A day or two before you begin digging the trenches, water the soil thoroughly to soften it, but try not to create mud. As you trench, periodically water down the area to keep down the dust. Recent research indicates that airborne dust is a serious health risk, it's better to be safe than sorry! As previously mentioned, do-it-yourselfers should seriously consider hiring someone to do the trenching if you have more than 100' or so to trench. You can often hire someone to do it for little more than the cost of renting a trencher. See the previous discussion of tools for more on trenching equipment and recommended tools. If you are digging the trenches by hand, I recommend doing it in sections. This has a couple of advantages, first you don't completely wear yourself out digging, second it reduces the danger of having lots of open trenches at the same time. The bottoms of the trenches need to be as level and smooth as possible. If the trenches were dug by a machine don't think you're going to avoid a lot of shovel work! The trenching machines leave a lot of dirt in the bottom of the trenches that you will need to remove. You will need a narrow blade trenching shovel for this. If you have someone dig the trenches with a machine, have them over-excavate the trench by an extra 2". Then you can just leave the loose dirt in the bottom of the trench and save a lot of work!
The depth of the trenches will vary dependent on a number of factors. All plastic mainline pipe must be at least 18" deep as measured from the top of the pipe to the soil surface. So if you are installing a 1 1/2" mainline the trench is going to need to be about 20" deep. A mainline is any pipe which is under constant water pressure. In other words the pipe between the water source and the control valves is a mainline. Any mainline pipe that is not buried at least 18" deep should be metal, such as copper, brass, or steel. In most areas this is required by law (the Uniform Plumbing Code) but it just makes good sense to do it everywhere. You don't want to hit this pipe with a shovel when planting a new shrub! As a side note if you use steel pipe, be sure to either use the plastic coated type or wrap the pipe with a special protective plastic tape (ask your supplier for the tape). Even the galvanized steel pipe will rust with time, I wrap it with plastic tape just to be safe. I don't recommend using polyethylene for mainline pipes. Again, polyethylene is not legal for use as mainlines in most areas.
Lateral trenches for home irrigation systems can usually be about 10" deep. For commercial systems the standard is that the pipe is 12" deep, so the trench needs to be about 14" deep. For the do-it-yourselfer the argument is more work now or more work later. The deeper the pipe, the less likely it is to be damaged.
See the full tutorial on Winterizing your sprinkler system.
In areas where freezing temperatures occur precautions must be taken to prevent water from freezing in the pipes, valves, emitters, and sprinkler heads. In most cases this is done by installing drain valves on the pipe and making sure the pipe slopes down to the drain valve so that the water can drain out. You need a separate drain valve on the section of pipe on both sides of every valve (the water can't drain through the closed valve, right?). Automatic drain valves work well but there are a couple of important points to consider before you use them.
- Automatic drain valves open and drain all the water from the pipes every time the irrigation system is turned off. So you must provide someplace, such as pit filled with gravel, for this water to run into or you will create a mud hole!
- Automatic drain valves waste water. If you pay a lot for water or live in one of the many areas where water is in short supply, you may want to use manual drain valves that you open by hand at the end of the irrigation season.
- Because the automatic drain valves drain the pipes after each use, the pipes must refill each time you turn on the irrigation. This takes time and creates stress on the piping which can lead to premature failure.
- If you just once forget to open the manual drain valves before the first freeze, you will wish you had installed the automatic drain valves instead. Can you faithfully remember to drain the system every year before the first frost? If not you better use automatic drain valves.
For large irrigation installations such as parks and golf courses we don't use drain valves. We install special taps on the pipes which are connected to a high capacity, low pressure, air compressor prior to the first hard freeze. The irrigation system is then run through its cycles using air in place of water. This blows out all the water from the system. As an added precaution I also install manual drain valves on all the mainlines just in case some water remains in the pipes. The drain valves are also handy for draining the system if you need to work on the pipes.
Trench depth is also an important consideration in freezing climates. Installing the pipe deeper gives it more protection against frost. Where possible, all the pipe should be installed below the soil frost line. Remember that you will still need drain valves in order to drain the water out of the sprinkler heads which aren't below the frost line.
The easiest way to install the pipe is to start at the water source and work out to the valves, then continue to the end of each lateral. Assemble the pipe outside of the trench and then lower the assembled sections into the trench as you move along. Try to avoid getting dirt into the pipe as you assemble it. The best way to do this is to keep the open end of the last pipe section glued out of the trench. Unfortunately it will want to fall into the trench if you don't hold it. A good way to prevent this is to place a couple of short pieces of scrap pipe cross-ways across the top of the trench to hold the last section of pipe up out of the trench while you glue the new length of pipe or fitting onto it. If you need to stop work for the night stuff a rag into the open pipe end to keep out dirt and small animals. Just don't forget to remove the rag before you start work again! Unfortunately, many problems with new sprinkler systems turn out to be caused by a rag left in one of the pipes!
Important! PVC cement, PVC primer, and acetone are flammable. Do not use them around any ignition source. Do not breath the vapors. Close the containers when not in use. Avoid contact between these chemicals and your skin. Don't get any of these in your eyes and avoid getting them on your clothes. You are working with an aggressive chemical solvent, so use common sense! One last warning. Look inside the pipe lengths before you connect them together and make sure nothing is in there. I've found rocks, cloth, and toy cars inside pipe that has been stored where kids can play with it.
- Cut the pipe ends square using a hacksaw or preferably, a PVC pipe cutter. It is extremely important that all the burrs be removed from the cut pipe end. These burrs will scratch the pipe as the sections are pressed together creating small channels that will become leaks someday. Smooth the burrs and rough edges using a pocket knife, file, or a piece of sandpaper.
- Insert the pipe end about 1/4" into the end of the fitting to check the fit, then pull it back apart. Sometimes the pipe isn't perfectly round or isn't the correct diameter, and it won't go into the fitting. Now is the time to discover this, not when it's coated with glue. Don't push it together any farther than 1/4" or it will be very hard to remove. Fittings are made so that the back of the fitting socket is smaller in diameter than the front. This is true of both threaded and solvent welded fittings, metal and plastic. The reason is to create a tighter joint which is less likely to leak. This is also the primary reason that it becomes harder and harder to turn a threaded joint the farther it goes into the fitting! It is also the reason that plastic fittings split in half if you over tighten them. So one more time (I know I keep saying this) hand tighten plastic fittings only -- no wrenches!
- This step is optional, many people don't use primer. But almost every professional installer does, so does this suggest anything to you? In cold weather (if you need to wear a jacket) you should always use primer as it makes the glue work better in cold temperatures. Coat the mating surfaces of both the pipe and the fitting with PVC primer where they will be joined. Use the primer like it was paint, don't allow it to puddle or drip. The purpose of the primer is two fold. First it helps clean any foreign materials off of the pieces. Second it starts to soften the PVC plastic, or in industry terms, it breaks the glaze. When PVC pipe is extruded a thin, hard layer of plastic develops on the surface which is known as the glaze. This is what gives the pipe that shiny look. PVC primer is pretty much just acetone with a pretty purple dye in it so you can see where you've used it. If you've been paying attention as you read this tutorial (wake up!!!) then you know that acetone is the solvent for PVC plastic. If you place a piece of PVC pipe in acetone it will get softer and softer until it pretty much just melts into a blob. Acetone will also melt most other plastics, so watch that you don't spill it (or the primer) onto anything you don't want melted.
- Coat the outside of the pipe and the inside mating surfaces of the fitting with PVC cement. Be generous with the cement but if it starts to drip or run off you're using too much. The goal is to use the thickest layer of cement possible without it running or dripping off. Press the pipe and fitting together using a twisting motion so that the pipe makes a 1/4 circle turn as it goes into the fitting. Twisting helps distribute the cement more evenly. Remember the cement dries extremely fast so don't waste any time and be sure to get the fitting aligned the way you want it before the cement sets. If you have never glued pipe before I strongly suggest buying a few extra couplings and using some scrap pipe to practice before taking on the real thing. Press the pipe into the fitting all the way until it bottoms out. If you don't get it all the way in before it sets, you greatly increase the chances of a leak. So practice, practice, practice! Hold the glued joint in place for about 30 seconds after shoving it together to allow it to set up. Try not to put any major stress on the joint for another few minutes after gluing (its OK to lay it down, just don't yank on the ends or twist the pipe). You really shouldn't put water into the pipes for 24 hours after gluing, but this isn't always practical in the real world. The reality is that the joint develops the majority of its strength in just a few minutes.
- Too much glue can be just as bad as too little. Remember the glue is a solvent, it works not just by bonding the pipe like standard glue, but by also melting the pipe and fitting together as well. But it will also melt a hole through the pipe if a large amount of it sits in one place for a long time. Why am I telling you this? After all, you're not planning to put a large gob of cement on the pipe! As you shove the pipe into the fitting the end of the pipe pushes a "bead" of glue in front of it, much like a bulldozer bushes dirt. If you use way too much glue this bead gets very large. I have seen beads so large that they completely blocked the flow path through a 1/2" fitting! Besides blocking the flow the bead also creates a weak point in the PVC. A small bead is normal and is expected. But if a glob of cement drips off from the joint when you push the pipe into the fitting, then you are using too much cement. Remember, too little glue is also a problem so don't over-react! If the cement doesn't drip you're OK. If you can't get the pipe fully into the fitting before the glue sets, then you're probably not using ENOUGH glue.
Assembling polyethylene pipe is pretty simple. Press the barbed ends of the fittings into the pipe and place a clamp on the pipe directly over the barbs. Tighten the clamp to lock the pipe on. That's pretty much it. If the polyethylene is cold and stiff you can soften it by leaving it in the sun, or dipping it in hot water, but don't use a flame of any kind.
Once the pipe is assembled you can place it in the bottom of the trench. For sprinkler systems install the sprinkler risers, but not the sprinkler heads. Temporarily place threaded caps on the risers.
Don't forget to install the control wire for the valves while the trenches are open (unless, of course, you don't plan to bury the wire). The wire should be at least two inches away from the pipe, and either next to, or under the pipe. Never place the wire above the pipe. My preferred location for the wire is 2-4" below the pipe. This way the pipe protects the wire from damage and you are less likely to accidentally cut the wires when making repairs to the pipe. If you are using numerous wires rather than a multi-wire cable, it is easier to handle the wires if you tape them together into a bundle every 10' or so. The size of each wire is determined from a chart provided by the valve manufacturer. Every valve model is different, but we can make some generalizations. For residential systems where the wire length between controller and valve (not the distance from controller to valve, but the length of the wire between the controller and the valve) is less than 200', #18 wire size will work. #18 is the size of wire provided in most multi-wire (or multi-conductor) irrigation cable. Check the wire packaging or look at the small print on the wire itself to find the wire gauge (size). The standard wire gauge for commercial irrigation systems is #14 or sometimes #12. As the numbers get smaller the wire gets bigger, so #12 wire will carry more current than #14. The wire should be made for direct burial in the ground, if it is the wire will be labeled "UF-AWG" on the wire insulation. If you don't mind a little extra expense, place the wire in a PVC pipe sleeve to protect it. If there are a lot of burrowing rodents (like gophers, moles, and ground squirrels), in the area I advise that you always sleeve the wires. They will chew on the wire and cause you no small amount of grief. While we're talking about wiring this is a good time to warn you against a common temptation that has disastrous results. Never connect the wires to the irrigation controller and then "spark" the wires against each other to test them! This can damage the circuits in many of the irrigation control clocks. If you need to test the wire circuits the best cheap way to do it is to connect a valve to the wire with the controller turned off, then have someone turn on the controller station while you listen for the valve solenoid to click when the circuit is activated. The best professional way is to use an ohm meter to test the wire continuity.
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