before quoting a volume, by probing, drilling test holes, or scanning. Where the cores are open, the honest options are to grout-fill the cores first as a separate item, to inject a gel that permeates rather than a foam that expands into voids, or to accept that the wall is a drainage job rather than an injection job. All three are legitimate. Quoting a per-linear-metre injection rate over an unknown void network is not.

Sealing the face raises the water behind the wall

This is the part that most pages about leaking retaining walls do not say, and it matters more here than on any other structure we work on.

Sealing a wall from the dry side does not remove water. It stops water passing through and leaves it in the ground behind. On a basement wall that redirects the leak sideways, which is an inconvenience. On a retaining wall it does something else: it raises the height of water standing in the backfill, and water standing in backfill is load. A saturated backfill can push considerably harder on a wall than a drained one.

Many older domestic retaining walls were designed — or built without design at all — on the assumption that drainage would keep the backfill from saturating. Sealing the face of one of those walls without restoring its drainage transfers the water it used to leak into pressure it was never asked to carry. That is why we check that the ag line and its outlet exist and flow before we seal anything, and why on a marginal wall we will recommend an engineer nominate what the wall can tolerate before we proceed.

The rule we work to is simple. Before sealing anything, ask where the water will go instead. If the answer is nowhere, that is a design problem and injection is not the tool.

How a block wall is injected

1. Confirm the drainage decision firstDrainage cleared and still leaking, drainage absent and injection accepted as a compromise in writing, or drainage restored and injection unnecessary. That decision is made and recorded before the drill comes out.
2. Establish what is behind the faceProbe or scan the cores. A wall with open cores needs a different scope and a different price basis from one that is solidly filled, and finding that out at quoting stage rather than mid-injection is what separates a real price from a guess.
3. Set out tighter than you would on concretePackers at 150 to 250 mm centres, targeted at the mortar joints rather than the middle of blocks, because the joints are the water path. Blockwork paths are diffuse, so wider spacing simply leaves untreated gaps between treated ones.
4. Flush and watch where the water comes outWater pumped through each packer tells you how the wall is connected internally, which on blockwork is rarely what you would guess from the outside. It also clears the salt deposits that block resin. Where the flush water appears is the map.
5. Inject at low pressure and escalate slowlyStart at the bottom of the pressure range and raise it only as the wall demands. The warning signs of over-pressure are visible: a mortar joint opening, resin appearing somewhere unexpected, spalling around a packer, or a sudden pressure drop with no resin return. Blowing out a mortar bed makes the wall worse, and it is not recoverable with more resin.
6. Cut back at 24 hours, then make goodPackers stay in for a minimum of 24 hours. Shafts are cut 10 to 20 mm below the face, recesses filled with a non-shrink mortar and finished flush. Re-rendering and painting the exposed face is quoted separately.
7. Reinstate the weep holesThe wall still needs a way to relieve water. Weep holes that were blocked get cleared and kept clear, and where the finish is being reinstated they are protected rather than rendered over again.

Where a reinforced concrete wall differs

On a designed reinforced concrete retaining wall the work is closer to standard crack injection. Holes are drilled at 45 degrees, alternating each side of the crack and offset from it by roughly half the wall thickness so the hole crosses the crack at mid-depth, drilled to two-thirds or three-quarters of the thickness and never through. Mechanical packers at 200 to 400 mm centres, water flush to prove the crack is connected, injection from the bottom up, cut back at 24 hours.

The judgement that matters is what kind of crack you are looking at. A vertical shrinkage or thermal crack in a wall that has stopped moving is straightforwardly injectable. A crack from load — flexural cracking, which typically appears on the retained face in tension and can widen toward the top — is the wall bending. Sealing it is sealing a warning light.

Where the seepage is diffuse across a whole wall face with no single crack to inject, there is a second option: curtain injection. Instead of injecting into the concrete, the wall is drilled right through on a grid, typically 250 to 500 mm in each direction, and a low-viscosity acrylate gel is injected into the soil behind to form a continuous barrier between the ground and the structure. It is priced from a trial section and a rate per point or per kilogram of gel, never a lump sum, because you are injecting into ground you cannot see. It also travels, so neighbouring properties, service trenches and stormwater lines are part of the planning.

Cost and how long it takes

Polyurethane crack injection on an accessible wall runs roughly $150 to $450 per linear metre of crack. Long repetitive runs on one site trend toward the bottom of that band because the set-up is shared. Difficult access, height, or working around an occupied space pushes it well above.

Construction and cold joints are priced higher, at roughly $200 to $500 per linear metre, because a joint is wider, more connected, takes more resin and needs closer packers than a discrete crack. Discrete defects and individual holes price per point at roughly $35 to $95 each.

Every job carries a site attendance minimum of roughly $450 to $900 plus GST in business hours in the metro area. That covers travel, set-up, plant and a minimum block of labour, and no legitimate contractor attends for less. Outside the metro area, mobilisation runs roughly $1.50 to $3.50 per kilometre or $850 to $1,600 per travel day, and it is worth designing the programme so the cut-back visit does not double it.

On site, a discrete crack in a concrete wall is half a day to two days. A block wall of any length, particularly one where drainage is being remediated at the same time, is one to three days. Drainage works — excavating, jetting, replacing an ag line, connecting an outlet — is separate work with its own programme, and where excavation is involved it can exceed the injection cost.

What you should be handed at the end

The deliverable on a retaining wall is not just a dry face. It is a record you can produce in two years when something changes.

That means photographs before and after, taken from the same positions, and a written description of what we found and what we did.

It also means before-and-after moisture readings at marked points, so there is a measurement rather than an impression, and observation over at least 24 to 48 hours and ideally through a genuine rain event. In South East Queensland the honest test is the wet season between November and March, when the ground stays saturated for days rather than hours.

Finally, it should state in plain words what was not done. If the drainage behind the wall was not restored, that belongs in the handover document, because it is the thing most likely to bring the water back and everybody should have agreed on it in advance.

What it costs

Indicative only, and excluding GST. Build a line-by-line estimate for your own job with the price book, or read the working behind the rates in the 2026 cost guide.

Questions we actually get asked

There is white powder all over the face of my retaining wall. Is that damage?

That is efflorescence — salt left behind when water carrying dissolved minerals evaporates out of the wall. The salt itself is not damage, but it is proof that water has been moving through the wall, which makes it a useful diagnostic rather than a cosmetic problem. It matters technically as well: efflorescence and calcite deposits build up inside the crack or joint and physically block resin from penetrating, which is exactly why we flush with water before injecting rather than pumping resin into a blocked path.

Can you inject a block wall that was never core-filled?

Sometimes, but not by pumping expanding foam into it and hoping. Open cores are a void network of unknown volume connected to the ground behind, and a foam resin will disappear into them without ever sealing the face. The workable approaches are to grout-fill the cores as a separate priced item first, to use a permeating gel rather than an expanding foam, or to treat the wall as a drainage job instead. What we will not do is quote a per-linear-metre injection rate over a void nobody has measured.

My weep holes were rendered over when the wall was painted. Does that actually matter?

Yes, and it is one of the most common causes we find. Weep holes are the wall's pressure relief. Blocking them means water that gets behind the wall has nowhere to go except through the wall, which is the leak you are looking at, and it stays in the backfill, which is load the wall has to carry. Clearing them is cheap, it is reversible, and on more jobs than most contractors would like to admit it is a large part of the fix.

How much pressure is safe on blockwork?

Lower than on concrete, and the honest answer is that the wall sets the limit rather than the pump. Mechanical-packer polyurethane injection in reinforced concrete commonly runs 35 to 140 bar. Blockwork will blow a mortar bed out well below that, and a wall with partly filled cores gives you very little warning. We start at the bottom of the range and raise it only as the wall takes resin, and we stop the moment we see a joint opening, resin turning up somewhere unexpected, or spalling around a packer. On a slender or marginal wall, the maximum injection pressure should be nominated in writing by an engineer.

Can you inject the wall from the garden side instead of the exposed face?

If you can get to the retained face you usually do not need injection at all — that is the wet side, and the correct repair from there is a positive-side membrane applied to the back of the wall with the drainage cell and ag line reinstated behind it, which is a longer-lasting fix than sealing from the dry side. The entire reason injection exists is that on the great majority of sites the retained face is under a garden, a driveway or a neighbour's yard and excavating it costs several times more than working from the front.

The wall leaks all summer and is dry by August. Which resin should be used?

That seasonal dry-out is the deciding factor. Hydrophilic resin holds water inside its structure, which is what makes it grip damp concrete and chase fine paths — but a crack that dries out completely for months can cause it to shrink, and while it should re-swell when the water returns, shrunken foam can lose its grip in the crack and never fully recover. For a wall that dries out every winter, a hydrophobic resin is generally the safer selection because it is dimensionally stable through wet and dry cycling. There is a genuine, unsettled disagreement in the industry about this, and both sides of it are commercially motivated, so treat anyone who tells you it is a settled rule with some caution.

How long before I know whether it worked?

Immediately for the obvious part — resin appearing at the next packer tells us it travelled, and an actively running leak stops the same day. The real answer takes a wet season. A retaining wall is loaded by groundwater, and groundwater takes days of sustained rain to build to the level that caused the original leak. That is why we observe for 24 to 48 hours as a minimum, take moisture readings at marked points before and after, and treat the November to March wet season as the honest test.

Do I need council approval or a licensed contractor for this work?

For the injection itself, in Queensland trade work valued over $3,300 including labour, materials and GST requires the appropriate QBCC contractor licence, and it is worth checking the number against the QBCC register rather than a logo on a vehicle. Structural work on the wall itself, changes to its height or retained load, or work affecting drainage that crosses a boundary can raise separate approval questions, and those depend on your council and the specific wall. If your wall is in that territory we will tell you before quoting rather than after.