Selection of a the proper size hammer is always important - the pile needs to keep moving with each blow. Movement of 1/8" to 1/4" per blow is often recommended.
In some circumstances, jetting or preboring can help. You may want to look thru some of the pile driving documents posted at the link below.
Can you give us more specifics about your project?
Is the problem with vibrations human perception or are you concerned that the vibrations will cause actual damage?
The obvious solution is to use augercast piles so there are no vibrations from pile driving.
I was recently involved with a job where piles were driven within 5 feet of an existing sewerline. Vibration monitoring was done and the actual vibrations were low enough that no action needed to be taken to reduce the vibrations. The piles were driven H-piles to rock and the soil above rock was loose to medium dense alluvial deposits. If vibrations were a problem, we were planning on preboring to loosen the soil and reduce the energy required to drive the piles.
Thank you for the reply. Augercast piles/preboring might not be possible options since sheet piles are going to be driven. The sheet piles are to be driven about 5 feet into sand and 2 to 4 feet into stiff clay. The problem is that there are structures nearby (100-150ft). We had previously assessed the situation and recommended a pre-construction survey and construction monitoring as pile driving vibrations might be an issue. The client wants to know if there are any measures that can be taken during construction to reduce the vibrations.
Cut off trenches help reduce (but don't eliminate) vibration. 100 to 150 ft is fairly far from the action. it is unlikely that driving 7 feet will cause detrimental vibration, althouh the vibration will be perceptible. Drivig with an impact hammer instead of a vibratory will reduce vibration.
I would try driving with a small vibratory and do some vibration monitering. If the results are not good you can try a pilot trench. If you still are not satisfied, you can switch to an impact hammer.
My feeling (with out knowing all he particulars) is that you will be okay with a small vibro.
I agree with DRC1. The project I was involved with had us driving sheet pile 30 feet from residential structures and much deeper than 9 feet. We drove some test piles in a location where the piles were further from the houses and monitored the vibrations (using a good size vibratory hammer I might add.) The vibrations were not as bad as we feared, so the project could go ahead. I would still do the pre-construction survey though. People will feel the vibrations and suddenly discover cracks in their walls that they never noticed before.
We found the opposite is true. In downtown Chicago, it is better to quickly drive a larger vibro for a shorter duration than a smaller vibro for a longer duration when it comes to the human perception issue (got less complaints).
Fortunately you are quite a ways from the action as others have noted. Set up a vibration monitoring device (portable seismograph) and set triggers for the vibration limits you want to prevent. Set the transducers between the pile driving and the structures at question, as well as on the structure itself. This will give you a record of actual vibration levels for future reference in the event damage occurs or is "perceived" to have occur. Do a thorough pre- and post-event condition assessment, including monitoring the crack widths and lengths of any observed cracks in the structure.
