CROSS Safety Report
Loading reinforced masonry wall
This report is over 2 years old
Overview
A 2.5m high retaining wall showed signs of movement not long after backfilling, and it was found that the deflection was due to creep in the bed joints of the immature wall.
Key Learning Outcomes
For the construction team:
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Backfilling of any retaining wall should not be done until after adequate strength has been achieved
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Early age calculations can be undertaken to assess this if required, as when backfilling needs to be early
For civil and structural design engineers:
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Contract specifications should stipulate the basis on which loading is permitted on new construction where there are hardening processes involved to avoid this type of problem
Full Report
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A 2.5m high retaining wall was constructed as a reinforced masonry wall incorporating reinforcement in pockets on the back. Access requirements meant that the wall was backfilled two days after construction. A week later it was noted that the wall was no longer plumb. Investigations included excavating back to the base slab which had remained horizontal and it was found that the deflection was due to creep in the bed joints of the immature wall.
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Expert Panel Comments
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This is another example of early age loading - see report 177. With any retaining wall backfilling has to be done after adequate strength has been achieved. Early age calculations can be undertaken to assess this if required, as when backfilling needs to be early.
This may have consequences for the original design (e.g. thicker wall, higher strength materials used, higher designation mortars used). The reporter suggests that there was ‘creep’ in the bed joints but it may have been horizontal slippage (bond failure) or shear strain. Contract specifications should always stipulate the basis on which loading is permitted on new construction where there are hardening processes involved to avoid this type of problem.
Over the years a recurring theme has been trouble with small retaining walls and how they perform, and a number of collapses have been reported to CROSS and others. These are not major structures but the failure of a 2.5m high wall is sufficient to kill (as has been seen in previous reports), and such retaining walls, and indeed free standing walls, warrant proper engineering design and construction which includes consideration of backfilling. Failure can occur just after construction or can take years to occur.