Performing the percolation test – a Step by step guide
This guide is part 1 of 2 guides which detail how to design and install a drainage field. Part one lists how to perform a percolation test, the second part takes the readings from the test and describes how to size and construct a drainage field from those readings.
Put simply, a percolation test is a method of testing how quickly water passes through soil.
Following this step by step guide will help you learn to perform the test
Finding the right location for the drainage field is critical. The drainage field will go after the septic tank, and therefore if you have a fall on your site, it is worth using this fall and putting the drainage field at a slightly lower point than the septic tank or treatment plant.
You may not know which areas of your garden are drain well; therefore it is advisable to dig a few trial holes (I suggest 4). Performing test in 4 holes allows you to find the best 2 or 3 and locate your drainage field at this location. When you come to calculate the size of drainage field, you will need the average results from at least 2 holes. Therefore the more you start with, the better and more accurate your location will be.
The best time to do a percolation test is during a wet spell. Doing the test during the wettest months gives you the “worst case scenario”. By contrast, Performing the test in the summer will most likely give you good readings. A drainage field designed in the summer will work in the summer, however you will not know what will happen during winter when the ground is wetter. If you have to do the test in summer, take this into account by oversizing your drainage field and raising it in the ground if local knowledge suggests the ground water might be high.
It is important to know where the natural water table is. You may have local knowledge, and you may know where it is, in which case you can skip this step. However, if you don’t know, dig a hole 2 metres deep (or at least 1.5m below the depth of the pipes coming from your treatment plant or septic tank). Leave this for a few days and check to see if there is water in it. You need to ensure the bottom of your drainage field is 1m above any groundwater present in the hole.
Note: This may lead you to have to raise your pipes or install a pump after the treatment plant or septic tank to lift the waste above the water table.
By now you should know if you are above your natural water table. Providing you are above it it is ok to continue to perform the percolation test.
The test measures the time it takes water to drop by 150mm in the ground at the depth you intend to have the pipes in the ground. Therefore, you must establish what that depth is. If you do not know this then you will need to consult your groundworker to help you establish your levels.
In each hole, dig down to the level of the your drainage pipes. Then dig a 300 x 300 x 300mm hole at the bottom of the pit. This is your testing hole.
Since you are testing the drop of water over a given time, you should get 4 long straight poles or canes to use as markers. mark a 300mm section with some tape, and within that section, mark 75mm from the top, and 75mm from the bottom (this will give you a 150mm "testing section".
Fill each of the 300 x 300 x 300 holes fully and leave to drain over night.
The next day, insert the canes with markers on so that you can clearly see the start and stop markers.
Fill each hole with water to the 300mm point.
Wait for the water to drain to the first marker and start a timer. Repeat this on all holes.
Stop the timer when the water reaches the lower marker.
Record the reading for each hole. And repeat the test 3 times in each hole. At this point you can discard the worst one leaving you with 3 reading for 3 of the best holes. You should note down the readings in a table like this. (Alternatively use our excel sheet at the bottom of the page)
| Test | Hole 1 | Hole 2 | Hole 3 |
|---|---|---|---|
| Test 1 | 78 minutes | 56 minutes | 80 minutes |
| Test 2 | 84 minutes | 68 minutes | 87 minutes |
| Test 3 | 96 minutes | 75 minutes | 98 minutes |
| Total: | 258 minutes | 198 minutes | minutes |
| Average (Total divided by number of tests) | 86 | 66 | 88 |
Finding the average draining time is a simple calculation. It is the total of all the averages, divided by the number of tests, using the above example we obtain the following reading:
| Hole | Average time to drain |
|---|---|
| Hole 1 | 86 |
| Hole 2 | 66 |
| Hole 3 | 88 |
| Total | 240 |
| Average | 80 |
This now also needs to be converted to seconds
Number of seconds = minutes x 60
80 minutes x 60 = 4800 seconds
The rate of percolation is what we call the “VP” (V = velocity (or speed) P = Percolation). To calculate the VP, add up the. The rate of percolation is:
Average seconds (from step 6, divided by 150)
Example: 4800 ÷ 150 = 32.
Congratulations, you now have your VP (rate of percolation)
You van now use your VP Rate to design and construct your drainage field.
In certain areas of the country where ground is very porous, the water in your percolation test may drain too fast. If the water drains too fast, there will not be enough contact time on the gravel to adequately clean the water. Subsequently you must slow the water down. Using sand at the base of your drainage field should slow down the water enough. You must redo the percolation tests with the sand to correctly size the drainage field using the following steps before the test:
Step 1: dig another 300mm in your percolation test hole.
Step 2: fill the extra 300mm with sharp sand
Step 3: compact sand with hand tamper
Step 4: Repeat percolation test
If the test now gives a VP of above 15 you can design your drainage field with the same compacted sand as a 300mm layer underneath the field.
Adding or removing sand will allow you to adjust the VP, and if it is still too slow you can try again with different sand until you reach a result that is not too fast.