Inspection of a leaning retaining wall and uneven floors in Mount Duneed

The brief

We were asked to inspect a house in Mount Duneed ahead of further renovations. The job covered a timber retaining wall along the side yard and floors that were out of level, along with other visible defects.

The inspection was to assess the structural condition of the house, look at the visible defects inside and outside, consider what might be causing them, and recommend the next steps: further investigation, monitoring and repairs.

Why it needed a structural engineer

A leaning retaining wall, out-of-level floors and cracked cornices can share a cause, or have separate ones. The report weighed several possible factors: movement of the filled ground the house is understood to have been built on, differential foundation movement, moisture and drainage near the house, pressure behind the retaining wall, and localised building movement and material deterioration. Sorting these out shows what needs repair and what needs monitoring first.

Drainage is part of the retaining wall question. The explanatory information to Part 3.3 of the ABCB Housing Provisions in the National Construction Code says subsoil drainage may be needed where subsurface water could cause damage through a build-up of excessive moisture or lateral water pressure, and it lists behind retaining walls as a typical location. At this wall, no drainage provisions were visible.

What we checked

The inspection was visual and non-destructive, and it covered only the elements exposed at the time. It included:

• the accessible areas inside and outside the house;
• the reported floor-level survey measurements, and laser-level readings of the floors;
• visible cracks, cornice and skirting separation, and how the doors operated;
• the side yard and the timber retaining wall.

The original structural design capacities were not verified, because that was beyond the scope of a visual inspection.

What we found

The floor levels varied by about 30–35 mm across the house, with minor cracking and separation inside, and the timber retaining wall had moved locally. No significant stepped cracking or other visible evidence of major structural distress was seen.

Area	What was seen	Possible cause, according to the report
Floor levels	A variation of about 30–35 mm across the house, with levels generally lower toward one side	Fill that varies in depth, composition or compaction may settle unevenly over time; the lower side may also point to differential movement from changes in ground moisture, localised softening, erosion or differences in footing support. The fill's condition and compaction were not verified.
Cornices and ceilings	Separation and cracking at cornices in the living area and bedrooms, and localised ceiling cracking	Building movement, localised movement of the finishes, or a combination of these
Skirting boards	Minor separation between sections of skirting and the adjoining surfaces	Building movement, localised movement of the finishes, or a combination of these
Doors and windows	Minor cracking around some openings; minor sticking reported at some doors	Building movement, localised movement of the finishes, or a combination of these
Articulation joints	Deteriorated sealant at some joints	Deteriorated sealant may reduce a joint's ability to take up movement and keep water out
Drainage beside the house	Surface drainage pits in the paving next to the house	Water near the house could affect the supporting soil; the pits' condition, connection and effectiveness were not verified
Timber retaining wall	Leaning or bulging outward; several nails connecting the boards to the posts partly withdrawn or disengaged; no visible drainage	Lateral pressure from the retained ground, which water collecting behind the wall could increase. The part played by drainage, post condition and the wall's construction can't be determined without further investigation.

What we recommended

The report recommended rectifying the retaining wall and adding drainage to it, and measuring and monitoring the floors and cracks before any decision about the foundations.

• Retaining wall. Assess the affected section and repair or rebuild it, rather than just replacing the withdrawn nails. Check the timber posts, boards and connections for deterioration, splitting, decay and loss of structural capacity. Replace loose or disengaged fixings with suitable corrosion-resistant structural fixings, and replace posts or members that are significantly deteriorated, displaced or inadequate. Reinstate the wall to an appropriate alignment with enough capacity to resist the retained soil pressures. If it can't reasonably be returned to a structurally adequate condition, replace the affected section with a properly engineered retaining wall system, based on the actual retained height, ground conditions and surcharge loads next to the wall.
• Temporary precautions. Consider them if the wall shows significant progressive displacement, or if people regularly use the area right next to it.
• Retaining wall drainage. Build drainage into the repair: free-draining granular backfill directly behind the wall, geotextile filter fabric where required, a subsoil drainage pipe at the base of the retained ground, and positive discharge to an appropriate lawful drainage point. Direct surface water away from the wall where practicable. The report notes that water pressure behind a wall can substantially increase the lateral load on it.
• Floor levels. Carry out a detailed floor-level survey across the house as a baseline, then monitor the floor levels and representative cracks for about 6–12 months, where practicable, to see whether movement is continuing. If progressive movement is found, a geotechnical investigation should establish the nature and depth of the fill, the natural material below and the founding conditions. If ongoing differential movement is confirmed, a structural engineer should use those results to decide whether footing strengthening, underpinning or other foundation work is needed. Major foundation work or underpinning shouldn't be done on the strength of the visual inspection alone, without confirming the cause and extent of the movement.
• Cornices, walls and skirting. Monitor the cracks before covering them up: photograph them and record their approximate widths. If they stay stable, remove loose material and repair them with flexible fillers, plaster repair systems or sealants, and refix the skirting where it has separated. Leave repainting until the cracks have shown reasonable stability. If cracks keep widening or come back after repair, further structural investigation is needed.
• Doors. Check sticking doors for frame distortion or out-of-square frames. Minor adjustments to hinges, latches or clearances may be made where the surrounding structure is stable, but avoid significant trimming until the movement is known to have stopped. Doors that get progressively worse are another sign to include in the monitoring.
• Articulation joints. Remove the deteriorated, cracked or debonded sealant, clean the joint, add backing material where required, and reseal with a flexible external-grade sealant that suits the masonry or render. Don't fill the joints rigidly with mortar or anything else that stops them moving.

The result

The report concluded that, at the time of the inspection, the house had a floor-level variation of about 30–35 mm, minor cornice cracking, skirting separation, door operation issues and localised movement of the timber retaining wall, with no significant stepped cracking or other visible evidence of major structural distress.

It recommended the retaining wall repairs, the drainage works and the ongoing monitoring to prevent further deterioration and reduce the risk of future structural or safety issues. That gives the client a clear plan: rectify the wall and add drainage, and set up a baseline survey and monitoring before any decision about the foundations.

Leaning retaining wall or uneven floors in the Geelong region?

Our structural inspections cover cracked houses and building movement across Geelong and the surrounding region. Our guide to retaining wall failure explains the common warning signs and how poor drainage builds up pressure behind a wall, and our guide to foundation investigation covers what a foundation investigation looks at, including crack patterns and floor-level variations, and when monitoring or soil testing is added. Send us photos of the wall, the cracks or the floors, and tell us when you first noticed them, through our contact form, or call 03 4250 8105.