Before constructing a soakaway, it is important to know whether the soil can absorb water properly. A soil percolation test helps determine how quickly water passes into the ground. The result helps builders and engineers decide whether a proposed location is suitable for a soakaway and how much space the system may require.
A soakaway that does not match the soil conditions can cause serious problems. Wastewater may collect inside the pit, produce unpleasant smells, overflow or appear on the ground surface. In some cases, the system may fail shortly after construction.
Testing the soil before building helps prevent these problems and reduces the risk of spending money on an unsuitable drainage system.
What Is a Soil Percolation Test?
A soil percolation test is a simple field test used to measure how quickly water enters the ground.
The test involves digging a hole at the proposed soakaway location, filling it with water and measuring how quickly the water level drops.
If the water level falls very slowly, the soil may not absorb enough wastewater to support the proposed soakaway. If it falls too quickly, wastewater may pass through the soil without receiving enough natural treatment.
The aim is to determine whether the soil can absorb wastewater at a suitable rate without becoming saturated or allowing contamination to reach groundwater too easily.
Why Is a Percolation Test Important Before Soakaway Construction?
A percolation test provides useful information before construction begins.
1. Determines Whether the Soil Is Suitable
Not all soil types can support a soakaway. Some absorb water quickly, while others hold water for long periods.
Testing helps identify whether the proposed location can receive wastewater from the septic tank.
2. Helps Determine Soakaway Size
The amount of wastewater a household produces and the speed at which the soil absorbs water both affect the required soakaway size.
A larger household generally produces more wastewater and may require a larger drainage area. Soil that absorbs water slowly may also require more space, provided it meets the applicable design requirements.
3. Reduces the Risk of Soakaway Failure
A poorly designed soakaway may become full because the surrounding soil cannot absorb water quickly enough.
This can cause wastewater to rise inside the pit, overflow or spread across the compound. A percolation test helps identify unsuitable soil before construction.
4. Helps Protect Boreholes and Groundwater
A soakaway must allow wastewater to pass through enough suitable soil before reaching groundwater.
If water moves through the ground too quickly, there may not be enough soil treatment. A high groundwater level can also increase the risk of contamination.
The test should therefore form part of a wider site assessment.
Factors That Affect Soil Percolation
Several conditions determine how quickly water enters the ground.
Soil Type
Different soils absorb water at different rates.
| Soil type | General behaviour |
|---|---|
| Coarse sand | Usually absorbs water quickly |
| Fine sand | Often drains well, depending on its composition |
| Sandy loam | May provide suitable drainage |
| Clayey soil | Usually absorbs water slowly |
| Compacted soil | Restricts water movement |
| Rocky ground | May prevent suitable water absorption |
These are general descriptions. Actual soil conditions must be checked at the proposed soakaway location.
Soil Compaction
Heavy construction equipment, vehicles and previous building activities can compress the soil.
Compacted soil has less space for water to move through, which may reduce its ability to absorb wastewater.
Soil Layers
A property may contain different soil types at different depths. The upper layer may absorb water quickly, while a deeper clay layer may prevent further movement.
This is why the test should reflect the depth at which the soakaway will operate.
Groundwater Level
The level of underground water affects how much space remains for wastewater to pass through the soil.
Where groundwater is close to the proposed soakaway, the system may not provide enough space for proper wastewater treatment.
The assessment should consider seasonal changes because groundwater levels may rise during the rainy season.
Equipment Needed for a Soil Percolation Test
A basic field test requires:
- Digging tools.
- Measuring tape or ruler.
- Water supply.
- Stopwatch or timer.
- Measuring stick for checking the water level.
- Notebook for recording results.
- Appropriate protective equipment.
The test hole dimensions and testing procedure should follow the applicable design guidance.
How to Carry Out a Soil Percolation Test
The exact procedure may vary depending on the design standard being used. However, the general process involves the following steps.
Step 1: Choose the Proposed Soakaway Location
Select the area where the soakaway is expected to be constructed.
The test should take place within the proposed drainage area because soil conditions can change across a property.
Before digging, check for underground water pipes, electrical cables and other buried services. Also consider nearby buildings, boreholes and wells.
Step 2: Dig the Test Hole
Excavate a hole to the dimensions and depth required by the testing procedure.
The depth should represent the part of the ground that will absorb wastewater from the proposed soakaway.
Remove loose soil from the bottom and sides without unnecessarily compacting or damaging the exposed surfaces.
Step 3: Examine the Soil
Look at the soil removed during excavation and the exposed surfaces inside the hole.
Check for sand, clay, rock, compacted layers and signs of underground water.
If the soil changes significantly with depth, additional testing may be necessary.
Step 4: Fill the Hole with Water
Pour water into the hole to wet the surrounding soil.
Allow the soil to absorb water before taking the main measurements. This step helps prevent dry soil from producing an unusually fast initial result.
The soaking period should follow the selected testing procedure because different soil types require different preparation times.
Step 5: Refill and Measure the Water Level
After preparing the soil, refill the hole to the required starting level.
Use a ruler or measuring stick to record the water level. Start the timer and observe how far the water falls within a measured period.
For example, you may record the water level at the beginning and check it again after 30 minutes.
The actual measurement intervals should follow the selected testing method.
Step 6: Repeat the Test
Repeat the measurements as required by the procedure.
Several readings help show whether the soil continues to absorb water at a similar rate.
Where the proposed soakaway covers a large area, tests at different locations may be necessary because the soil may not have the same drainage characteristics throughout the property.
Step 7: Record the Result
Record the time taken for the water level to fall and the distance it drops.
For example, if the water level falls by 25 mm in 30 minutes, divide the time by the distance:
30 minutes ÷ 25 mm = 1.2 minutes per mm.
This means the water level takes 1.2 minutes to fall by each millimetre.
The result must be compared with the limits in the applicable soakaway design procedure. This example does not establish whether the soil passes or fails.
How to Understand Percolation Test Results
The result shows whether water enters the soil slowly, moderately or quickly. However, the correct interpretation depends on the design requirements.
| Test result | What it may mean |
|---|---|
| Water falls very slowly | The soil may not absorb enough wastewater |
| Water falls at a suitable rate | The soil may support a soakaway if other conditions are satisfactory |
| Water falls very quickly | Wastewater may pass through without adequate treatment |
| Results vary between tests | Soil conditions may differ across the site or further testing may be needed |
There is no single result that guarantees a successful soakaway in every location. The designer must also consider groundwater level, soil layers, household wastewater volume and available land.
How Percolation Test Results Affect Soakaway Size
The test helps determine how much ground is needed to absorb the wastewater coming from the septic tank.
Where the soil absorbs water at a suitable rate, the designer can use the test result and the expected daily wastewater volume to calculate the required drainage area.
A house occupied by eight people will generally produce more wastewater than one occupied by three people. The larger household may therefore need a greater soakaway area.
The final design should consider:
- Number of people living in the house.
- Estimated daily wastewater production.
- Soil absorption rate.
- Depth of suitable soil.
- Groundwater level.
- Available space.
- Required distances from buildings and water sources.
A percolation test alone cannot determine the complete soakaway dimensions. It must be used together with the household wastewater estimate and other site information.
Common Mistakes During Percolation Testing
Mistakes during testing can produce misleading results and lead to an unsuitable soakaway design.
| Mistake | Possible problem |
|---|---|
| Testing dry soil without proper preparation | Water may disappear unusually quickly at first |
| Digging the test hole too shallow | The result may not represent the actual soakaway depth |
| Testing only one location | Other parts of the property may have different soil conditions |
| Ignoring clay or rock layers | The soakaway may fail when it reaches the unsuitable layer |
| Recording measurements incorrectly | The calculated result may be unreliable |
| Ignoring groundwater levels | Wastewater may contaminate underground water |
| Using the wrong acceptance limits | The design may not meet applicable requirements |
What Happens If the Soil Fails the Percolation Test?
If the soil cannot absorb water at a suitable rate, a conventional soakaway may not work at the proposed location.
However, this does not automatically mean that the property cannot have a wastewater disposal system.
Possible solutions include:
Relocating the Soakaway
Another section of the property may have better soil conditions. Additional tests can help identify a more suitable location.
Using a Larger Infiltration Area
Where the soil has an acceptable but relatively low absorption rate, a properly designed system may distribute wastewater over a wider area.
The design must still meet the required standards and provide adequate treatment.
Considering an Alternative Wastewater System
Where the ground cannot safely receive septic tank effluent, an approved alternative treatment and disposal system may be necessary.
The appropriate option depends on the site conditions and local requirements.
Do not simply dig a deeper pit or increase the soakaway size without professional assessment. A deeper excavation may encounter rock, unsuitable soil or underground water.
Safety Precautions During Testing
Although a percolation test is relatively simple, excavation still presents safety risks.
- Check for underground pipes and cables before digging.
- Keep children and unauthorised people away from the test area.
- Do not enter deep or unstable excavations.
- Protect open test holes against accidental falls.
- Use appropriate protective equipment.
- Secure or backfill the hole after completing the test.
Deep excavations require proper support and professional supervision.
Conclusion
A soil percolation test helps determine whether the ground can absorb wastewater from a septic tank. It measures how quickly water enters the soil and provides information needed to plan a suitable soakaway.
The result helps reduce the risk of wastewater overflow, unpleasant smells and premature soakaway failure. However, soil absorption is only one part of the design. Household wastewater volume, groundwater level, soil depth and available space also matter.
Before constructing a soakaway in Nigeria, test the soil at the proposed location and have the results assessed by a qualified professional. This helps ensure that the wastewater system suits the property and protects nearby water sources.