How to Install Posts in the Water for a Dock or Pier. Installing posts in the water isn't as crazy as it sounds. When you build a dock, the posts (pilings) need to be installed under the water to keep the dock stable. Aside from hiring a pile driver to install the posts, there are several ways for experienced do-it-yourself enthusiasts to accomplish this labor-intensive task. So, get some eager volunteers and be prepared to get wet.
Choose the best pilings you can afford. Pressure treated wood is the most cost effective to use as posts for your dock. Most professional dock builders believe that round posts are much easier to install and last longer because the wood isn't cut off. Cut off wood exposes the interior of the post and causes it to weaken. Other types of posts to use under water include plastic coated wood, heavy-duty aluminum and concrete.
Jet the water out if you're building the dock in sand or silt. This is the easiest way to install posts for your dock if you're doing it yourself. Rent an industrial type of power washer to remove the sand or silt. The process is done by placing the posts on the lake bed bottom and pound them down with a sledge hammer to a depth of at least 2 feet. Aim the jet at the bottom of the post to jet out the sand and water. As the sand is removed, the post will sink deeper.
Use heavy-duty PVC pipe to form concrete posts. The process is done by having two different diameters of pipe, one to fit inside the other. Make sure the length of pipe is long enough to go into the lake bottom by several feet. For ease of handling, it's recommended that shorter pieces of pipe are used and coupled together as it goes deeper. Place the 12-inch diameter pipe on the floor of the lake bed and hold steady (this is where the eager volunteers come in handy). Use a sledgehammer to pound the 12-inch pipe deep down as far as possible. Clean out the muck from the pipe with post hole diggers.
Place the smaller pipe, 6 to 8 inches in diameter, inside the wider pipe. Slowly pour concrete into the smaller diameter PVC pipe. As the pipe fills from the bottom up, pull the smaller pipe out in increments. This allows the concrete to go all the way to the bottom and form the base of the posts. The PVC pipe keeps the water out while the cement cures. The PVC pipe can be removed when the cement posts are cured. Holes can be drilled into the cement to attach the frame of the dock to the posts.