The capability of hydraulic vibratory pile driving hammers and other techniques using the principles of vibration have so far been underestimated by offshore foundation engineers. In many installation projects vibratory hammers should be considered as a real alternative, in other projects as a complementary tool to impact hammers.
A simple approach for the prediction of vibratory pile driveability is presented and some considerations are given on the static bearing capacity of vibratory driven piles.
Possible laboratory tests, in-situ tests and mathematical modellings are discussed which will make the use of vibratory hammers as reliable ~s the present confidence level in impact hammers.
As the techniques of vibratory pile hammers have not enjoyed wide use within the offshore industry, a short review will be given on the principles of operation, the basic hammer characteristics and the main differences between conventionally used impact hammers.
The paper concludes with a discussion on the use of vibratory hammers for the purpose of ground-treatment in a similar manner as is widely accepted on onshore construction sites.
Vibratory pile driving hammers are onshore as widely used as impact hammers. In all cases where low-displacement piles such as sheet piles, H-beams and open-ended tubular piles have to be driven, the vibratory hammer has proven to be a much more economical tool than an impact hammer.
Apart from being much faster, it operates at a low noise level, introduces much lower accelerations in the surrounding soil or guiding structures and is considerably lighter in weight than an impact hammer. For offshore piling operations the hammers have a number of specific advantages over impact hammers, such as:
pile and hammer are clamped together and can be handled as one unit;
hammer easily adjustable to different pile diameters up to 3.5 m;
no offshore cage required for surface pile driving;
no pile-sleeve follower required for underwater operations;
capability of extracting piles;
no temporary guide frame required to drive free-standing piles. The vibratory hammer can be activated even before the pile-tip reaches the seabed;
the same vibratory hammer can be used for deep compaction of soils and other soil improvement projects.
A limitation of the vibratory hammers is the maximum pile weight. The heavier the pile the more energy is required to induce vertical vibration. This has been the main reason why these hammers have only had limited use so far on offshore installations.
The steadily increasing waterdepths at which oil- and gas reserves could economically be exploited, accelerated by the oilcrises, forced the offshore industry to consider other platform (production) concepts as an alternative to the huge, heavy and expensive conventional jackets.
Small modifications are necessary to operate with these hammers in a tandem combination doubling above values and being capable to drive piles up to 200 tons. (Fig. 3).