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Plunge Columns (for top-down construction)
Construction Sequence
1. From ground level, construction of the rotary bored pile to the required design depth.
2. Lower the reinforcement cage into position.
3. Plunge Column Frame on the temporary casing:
- Position and fix locating pins onto casing in line with plunge column axis.
- Use locating pins to centralise the plunge column frame within the pile bore.
- Use the hydraulic "jack out" legs at the base of the frame combined with verticality lasers to align the frame to vertical.
4. Pour concrete into the pile bore to the concrete cut off level.
5. Lowering the plunge column through the frame, using the frame's hydraulics to centralise the plunge column to structural tolerances as required within the frame.
Complete 'plunging' the column into wet concrete to the correct level and carry out a final check on position.
6. Withdrawal of frame
7. To keep the column in position, backfill pile bore with pea gravel above the top of concrete level and evenly around the plunge column.
8. Remove the temporary casing.
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Fields of application
Effective for top-down construction allowing for simultaneous deep basement excavation as well as superstructure erection, prior to the completion of basement excavation. Used in deep basement construction to form structural steel column projecting from the lower basement level foundations up to the approximate ground level.
Heavy and extensive strutting system minimized as the plunge columns may incorporate connection fixing points to allow the attachment of struts or beams. Plunge columns are also used to support the basement floor slabs which prop the excavation sides in the permanent condition. By propping the excavation with the permanent basement floor slabs in lieu of temporary props, an efficient design for the basement is created.
Environmental impact
Accelerated construction: creates significant saving of the overall construction time by realizing the superstructure at the same time as the basement excavation, thereby reducing disruption to the local area.
The top-down construction technique creates a more efficient permanent works design for the basement walls, saving significant quantities of steel and concrete.
No large working space required
Technical specifications
Usually incorporated in pile diameters of 1200mm and above, when positioning frames are used.
Plunge column frame accommodates plunge columns sizes up to 456 x 406mm.
Complicated construction process (correct positioning of plunge column frame and plunge column).
Very high levels of accuracy achieved: a vertical tolerance of 1:400 and a plan positional tolerance of 25mm are achievable with the benefit of the plunge column frame.
Substantial cost-saving in the overall project.