Once the pipes are assembled you can start putting the dirt back into the trenches. Only fill the trenches half full at this time. For sprinkler systems don't back-fill the area around the sprinkler head risers yet. Don't worry about compacting the dirt yet either, just toss it in on top of the pipe.
Important: A few years ago I had a guy email me and argue and argue with me over the issue of flushing. It would just take too much time! It was going to turn his yard to mud and make a mess. He knew he had done a good job and one quick flush would do the trick. I tried and tried to convince him and finally said, "fine, only flush it once." A few weeks later he was wrote to say his valves were not closing. He couldn't turn off the water and his yard was a muddy mess! Was I laughing? You bet I was! Guess what was wrong with his valves? They had sand in them! All it takes to make a valve stick open is one tiny grain of sand. He learned the hard way. Now you have the opportunity to learn from his mistake. Don't blow it!
Proper flushing of the irrigation system is one of the most important steps you will take. It's easy to do, but takes time so there is a temptation to shortcut the flushing time. After all, you were careful not to get a lot of dirt in the pipes when you put them together, right? You might be surprised at the things I have pulled out of so called "clean" installations over the years! Dead rats, fish (yes, fish, and fairly good sized ones!), frogs, toads, rags, plastic bags, the remains of someone's lunch, rocks, pebbles... But by far the worst enemy of your new irrigation system is plain old sand. It will get into your sprinkler nozzles, valves, or drip emitters. Then they will stop working. It is extremely frustrating when a two week old irrigation system needs to have all the nozzles in the sprinklers replaced. Further more, it is very likely that the sand is not coming from the irrigation system. How's that you ask? Well, it's really pretty simple. The water system has been happily supplying water at a very low flow to your house or building, perhaps for years. At this low flow rate lots of sand has settled out of the water (yes, even the cleanest water supply has sand in it, and most have LOTS of sand in them). In fact, it would not be unusual if there was 1/4" or more of sand in the bottom of the pipes. Now you just installed an irrigation system that is going to demand a lot of water, and when it does the velocity of the water is going to stir up all that sand. Guess where the sand's going to go? That's right, straight into your new irrigation system. So you want to flush it all out BEFORE you install your sprinklers or emitters. In short, you're not just flushing out the irrigation system, you're flushing out the entire water system!
So now you're convinced and you're going to do a great job of flushing the system, so let's get on with it. You will be flushing one valve circuit at a time. The top of the risers should be higher than soil level outside the trench, so that when the flush water ponds around them the water can't run back into the pipes after you stop flushing! If the risers aren't high enough add the shortest possible temporary extension that will extend them above the ground level.
Start flushing! Open the valve to full open position and let the water run for at least 5 minutes. The water should run down into the trenches (which you only filled halfway with dirt, right?), and the water will cause the back-fill dirt to settle around the pipes. Wow! You're pretty clever, doing two things at once! Walk around with a shovel and add more dirt to the trenches as they start to settle. Poke the dirt gently to loosen any air bubbles. Pretend you're a kid again and make mud pies!
Once you've finished flushing the entire valve circuit you're ready to flush the individual risers. That's right, you're going to flush it again. And again. And again... Cap all the risers except for one (the caps don't need to be tight, if they leak a little it won't hurt). Make sure the riser is braced, if you don't it's going to flop around getting everyone and everything wet. Now slowly turn on the water again until you have a small geyser coming from the riser, maybe 4 or 5 feet high. Let it run for at least 2 minutes, then repeat the process for every riser. When you've finished all the risers on the valve circuit, repeat the whole process for the other valves, starting each time with a 5 minute flush of all the risers together.
Explanation: The first flush cycle with all the risers open serves two purposes. It provides water to settle the trenches and it creates a high water demand (velocity) on the water supply, flushing out the "sands of time". The following flushing of individual risers is to remove the sand and debris from the irrigation system pipes and identify any problems (for example, say no water came out of one of the risers -- what's blocking the flow? Probably a rag or large rock in the pipe. Hope this doesn't happen to you. I feel like we're becoming friends here, and I would hate to see you go through the misery of finding and correcting this problem.)
If you haven't yet, finish back-filling the trenches. Compact down the dirt by stomping on it or tamping it with the head of a sledge hammer. I've seen some people use the wheels of their truck to compact the trenches. I've also seen these same people crush the pipes, valves, and risers with their truck tires. Do not be like these people! You don't really want to replace the new irrigation system you just installed do you? No, of course not, you're too smart for that!
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