CROSS Safety Report
Partial failure of precast concrete tank unit on installation
This report is over 2 years old
Concerns are raised after the foot of a precast concrete unit failed during installation, making the unit unstable.
The use of precast concrete units to construct water retaining structures is becoming more common but their safe installation is not as straightforward as may be suggested.
Key Learning Outcomes
For precast installers and temporary works designers:
Consider temporarily propping precast units prior to their incorporation into the permanent structure, particularly when shims are required under the main body of the units
Heavy precast units should be propped and secured prior to disengaging the lifting slings
Competent supervision on site to ensure the even distribution of shims under the precast units can assist in their safe installation
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The use of precast concrete units to construct water retaining structures is becoming more common. Their safe installation is not as straightforward as may be suggested, says the reporter.
During construction of such a tank, one of the precast units’ integral stabilising feet failed as the unit was being installed. The tank comprised units up to 8m high which were placed on an in-situ base slab. The foot failure was attributed to the use of shims and a steel wrecking bar used when attempting to plumb the unit.
Prior to the incident, eight units had been placed without damage. A unit was then lifted into place and with the load still on the crane, it was plumbed by placing shims under reinforced concrete (RC) stabilising feet. A steel bar was used as a pry to help placing the shims. As the load from the crane was released, one of the feet failed, making the unit unstable (Figure 1 & 2).
The use of shims, particularly under the body of the unit (as well as the feet) raised concerns regarding the temporary stability of the units prior to their incorporation into the parent structure. After an investigation, the following measures were taken:
Where shims were required under the main body of the unit, temporary props were provided to stabilise units
Construction supervision was increased to ensure even distribution of shims under the units and the addition of grout where necessary
The use of shims, particularly under the body of the unit (as well as the feet) raised concerns regarding the temporary stability of the units prior to their incorporation into the parent structure
Many of the issues might have been negated by the provision of a flatter base slab making excessive shimming unnecessary. That said, the provision of a suitably ‘super flat’ slab and the residual requirement to place shims under the unit’s feet would negate many of the advantages otherwise attributable to precast concrete construction of tanks.
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This shows the need to consider the practicalities of installation when developing an overall design. All construction has tolerances, and all items (whether in precast concrete or other material) will require adjustment to achieve alignment, so the fittings need to be ‘robust’.
A robust design also ought to ensure there is no gross change in state consequent on minor damage. In this case, there should have been no overturning risk if any of the items attached to aid stability had been damaged. Combinations of adverse tolerances need to be considered as part of component design.