Installing An Irrigation System

Now that we have developed an understanding of the irrigation design plan, the blocks can be demarcated (lay out) in the field. The assumption is made that the land has been cleared and soil preparation has been done, and that other infrastructure items, such as roads and waterways, has already been demarcated. Care must be taken when laying out the irrigation blocks that it is done correctly and precisely, even if it may take a day or two longer to obtain an accurate lay out. Remember, the result will determine the lifespan of the orchard. The same principles are used for the demarcation of other types of irrigation systems. 

The following tools and equipment are required for demarcating of blocks: 

• Pegs (one per tree in the block) 
• Hammer 
• Dumpy level or another surveying instrument 
• Measuring tape 
• Wire or rope 

Blocks are laid out as follows: 

• Marking the corners of the blocks 
• Marking the rows 
• Marking the plant positions 

Marking the Corners of the Blocks 

The corners of the blocks are first pegged out roughly to see the relation of the blocks to one another. The lengths of the sides of the blocks are now measured on the irrigation design plan, and the scale of the plan is used to calculate the actual length. The scale is a ratio that shows the relation between the measurement on the plan and the actual distance on the ground. The scale of the plan in the example in section 1 is 1:1500, meaning that for every centimetre on the plan, the distance on the ground would be 1,500cm or 15m. If the side of a block measures 10cm on the plan, it equals a distance of 15,000cm or 150m on the ground. The corners of the blocks can now be marked precisely. Make sure the corners are in the right spot and that the pegs are driven in well. These corner-markers will be the used to lay out the rest of the block, so if they are wrong, the rest of the layout will also be wrong. 

Marking the Rows 

Start by measuring the rows on the side of the block with the mother line, using a measuring tape. Remember that poor-quality measuring tapes can stretch and expand. Use a good quality measuring tape made of fibre.

If the plant spacing on the irrigation design plan is for example 6x2, it means that there must be a 6m space between rows, and a 2m space between the plants in the rows. The row spacing is the bigger number. Set up the dumpy level at one of the corners of the block. To set up the dumpy, the legs of the tripod is extended and secured. The base of the dumpy is fastened to the tripod by means of the attachment screw on the base of the tripod. On the dumpy is a spirit level, located on the eye-piece or the base of the dumpy. By moving one leg of the tripod at a time, in or outwards will adjust the level of the dumpy until it is almost centred. On each corner of the base of the dumpy is an adjusting screw. Adjust these screws until the bubble in the spirit level is dead centre. In the case of the spirit level being located on the eye-piece, the dumpy has to be turned to face the leg that is being adjusted. Rotate the dumpy through all three legs after it is centred to check the levelling. When you look into the dumpy level you will note crossed hair lines (the same as a rifle’s telescope) on the inside. Turn the dumpy towards the other corner of the block. (If the other corner is out of sight due to elevation, have an assistant hold a long pole at the other corner.) Plant pegs between the corners at about 20m to 30m intervals. Make sure the pegs are in the vertical hairline and therefore in line with the other corner. These preliminary pegs give a straight line along the side of the block for the measuring of the rows. Use a tape measure to mark the end of each row with a peg. Make sure the straight line is followed. Check the pegs with the dumpy. They should all be in one straight line from the dumpy to the other corner. Move the pegs perfectly into line, and check the row spacing again with the measuring tape. Keep the dumpy at the one corner and swivel it to mark a line perpendicular to the one on which the rows have been marked. Apply the same method as above, and put down two marking pegs at about 50m to 100m intervals on this line. Move the dumpy to the first peg and follow the same levelling method as above. Turn the dumpy back to the first corner and zero the degree scale by adjusting the degree plate on the dumpy so that 0 degrees shows to the first corner. Now swivel the dumpy trough 90° and lock. Peg out a line to the opposite end of the block and use the same method as above to mark the rows at six-meter intervals on this line as well. Repeat the procedure for the remaining pegs that run parallel to the rows. When the rows are marked, the pegs in the rows must be checked to see that they are in line. Orchards are ridged in many cases. This means that the ground is cropped up into a row to improve the soil depth and drainage. Once the rows are marked, the ridges must be made. Once the rows have been ridged, the rest of the land can be pegged (demarcated).Check the rows by using the same method that was used to lay them out. 

Marking the Plant Positions 

Set up the dumpy at the end of a row and look at the other end. Use the tape measure to measure the inter-plant spaces. If the plant spacing is 6x2m, this would be 2m. Plant pegs at every spot where a tree must be planted. Check the pegs visually to see that they are in line and also check them at an angle, meaning from one corner to the corner diagonally opposite, to see if they are aligned.

Conveyance System Trenches

Conveyance system trenches are the trenches into which the pipelines are buried. Before the trenches are dug, they must be pegged out at first. The centre of the trench can be pegged with pegs about 50m to 100m apart. Tie a wire or a rope to the pegs to mark out the line between them and use ordinary lime to mark out the line of the trench. When the trenches are dug, the chalk line must be in the centre of the trench. The measurements of trenches vary from 400mm to 600mm in width, depending on the size of the pipe, and are usually about 600mm deep. Trenches for pipes that go through lands and roads, such as mainlines and sub-mainlines, should be 1,000mm deep. Trenches for mother lines can be a bit shallower, at about 400mm. Trenches must be free of protruding rocks on the bed since these rocks can break the pipes. Trenches can be dug by either using manual labour or a back-end loader, also called an excavator. Manual labour may be cheaper, but will take longer, while using a back-end loader will be faster but more expensive. Keep in mind that back-end loaders are also more suited for rocky terrain. The choice will depend on the budget.

Installing Pipes 

Pipes are installed in trenches as follows: 

• Before installing pipes in trenches, ensure that the trenches are free of stones and sharp edges. 
• Asbestos cement pipes and large uPVC pipes must be bedded in sand. 
• Place the first pipe into the trench and secure it by backfilling the trench near the ends of the pipe. 
• Place a collar over the end of the pipe. Make sure the collar and the end of the next pipe is clean. 
• Lubricate the inside of the collar and the end of the next pipe with pipe lubricant. Do not use oil because this will cause the rubbers to perish.
• Insert the end of the second pipe into the collar. 
• Place a wooden block over the tail end of the second pipe and tap the block with a hammer to force the pipe into the collar. The force that is required depends on the size of the pipe.
• Drive the one pipe into the next one as far as the depth mark on the pipe. Take care not to pinch the rubbers. 
• If the pipe refuses to go into the collar, remove the pipe and inspect the collar as the rubbers may have shifted.
• Ensure that there are no foreign objects inside the pipes that can cause blockages in the irrigation system. 
• As the pipes are fitted and positioned they can be backfilled in between joints, but the joints must be left open to check for leaks when the system is operated. 

Where bends, reducers or end caps are fitted, there is always the possibility that the joints can kick out, except in the case of steel joints. Bends and end caps must be anchored by concrete thrust blocks. Thrust blocks should be done according to specifications. It is a good practice to flush the pipes as the work continues. First, flush the mainline before the valves are fitted. Then flush the sub-main lines before the laterals are fitted. Next, the laterals can be flushed before emitters are fitted. After the system has been tested the trenches can be backfilled. The backfilling around and above the pipe for about 100mm must be stone free and compacted. The rest of the backfill also needs to be compacted to prevent subsidence of the backfilled soil. 

Installing Filters and Valve Clusters 

Pump and filter bank installation is a specialised job that should be carried out by a competent contractor. Inline filters can however be installed very easily. The filter is attached to risers so that it is above ground. The bottom end of the risers is fitted with riser outlet bends which are spigot to slide over the pipe. At the back of each riser leg, a Y-standard is hammered in and tied down to the riser to keep the leg from popping out. The valve clusters are installed in the same way.

Installing Infield Fittings 

Grommets are installed by drilling a hole in the mother line and inserting a rubber ring into the hole which has a groove that fits into the sides of the pipe. The coupling is inserted into the lateral and then pushed into the rubber ring. There is also another type of grommet that is pushed into the hole and the retaining nut on the grommet is tightened. Nylon couplings and reducers are pushed into the poly pipe with no clamps needed as long as the working pressure is within limits. Micros have a tube that is fitted with a barb and drippers have the barb moulded onto them. A hole is punched in to the poly pipe and the barb is inserted into the hole.