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Choose pilings made of pressure-treated wood. Rent a 2–3 in (5.1–7.6 cm) water jet from a home improvement store. Spray paint your pilings at 12 in (30 cm) intervals to monitor their depth. Measure the beams according to the height of your pier or dock. Stand the first piling in the water. Aim the tip of the water jetter at the bottom of the piling and turn on the pump.
In addition to remaining constantly waterlogged, your pilings will be subjected to tiny organisms that feed on wood, so you need something that will hold up over time. Pressure-treated wood has been preserved with special chemicals that will help it last longer when it’s exposed to harsh outdoor conditions. It's also the most cost-effective option.  Other options for pilings include plastic-coated wood and heavy-duty aluminum. Typically, pilings should be 6–8 in (15–20 cm) in diameter, but if your deck will weigh more than 10,000 lb (4,500 kg), choose 10–12 in (25–30 cm) pilings.  You’ll need to place a post about every 10 ft (3.0 m) along the dock in order to support its weight. Often, your local home improvement store will allow you to rent high-powered tools like a water jetter (which may also be called a hydro jet) for a short amount of time. This will prevent you from having to pay full price for a tool that you won’t use very often.  The cost of renting a water jet will vary depending on your location and the length of time you need it. These high-powered water pumps typically run on gasoline, and they come with either a 2 in (5.1 cm) or 3 in (7.6 cm) diameter hose. Either size will work, but since the hose gets heavy when it’s full of water, the 2 in (5.1 cm) hose may be easier to use.  If you don’t have access to a water jetter, you may be able to use a high-pressure garden hose instead, but the process will probably take longer. As you start to sink the pilings, it can be hard to keep track of how far they’ve gone into the ground. By marking them with spray paint, it will be easier to make sure your pilings are all installed at the same depth. Burying the piling 4–6 ft (1.2–1.8 m) into the ground should give you a tight, secure fit. Your pier should sit about 3–4 in (7.6–10.2 cm) above the highest level the water will reach. Measure from the bed beneath the water to the maximum water line, then add on the depth that you plan to bury the pilings to get their total height.  Even in a body of water that isn’t affected by tides, the water level may still fluctuate. If you’re not sure what the maximum water line is, ask other pier or dock owners in your area. Cut the beams a little longer than you’ll need them. You can always trim them later if you need to. You’ll need to have 1-2 strong people to help you hold the piling in place, so bribe a few friends with lemonade and the promise of spending sunny days on the water. If you don’t have anyone who can help you, use sturdy pieces of wood and a pulley system to create a tripod that can lift and hold the pilings in place. The water will blow out of the hose with enough force to push the sand and silt out from under the piling. As this is happening, guide the piling further down into the earth until you reach the desired depth. If you need to, move the tip of the hose from side to side so the piling can go in evenly.  Repeat this process for the remaining pilings. If the force of the water isn’t enough to remove the sand, attach a PVC pipe with a 1 in (2.5 cm) diameter to the end of the hose to increase the pressure. Use the water jetter to push some of the dirt back into place after the piling is set.