CROSS Safety Report
Accidental concrete infill around a boxed out plunge column
Overview
A problem occurred during a concrete pour to a boxed out temporary column resulting in an area of unsupported suspended concrete slab.
Key Learning Outcomes
For the construction team:
- Concrete pours must have formal planning and experienced supervision
-
The installation of plunge columns is a specialised operation requiring thorough planning and competent execution
Full Report
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A plunge column was boxed out for the pour of a slab. Couplers were installed around the box out so that the rebar could be reinstated once the plunge column had been removed. During the pour, the box out (which contained no rebar at the time) was accidentally filled with concrete, so when the formwork was struck, the only support for that area of slab was the adhesion to the faces of the ply box out and the plunge column. When this was noticed, the area was hoarded off and the concrete inside the box out was safely removed.
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Plunge column description
A plunge column is a structural steel or concrete section embedded in a freshly poured concrete pile, eliminating the need for baseplates and holding-down bolts. The technique is especially suitable for top-down construction and is usually carried out by, or in association with, specialist groundworks contractors. References and examples may be found on the internet.
For example the following his description is taken from a technique sheet produced by Balfour Beatty Ground Engineering: 'Generally the construction process is as follows; a rotary bored pile is excavated to the required depth, a reinforcement cage is then installed and then the void is filled with concrete stopping the required distance below ground level. Whilst the concrete is still wet a hydraulic plunging frame is installed within the temporary casing and then the plunge column is installed to structural tolerances. The concrete is then left to set in the pile and once this is complete the empty bore above is backfilled. When all the plunge columns are installed the construction of the structural frame above ground level can begin.'
Leaking box outs are fairly common and the risks to later construction should be noted in the method statements and procedures put in place to check and carry out remedial works before the next stage of construction. This was an accidental filling rather than a leak but the principles would be the same.
The safety issue was that a portion of structure, a slab, was temporarily unsupported from below. This emphasises the benefit of experienced supervision and having people around who can assess things on the basis of common sense and intervene when needed.
CROSS will be interested to know of any safety problems associated with the installation of plunge columns.