Install Posts in the Water for a Dock or Pier
Installing posts or "piling" in the water to support a dock or pier can be done either by "jetting" with water, or driving with a pile driver, if the earth under the water is not too rocky, yet is solid enough to be load-bearing. Because "driving" pilings requires heavy equipment, this article will focus on jetting them.
Steps
- Determine the soil characteristics of the lake, lagoon or stream bed you are installing your piles into. Rock will be unyielding, obviously, so this technique will not work, and silty, muddy material will not support the pier structure. Sandy material is ideal for jetting pilings, but any firm soil will allow jetting.
- Obtain your pilings. These need to be chemically treated to resist rot, usually with sodium pentachloride at .25%, copper-chlorinated arsenate, at .25%, or creosote. You will need the pilings to be large enough to attach your support joists to, and long enough to allow them to be sunk deep enough in the round to bear the weight you will be putting on them. Because you will achieve a different load bearing in different bottom materials, there is no rule of thumb for how deep the piling should be installed, but a minimum of four feet would be reasonable if the piling seems "tight", or secure. Try for 6' deep in the sand. Mark the piles with spray paint at 12" intervals so you know the depth of penetration. 6" to 8" diameter piles are used for small (up to 10,000 lb.) watercraft docks.
- Rent a {{safesubst:#invoke:convert|convert}} gasoline-powered water pump, a suction or "pickup" hose long enough to draw water from the lake or seawater that you are installing pilings in, and a discharge hose long enough to reach the end of the pier from the location where you plan to set the pump. An alternative to the pump may be connecting to a fire hydrant, if your local utility will allow you to do so, but a back flow preventer must be used in this case.
- Build a "jetting pipe". Depending on the depth of the pile jetting, this will be a 6' to 12' section of 3/4" pipe with an elbow at one end to attach your pump discharge hose. That will supply water for jetting. Connect a reducing bushing or flatten the other end. That will be your "jet", providing a stream of water to wash and blow out a hole for the piling to set into.
- Lay out the location of your pilings by using a 2X4 wood stake driven into the ground on the bank, and another driven into the pond bottom beyond the location of the last piling. Tie a length of builder's line from the stake on shore to the stake in the water, and measure the distance from piling to piling, marking them with a permanent marker. The string line should be about the height above water you will want the top of the pilings to be when they are set. Set out the offset from other pilings with a 45 degree plastic square, and cut to length bamboo poles snap-tied to it.
- Stand a piling up in the water, raise your jet pipe alongside it, and turn on the pump. The water rushing through the jet pipe will "blow out" a hole underneath the piling, and it should immediately begin to sink. By moving the jetting pipe around the bottom of the piling, you can guide the direction that the hole is created underneath it. By moving the jet from one side of the piling to the other, you can "steer" it down straight. In good material, such as sand, you can jet down a piling in a few seconds, provided you have sufficient water pressure and velocity. The sand is seldom clean and can contain roots, mud etc. so use a 6' ice scraper to break through. Roots can cause piles to seat off their desired location. The pilings float so you may need to push them down to obtain better depth (see tips below). 12'x 6" or 16'x 6" pilings can be stood up easily by one or two persons. Up to a 12'x 8" dia. piling can be installed with two people. Larger piles need a crane as they weigh 500lbs (20' x 8"dia.). A crane can be built simply with a 24' pontoon boat, 2" steel water pipe, steel cable and a 12v car winch. The design and manufacture would have to be checked for safety before using it. When using a crane you need one person jetting, one guiding the pile and digging and a crane operator. Try to work when the water is calm. However if the pile is out of line, it is easy to jet it out and put it in the correct position. It is possible to install about 10 piles per day. Protective wet suit pants, boots and gloves should be used to prevent abrasion loss of skin especially when you have been in the water for some time.
- Plumb up the piling with two spirit levels and a bungee chord, and quickly make sure it is vertical before the soil settles back into the hole. Once the settling occurs, it will be difficult to change the orientation of the piling.
- Wash sand or dirt back around the piling with the jetting pipe after the piling is sunk, to refill the hole what you washed out. This may not be necessary in sand as it settles back in the hole pretty quickly.
Tips
- Most water ways, rivers, and bays are jurisdictional, and permits are required for both construction of piers and using jetting to install piling.
- If you use a steel pipe for jetting, you can flatten the end with a sledgehammer to restrict the water flow, increasing the velocity of the jet stream. Or, screw on threaded fittings to downsize the end.
- Always wash the larger (butt-end) end of the piling into the ground. This makes it far more likely that the "mushroom-anchor" effect will prevent winter ice from lifting the pilings.
- Size your jetting pipe slightly smaller than the pump discharge hose, and clamp the connections or thread fittings securely. The discharge orifice may need to be as small as 3/8-1/2" so that the water jet has cutting velocity. This will depend on your pump capacity, so try several sizes and optimize.
- Mark an index mark on the piling and jet pipe so you can hold the pipe at the same depth as the bottom of the piling as you jet it down. The water jet should be close to the piling bottom for the best results.
- Some bottoms are tough, clayey, or full of shells. Pilings may need help "jetting" down by turning. Here's how: get a giant C-clamp (or cut one in half and re-weld it to extend its opening), weld onto it some grippy angle-iron feet to bite into the wood, and also an extra long handle. Use this to twist the piling around as you jet-wash. In toughest areas, cut the "digging" end of the piling but not sharp like a pencil, but like a flat-bladed screwdriver. That way when you twist the C-clamp, the "blade" helps cut through the muck.
- Pilings will become buoyant and "lose the weight" necessary to push them down deeper than 5-6 feet (that's below water level, not below bottom level). In areas where winter ice uplift is a problem, you will need to set the pilings deeper. To do so, drive a nail into the top of the piling before you lift it upright. Temporarily attach two small chains and S-hooks and let them dangle. Once the piling is part way down, attach two cinder blocks previously filled solid with concrete and an eye-bolt. This extra weight forces the piling deeper.
- This technique is fairly simple in shallow water, but it is very difficult to accomplish if you must work from a boat.
- Move the jetting pipe around the piling so you are washing a hole out large enough for the piling to drop into. If the pump has high enough capacity, or if the soil is very soft, the jet pipe will not even need to be moved around very much, just down, as the jet blows out a hole "underground."
Warnings
- Use common sense jetting pilings, as with any activity in or on the water.
- Any lifting equipment must be checked by a registered engineer for safety
- Be careful when you are jetting a hole for the piling. The jet can wash a large hole in moments, which could be deep enough to slip into and cause drowning.
Things You'll Need
- Permission from the landowner
- Water pump or water supply
- Jetting pipe
- Piling
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