The technique of installing a dock piling is known as jetting. The process involves using a high-power water pressure hose to literally dig a hole out of the soil that you can sink a piling into. You will need at least two people to complete this task, and this article only deals with repairing a piling in an existing dock where the soil below is sandy or soft.
Things You'll Need
High power hose, such as a gas powered water pump with a 2-inch or 3-inch hose
Obtain your pilings. These need to be chemically treated to prevent water damage and early failure. Make sure that the pilings are long enough to reach the dock above the water line after they are driven a minimum of 4-feet into the soil. Ideally, you should strive for 6-foot or 8-foot pilings to obtain optimal load bearing. The deeper you sink them into the soil, the better. If you don't go deep enough, the piling may float out of place. Make sure that your piles are of sufficient diameter to bear the load as well; if you are replacing an old pile, match or increase the diameter of the old one for small to medium watercraft.
Set up your water pump so that the suction hose is in clean water and will not be disturbed by the jetting. The discharge hose needs to be long enough to reach the bottom of the hole at the furthest pile from the shore.
Mark your jetting pipe and pile with index marks so that they can be kept at the same level while you are setting the pipe. At the end of your jetting pipe you will need an elbow at 90 degrees to direct the water underneath the piling as you drive it down.
Jet the water below the pile to displace the soil. As the soil becomes displaced, you can force the pile down in to the hole. You will need to work quickly enough to drive the pile down before the soil settles back into the hole. Work the jetting pipe fully around the pile to keep it centered on the location you desire.
Once the pile is in place, strap it to the dock with a rope or bungee cord and use two plumb levels or spirit levels to make sure it is true. Be sure to do this before the soil settles in, otherwise it will be very difficult to correct it. Once you are satisfied with the position, you can jet soil to back-fill the hole.
It will take at least two people to hold up a 12-foot by 8-inch piling, anything larger will require a crane. For depths beyond 6-feet, additional weight may be required to overcome the buoyancy of the pile. Do this by attaching concrete at the top of the pilings to force them down into the hole.
Be sure to wear protective gear to prevent injury, especially abrasions