The Blue Mountains receives some of the highest rainfall of any urban area in NSW. East-coast lows regularly deliver 80-150mm in 24 hours to the region, and extended wet periods, weeks of above-average rainfall, are common in La Niña years. After any significant rainfall event, a post-rain inspection of your retaining walls is one of the most practical maintenance habits you can develop.
Retaining walls fail under load. The heaviest load they ever carry is after prolonged or intense rainfall, when the soil behind them is saturated. The post-rain window, typically 24-72 hours after the rain stops, is when failures are most visible, because the wall has been under maximum stress and any movement or drainage failure will have manifested as visible change.
When to Do a Post-Rain Inspection
Not every shower warrants a wall inspection. The trigger events that justify a deliberate inspection are:
- 50mm+ rainfall in 24 hours, this is enough to significantly increase soil moisture and lateral pressure on retaining walls
- Extended wet periods, more than 5-7 consecutive days with above-average rainfall, even if individual daily totals aren’t extreme
- East-coast low events, the Blue Mountains regularly receives 100-250mm during ECL events; always inspect after these
- First major rain after a dry summer, soil that has dried and cracked over summer can transmit water very rapidly when it first wets up, creating surge drainage loads
- After any nearby earthworks, earthworks on your property or an adjacent property change drainage patterns, which can affect established retaining walls
Safety First, Do Not Approach Unstable Walls
Before inspecting, observe the wall from a distance (10-15 metres) first. If the wall shows:
- Major forward lean that was not previously there
- Partial collapse or soil spilling from behind the wall
- A crack opening through the wall
- Differential settlement (the ground on the high side of the wall has dropped)
Do not approach the wall until it has been assessed by a professional. A wall under active movement can complete its failure suddenly. Keep people, pets, and vehicles clear of the collapse zone, the area below the wall face extending to approximately twice the wall height in front.
What to Look For, The Post-Rain Inspection
Work through the following checks from a safe vantage point and then, if no major structural concerns are visible, closer up:
Check 1: Wall Lean and Plumb
Look along the wall face from one end. The wall face should be approximately plumb (vertical) or slightly battered (leaning back). Any forward lean that was not present before the rain indicates movement.
- Minor new lean (5-15mm over 1.0m height): Monitor closely. Re-check in 48 hours. A small amount of movement that has stopped may stabilise, or it may be the beginning of progressive failure. Get a professional assessment within 4 weeks.
- Moderate new lean (15-30mm): Do not use the area above the wall. Get a professional assessment within 1-2 weeks.
- Significant new lean (>30mm) or visible progression: Clear the area and get an emergency assessment. This wall may be hours or days from failure.
Check 2: Drainage Outlet
Go to the drainage outlet for each retaining wall’s ag pipe system. After heavy rain, water should be flowing (or have been flowing) from the outlet. Check:
- Is the outlet free of debris (leaves, sediment, spider webs)?
- Was there any flow during or after the rain? (Wet soil or staining around the outlet indicates flow.)
- Is the outlet discharging normally?
A drainage outlet that shows no evidence of flow after heavy rain when the soil behind the wall should be saturated is a red flag. Either the ag pipe is blocked, it was never installed, or the drainage system is overwhelmed. All three situations need attention.
Check 3: Soil Settlement Behind the Wall
Look at the ground surface immediately behind the wall (the retained side). After a rain event that stresses the wall:
- New crack or gap between the soil surface and the back of the wall capping: This indicates the wall has moved forward, opening a gap from the soil mass
- Settlement or sinkhole forming behind the wall: Soil is being lost through the drainage system or through a gap in the wall, either fines are migrating through the drainage aggregate (filter is failing) or the wall has a breach
Check 4: Weep Holes
If the wall has weep holes (circular or rectangular openings near the base of the wall), check whether they are draining. After significant rainfall, active flow from weep holes is normal and healthy, it means the drainage system is working.
Weep holes that are dry after heavy rain despite a saturated hillside above the wall suggest the holes are blocked or the drainage aggregate behind the wall is saturated and bypassing the weep holes entirely.
Check 5: Wall Face Condition
Inspect the wall face for:
- New cracks: Horizontal cracks running along mortar joints in masonry walls indicate the wall face is trying to rotate forward, this is a structural warning
- New vertical cracks: Vertical cracks in block or masonry walls can indicate differential settlement between sections of the wall
- Displaced panels or stones: Any panel or stone that has moved from its original position
- Rust staining: Fresh rust staining below H-post positions indicates the galvanising on the posts is failing, this is a longer-term structural concern but worth noting
Check 6: Water Ponding Behind the Wall
If you can safely view the area behind the wall, look for water ponding. Water sitting against the back of the wall, rather than draining away, is a drainage failure indicator. This is a serious finding: the hydrostatic pressure from standing water against a retaining wall is very high.
Documenting What You Find
After any post-rain inspection where you find changes:
- Take dated photographs from multiple angles
- Note the rainfall event (date, approximate amount) that preceded the inspection
- Note any changes from previous inspection
- Record whether drainage was flowing
This documentation is valuable if:
- You need to make an insurance claim
- You’re having a professional assess the wall (they need to know when movement occurred)
- There is a dispute about when damage occurred (neighbour property damage, sale of property)
Acting on What You Find
| Finding | Action |
|---|---|
| Everything looks normal, drainage flowing | No action required. Log date of inspection. |
| Outlet blocked, clear it | Clear immediately. Monitor drainage flow. |
| New minor lean, stable | Monitor weekly for 4 weeks. Get assessment if progression observed. |
| New moderate lean | Restrict access above wall. Professional assessment within 2 weeks. |
| New significant lean or movement | Clear area below wall. Emergency assessment. |
| Water ponding behind wall | Excavate drainage outlet area to clear blockage or call professional. |
| Partial collapse | Clear area. Emergency call. Report to BMCC if public safety risk. |
| Timber post rot revealed by water movement | Arrange replacement quote. Restrict access above wall. |
See [/guides/retaining-wall-failure-warning-signs/] for the full guide to warning signs and [/guides/retaining-wall-drainage-failures-causes-fixes/] for drainage failure assessment.
After Major Events, Professional Assessment
After a significant East Coast Low or other major weather event affecting the Blue Mountains, many properties will have retaining walls under stress simultaneously. This can mean wait times for professional assessments are longer than normal.
If your wall shows moderate to major movement after a significant event:
- Act promptly, early contact with a contractor secures a place in the assessment queue
- Document thoroughly with photos to support your insurance claim
- Do not allow use of the area above or below a wall showing active movement until it is assessed
For urgent assessments after weather events, contact us at [/contact/#quote], we prioritise emergency calls after major Blue Mountains weather events.
FAQ
My retaining wall was fine before the rain and is now clearly leaning, has it failed? A wall that shows new lean after rainfall has moved under load. Whether this is a temporary movement that will stabilise or a progressive failure in progress depends on the extent of lean, the drainage condition, and the wall type. Do not assume it will settle back, get a professional assessment.
Should I prop my leaning wall with timber to prevent further movement? Temporary propping is sometimes done as an emergency measure for short walls (under 1.0m) while awaiting professional assessment. For walls over 1.0m showing active movement, temporary propping is not a reliable safety measure, the forces involved can fail timber props. Clear the area and wait for professional assessment.
My drainage outlet was flowing during the rain but the wall has still moved, what does that mean? It means the drainage system was functioning but the volume of water in the soil exceeded the drainage capacity, the soil became saturated despite active drainage. This is a capacity or design issue, not a blockage. The drainage system may need upgrading (larger ag pipe, interceptor drain above the wall) rather than just clearing.
How long after rain should I wait before inspecting? The best inspection time is 24-72 hours after the rain stops. At this point: the soil behind the wall is still at its maximum saturation (maximum load), any drainage flows are visible, and any movement that occurred during the event has stabilised enough to assess.
Do I need to call someone if my wall appears fine after heavy rain? Not necessarily, if everything checks out and the wall looks the same as before the rain, no action is required. However, if you notice anything new (crack, lean, soil movement), follow up. A brief call or email to describe what you’ve seen costs nothing and may save you from a future emergency.
Contact Blue Mountains Retaining Walls for a post-storm assessment, /contact/#quote