Backfill Trenches and Flush Pipes

This page leads you step-by-step through the backfill and flush process.  Quick summary:

  1. Backfill trenches 1/2 full. Do NOT install sprinklers yet.
  2. Flush entire system for 5 minutes with all valves open.
  3. Cap all sprinkler risers leaving only one open and flush for 2 minutes.  Cap the riser after flushing and repeat with the next riser, flushing the remaining risers for 1 minute each.  Repeat for each riser until all are flushed.
  4. Finish backfilling trenches, don’t backfill around the risers.

Shade Backfill Trenches

Once the pipes are assembled you can start putting the dirt back into the trenches.  Only fill the trenches half full at this time. This is called “shade backfilling” in the irrigation industry because it’s just enough dirt to cover the pipes and hold them down so they don’t jump around during flushing.  Do not 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.

Why Flushing of the Pipes is a Critical Step!

Why such a fuss about flushing?  All it takes to make a automatic valve fail is one tiny grain of sand stuck in one of the solenoid ports.  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?  Well even if you didn’t get any sand in it there would still be a need to flush.  Read on…

Any irrigation repair person will tell you that they have found amazing things inside pipes.  Stuff that could only have gotten in there during the installation.  Dead rats, fish (yes, fish, and fairly good sized ones!), frogs, toads, rags, plastic bags, the remains of someone’s lunch, rocks, pebbles… and lots and lots of sand and silt.   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.

The Sand Fairy put sand in your pipes while you were sleeping!  Well, maybe not the Sand Fairy, but the sand is there just the same.  Even if you were really careful and managed to not get any sand in the pipes when installing them you may still find sand in the pipes.  Where did it come from? Well, it’s really pretty simple. The water system for your house or neighborhood  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 that has settled 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 in the pipes. 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 all the pipes that bring water to it also.

How to Flush the Pipes

So hopefully now you’re convinced and you’re going to do a great job of flushing the system, so let’s get on with it.

  1. Install the sprinkler risers on your lateral pipes.
  2. The sprinkler heads should NOT be installed on the risers during flushing.  Yes some sprinklers come with “flush plugs” attached and you will read elsewhere that you can flush through the heads.  It’s a bad idea, the sprinkler body will restrict the flow and the flushing will not be nearly as thorough.  Just leave them off while you flush.
  3. The top of the sprinkler risers should be sticking up above the lip of the trenches.  The reason for this is so that when the flush water ponds around them the dirty flush water can’t run back into the pipes after you stop flushing! If the risers aren’t high enough add a short temporary pipe extension to them.
  4. Flush one valve circuit at a time.
  5. Start flushing! Open the valve to full open position and let the water run for at least 5 minutes. The water should flow out of the risers and into the trenches (which you only filled halfway with dirt, right?)  The water will seep down into the backfill soil and will cause the back-fill to settle around the pipes. Wow! We’re pretty clever, using the flush water to settle the backfill around the pipes in the trenches!
  6. Walk around with a shovel and add more dirt to the trenches as they start to settle.  Gently tamping the mud with an old 2×4 board helps to work any air pockets out of the muddy backfill.  Note that if you had your lateral poly tubing “plowed” or “pulled” into the ground you only need to backfill at the sprinkler heads.  You may want to tamp down the slit where the tubing went in to close it up.
  7. Once you’ve finished a 5 minute flush of the valve circuit you’re ready to flush the individual risers. That’s right, you’re going to flush it again. And again. And again… once for each valve circuit.  Cap all the risers except for the one farthest from the valve (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 this first riser flush for at least 2 minutes, then repeat the process for each of the other riser, flushing for 1 minute per riser, working back to the valve.
  8. 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 5 minute 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 both the mainline and the water supply pipes.   Flushing of the  individual risers is to remove the sand and debris from the lateral pipes and sprinkler risers.  This riser flush also helps 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.)

Finish Back-filling

If you haven’t yet, finish back-filling the trenches. Don’t fill around the risers yet, leave a hole to install the sprinklers in.  Compact the dirt by stomping on it or tamping it with the head of a sledge hammer or a tamper plate.  Do NOT use the wheels of your car or truck to compact the trenches as you will likely crush the pipes or tubes.


This article is part of the Sprinkler Irrigation Installation Tutorial Series
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