Background
A residential property in South-East Queensland had a concrete sleeper retaining wall constructed to provide support to a landscaped area. The retaining wall, approximately 1600mm in height and 25 metres in length, was constructed by a licensed concrete contractor engaged under the Queensland Building and Construction Commission (QBCC) framework.
Following completion of the retaining wall, a reinforced concrete slab was placed directly behind the wall to form part of a pool deck area. Shortly after loading the wall with the slab, significant cracking and deflection of the retaining wall were observed, leading to a partial collapse of the retaining wall.
Incident
- Within a short timeframe of the slab placement, distress signs developed in the retaining wall.
- Lateral displacement of sleepers, tilting of the wall, and eventual collapse of a section.
- Safety risk to occupants and loss of structural integrity of the retaining structure.
Cause Analysis
Our engineering investigation identified the following technical factors contributing to the collapse:
- The retaining wall was not designed to support surcharge loads from a rigid concrete slab.
- Additional vertical and lateral loads induced by the slab exceeded the wall’s capacity.
- The contractor neither engineered the slab nor the retaining wall in accordance with AS 4678 – Earth-retaining structures.
- Water build-up behind the wall during rainfall events further exacerbated the failure.
- No evidence of certification or RPEQ review of either the retaining wall or the slab engineering design.
- The slab construction commenced without technical verification of load compatibility.
QBCC Involvement
Upon complaint lodged by the owner to QBCC and an inspection, the QBCC directed the contractor to rectify the defective works, as the collapse demonstrated clear non-compliance with minimum construction standards and lack of due engineering consideration.
Rainbow Engineering was engaged to provide independent technical support to:
- Determine the root cause of the collapse,
- Develop engineering rectification solutions compliant with the relevant Australian Standards and NCC.
Engineering Rectification Solutions
Our recommended rectification strategy included:
- Controlled demolition of the collapsed retaining wall and removal of failed elements.
- Provide advise of the most suitable retaining wall solution as concrete sleepers wall is unsuitable in this case
- New engineered retaining wall design in accordance with AS 4678 to site geotechnical conditions.
- New engineered slab design in accordance with the relevant Australian Standards and NCC requirements.
- Incorporation of geotextile fabric, free-draining granular backfill, and agricultural drainage system.
- Surface drainage diversion away from the retaining wall structure.
- Rectified works to be RPEQ certified and inspected during construction for each certifiable stage.
- Post-construction monitoring is recommended to assess wall performance during settlement period.
Conclusion
The collapse of the retaining wall was directly attributed to inadequate engineering design and improper loading from the deck concrete slab. The incident highlights the importance of RPEQ-certified design, load consideration between structures, and compliance with Australian Standards in construction.
With Rainbow Engineering’s technical input, a robust rectification solution has been developed to ensure long-term structural stability, compliance, and safety.