Piling studies

As part of a full range of services IHC Hydrohammer B.V. can perform a piling feasibility study for its clients to determine the best possible hammer and helmet combination for a given piling project.

The idea behind this service is to help clients in the project phase or tendering phase to make the best choice of hammer. For piling feasibility studies in the execution phase of a project and for other geotechnical consultancy it is best to engage recognised geotechnical consulting engineers.

In recent years IHC Hydrohammer B.V. has been using the TNOWAVE simulation program devised by Profound. At the request of IHC, Profound has incorporated different soil fatigue models into this program so that the program is better and more readily able to take account of changes in soil properties as a consequence of the piling process.

The program calculates the calenders to be expected, the maximum tensile and compressive stresses in the pile, the simulated soil resistance, the energy transmitted to the pile, losses in the helmet configuration, etc.

IHC Hydrohammer B.V. has published a 'blue' book containing a detailed analysis of the background to pile driving theory and an indication of how IHC hammer properties should be entered in a number of “piling formulas”.

Piling graphs showing the relationship between the calender and the resistance during pile driving are also given for all the hammers in the S-series, in combination with the most common tubular steel piles.

Energy information

There is a great deal of confusion surrounding the indication of the energy level of hammers. Many manufacturers, especially those that make diesel and (hydraulic) free fall hammers, indicate the capacity of the ram as the maximum potential energy the ram has on reaching its highest point during a stroke.
During the downward stroke, depending on the type of ram, all kinds of losses occur (mechanical, system engineering, compression, etc.), so the kinetic energy (energy resulting from velocity) at the point of impact may be considerably less than the potential energy given in the folders.

IHC Hydrohammer B.V. has decided, and finds it more realistic, to give the capacity of its hammers as the energy the hammer can actually deliver, that is the kinetic energy at the point of impact. To this end two proximity switches measure the velocity of the ram immediately before impact for conversion into kinetic energy using the formula Ek = &Mac189;.m.v2.

Ultimately it is all a matter of the energy that the hammer transmits to the pile. This 'Enthru' energy, also called 'stresswave' energy, is a function of the kinetic energy of the hammer, the momentum of the hammer at the point of impact and the losses that occur in the helmet configuration.

In the case of 'steel-to-steel' transfer these losses are almost negligible. But when sinking concrete piles they can amount to 25% because of energy dissipation into hammer cushion and pile cushion. Research at IHC in recent years has shown that by using the correct materials these losses can be reduced to 10-15%, so that the energy transmitted to the concrete pile using IHC Hydrohammers is 85-90% of the maximum kinetic energy of the hammer.

Piling formulas

'Piling formulas' are still in widespread use in many countries. The best known formulas used internationally are those of Hiley, Engineering New Formula and the Wellington formula. In the Netherlands we use the Dutch and the improved Dutch Piling Formulas

In the opinion of IHC Hydrohammer B.V. the use of piling formulas, to work out the permissible or failure bearing capacity of the pile on the basis of the mean drop per blow over the last 100-300 mm, is only acceptable if it is verified by means of test loads (performed locally).

Most formulas are based on the equality that the (potential) energy of the ram is the same as the work done by the soil. This equality is then corrected for (mechanical) efficiency of the hammer and the elastic properties of soil, pile and helmet cushion.

The majority of these formulas were devised nearly a century ago, at a time when free fall hammers (possibly steam driven), generally with a height of fall of 1-1.2 m, were the most commonly used.

The operating principle of the diesel and hydraulic hammers developed since then has not even led to a theoretical adjustment of the existing piling formulas. In many cases (countries) the best that has been done is the introduction of a kind of "practice" factor incorporating all the local and hammer engineering data.

If you want to make use of piling formulas, we recommend that you contact our head office in Kinderdijk for the modelling of the Hydrohammers in these formulas:

T +31 78 691 03 02 
info.hh@spam-preventionihcmerwede.com