Underground irrigation

Benefits of underground irrigation

You’ll reap a host of benefits if you choose to install an underground automatic irrigation system:

  • the practicality of being able to water your garden automatically, even when you’re away or on holiday, with no need to find a kindly neighbour or friend to do the job;
  • the certainty that each area of the garden will receive exactly the water it needs, without waste and with economical running costs;
  • complete absence of above-ground tubes, which can constitute an obstruction for a lawnmower: even the sprinklers are perfectly flush with the ground surface and are barely visible at a glance;
  • noiseless operation, so no risk of disturbing your neighbours;
  • an increase in the value of your home; on the modern housing market a well-kept garden with an automatic watering system is a definite bonus.

In areas subject to frost, a drainage valve must be installed at the lowest point on each distribution line. When the circuit is closed and there is no flow, the drainage valve opens automatically to drain off any water left in the system, which might otherwise freeze and damage the tubes. When the water supply is turned on again the drainage valve closes automatically.

Check water availability

To ensure efficient operation, conventional underground irrigation systems based on a Ø25mm – Ø32mm PE pipe and pop-up or turbine sprinklers need a supply of water that will guarantee a minimum flow rate of 25-30 l/min at a pressure of no less than 3 bar.
If your water supply does not match these parameters you have three options:

  1. Consider installing a “Colibrì System” (as long as the requirements can be met, refer to the specific section).
  2. Consider installing an above-ground system.
  3. Follow this procedure:
    • prepare a tank of capacity equal to or greater than that which would be sufficient to water the entire garden at any one time;
    • connect the tank to a supply of water (e.g. main, rainwater barrel);
    • install a pump (underground or surface) of rated power between 1 and 1.5 HP and head not less than 35-40 m, capable of delivering a flow rate proportionate to the amount of water the connected sprinklers are expected to consume.

Measure the water pressure

Fit a pressure gauge on an outdoor tap and open the tap completely while ensuring that all other taps in the house are off (STATIC pressure). You can borrow or hire a pressure gauge from a plumber. Take pressure readings at different times of the day and night, when the water supply pressure is generally higher.

Measure the water flow rate

The water flow rate can be measured easily using a container of known volume and checking the time required to fill it.
For example, if a 10 litre container can be filled in 20 seconds, the flow rate is 30 litres per minute.

Plan the system

You’ll need a compass to draw the circumferences of the circles or segments served by each sprinkler until you have covered the entire area. For better results install 90° sprinklers at the corners of the area to be watered, 180° sprinklers along the borders and 360° sprinklers in the open spaces. The examples given here show the required positions to cover narrow and long gardens uniformly. As we can see in the examples, the circumferences of each watered area will overlap to ensure complete coverage of the surface area and perfusion of a uniform water quantity at all points.

Divide the system into circuits

Referring to the sprinkler performance data table, mark the water consumption in litres per minute alongside each sprinkler on the layout diagram. Now group together the sprinklers in different zones (use different colour pens) adding together the water consumption of each one progressively. Make sure the water consumption of each zone is no higher than the available flow rate. Try to create a balanced system: each circuit should have the same water consumption.

Plot the path of the tubes

Starting from the point at which the solenoid valves will be installed to control each circuit, plot the connecting path of each valve to the corresponding sprinklers grouped together in zones, thereby obtaining independently controlled circuits. Since you have to create a channel in which to bury the polyethylene tube, it is advisable to plot straight lines, avoiding paths, terraces and other obstacles that will be hard to cross when actually installing the tube.

Diagrams of irrigated areas

The diagrams below provide sprinkler positioning examples; bear in mind that:

  • 90° sprinklers should be positioned at the corners of the garden;
  • 180° sprinklers should be positioned along the sides of the garden;
  • adjustable sprinklers should be used for angles or 270° or irregular angles;
  • 360° sprinklers should be used for any central areas that are not yet covered

Pressure drops

Pay attention to the PRESSURE DROP in the tubes: this table shows the values to be subtracted from the static pressure available upline. The DYNAMIC pressure must be such as to allow the sprinklers to function as they are designed.

Practical DIY installation

Get out into the garden and transform your plan into a fully functional irrigation system.

1) supply pipe

The solenoid valves unit contains the individual valves required to open and close each circuit.
The supply line to which the valves are connected is subject to the entire static pressure (when the circuit is closed) and it must therefore be composed of an absolutely water-tight metal pipe.
As a general rule the diameter of the pipe used to connect the valves unit to the utility line should be no smaller than the diameter of the valves.

2) upline valve

It’s also important to install a gate valve on the main pipe upline from the circuit solenoid valves to shut off the entire system whenever necessary, such as for maintenance work.

3) tube path

Once you’ve connected the solenoid valves to the water utility pipe, take the layout drawing and mark out the path of the tubes in each sprinkler circuit. Insert a stake at each point at which a sprinkler is to be fitted. Use string or chalk powder to mark out the tubes path of each circuit, from the solenoid valve to the associated group of sprinklers.

4) digging the channels

Using a spade, dig out a channel to a depth of at least 20-25 cm: this should prove fairly easy, and it means the system will be sufficiently protected from winter frost.
To protect the lawn surface it is good practice to lay out a plastic sheet on which to heap the excess soil from the channel.

5) sprinklers positioning

Now place the required sprinkler for each position on the ground beside each stake together with the threaded bracket coupling designed to hold the threaded extension (before fitting the extension remember that you need to drill a hole in the tube with a 10 mm bit).

6) polyethylene tube

Unwind the polyethylene tube from the valves up to the sprinkler stakes, along the marked circuit path. Place the tee connectors, couplings and drainage valves on the ground at the points where they are to be installed. Carefully straighten out the tube required before cutting it to length (leave some excess). Cut the tube with a hacksaw and use a scraper to clean the edges to remove any trace of shavings and burr before fitting the compression connectors (this ensures a perfectly water-tight seal).

7) installation

Once all the circuits have been prepared on the ground and the required irrigation output has been obtained, it’s time to bury the tubes in the ground. Place the circuit tube in the channel, making sure the sprinklers are perfectly flush with the ground surface (you are aided in this by the threaded extension element supporting the sprinklers, which features cut-off segments to adjust the sprinkler height). Now it’s time to connect all the pre-cut tube lengths to all the sprinklers and the drainage valve,
as required in the layout.

8) testing

Once you have fixed the sprinklers in a vertical position (tying them to the stakes with string) test operation of the circuit and make any position changes required to ensure perfect coverage.
When you finish the job, it’s good practice to flush the circuit to eliminate any debris that may have entered during the installation procedures. Remove the head from each sprinkler by simply unscrewing it: after opening the valves the flow of water will flush out the circuit. Refit the sprinkler heads and screw them down fully then fill back the channel, levelling the surface with the ground surface (and with the top edge of the pop-up sprinklers). Your irrigation system is now complete and ready to perform the watering cycles set on the automatic timer.