IHC IQIP has identified and quantified a number of negative effects of driving on water. Losses in piling efficiency can lead to early refusal.
Unless special precautions are taken while driving submerged piles, the anvil will come into contact with the water column inside the pile. This may also be experienced when driving submerged conical piles with the hammer and the pile top protruding above water (see attached figures). The enclosed volume inside the pile decreases during penetration, forcing trapped water to elevate.
IQIP’s research shows that the hammer strike is distributed over both the pile and inner water column. Energy entering the water column does not contribute to pile penetration and as the impedance of pile and entrained water is larger than that of the pile alone, the efficiency of transferring the blow energy is significantly reduced.
The proportion of energy loss is dependent on the configuration. In the example analysed by IQIP (cylindrical pile with a diameter of 4 m) the two modes of energy loss compound to values of approximately 20%. For larger diameters, this number will increase. In the case of conical piles, a portion of the entrained water must be lifted, leading to losses of approximately 1%.
In addition, there is a loss involved in loading both the inner and outer water column by the peristaltic motion of the pile. First estimations indicate losses of 5%, but this loss may also occur without driving on water and is still under investigation.
The field data, where reductions in hammer blows and installation time were observed, confirms IQIP’s findings. During the field tests, hammer blow records from the installation of eight cylindrical piles, partly perforated with dewatering holes and partly without, were compared. The cumulative blow energy required for perforated piles was approximately 20% lower than for the piles without such holes.
In summary, contact between the anvil and water must be prevented. This can be achieved either by use of a follower or by maintaining an air gap between the anvil and water.
For the latter, IQIP can offer support by designing a system with a perforated pile wall, assuring a sufficient dewatering flow while maintaining the integrity of the pile design.