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Casting of concrete

Concreting in "dry" boreholes is usually a straightforward procedure. On completion of the borehole, the reinforcement is placed with suitable spacers to locate it centrally in the pile shaft. During concreting, the casing is withdrawn, always maintaining a head of concrete within the casing. A hopper and tube should be used to ensure that the mix is not directed towards the reinforcement.

The tremie method of concreting is often employed when casting under water . This is a practical method of forming a pile shaft provided certain precautions are taken. It is essential that a slump mix greater than 175 mm is used, and that the tremie pipe is always kept well below the water/concrete interface during concreting. The tremie pipe and hopper connections should be watertight and in clean condition to permit free flow of the concrete. A minimum internal diameter for the tremie pipe of 150 mm is suggested for use with concrete having a maximum aggregate size of 20 mm, increasing for larger aggregates. A plastic balloon or poly-styrene plug should be used in the tube between the water and first batch of concrete, and immediately before placing the tremie pipe in the pile bore, a check on sedimentation should be made. Excess sediments should be removed by air-lifting. When the pile cut-off level is above ground, concrete should overflow from the pile head on completion. The upper part of the pile consists usually of weak concrete and may also be contaminated from boring detritus. In the case of a pile cut-off below ground, the concrete level should be raised to allow for around 0.5 to 1.0 m for trimming off the weak concrete.

Sudden drop in concrete level, as the casing is withdrawn, should be avoided and such incidents be noted. This could indicate that the soil strength is too low to withstand the weight of the concrete column in the pile shaft. In soils, such as alluvial clays or loose water-saturated sands, it is possible for the fluid pressure of the wet concrete in the borehole to fail the soil, and a bulge can occur in the pile shaft. An undrained shear strength of about 15 kN/m2 has been observed to result in this effect. As a result, excessive flow of concrete can occur, resulting in a zone of concrete over-break and/or necking of the pile shaft. Also distortion of pile reinforcement can occur, whereby the steel cage is moved downwards at the zone of over-break. Consequently, contaminated or segregated concrete can occur in the pile shaft, a feature which sometimes is mistakenly ascribed to the cross-flow of ground water. To prevent this situation to occur in unstable ground, the bore hole should be cased and boring ahead of the pile casing should be avoided.

The risk of soil failure can be reduced by pulling the casing at a steady rate, rather than in short rapid intervals. Limiting the head of concrete in the casing is adisable, provided this can be done without risk of the concrete slumping below the bottom of the casing in the bore.

By using a self-compacting concrete mix, it is not necessary to vibrate the concrete in the borehole during casting. A vibrator should not be used inside the pile casing, as this can affect the stability of the soil surrounding the pile shaft. It is sometimes considered beneficial, however, to vibrate the casing while being withdrawn, but this should be done with suitable equipment, otherwise mixing of the soft concrete and the soil can occur.

In a stratigraphic succession in which water-bearing gravel overlies a stiff clay, a temporary casing is used to support the borehole through the gravel, and is then driven a short distance into the clay in order to seal off the water. Boring then continues under dry conditions.



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