Basic Definitions
Soak Pit (Soakaway)
A covered, porous-walled chamber that allows water to slowly soak into the ground. It is typically a deep circular pit filled with coarse rocks or lined with porous concrete blocks.
Drain Field (Leach Field)
A series of shallow trenches containing perforated pipes and gravel. It provides a larger surface area for wastewater treatment and is ideal for soils with slower percolation rates.
Infiltration Rate
The speed at which water enters the soil, usually measured in mm/hour. Sandy soils have high rates (fast drainage), while clay soils have low rates (slow drainage).
Peak Factor
A multiplier (typically 2.0–3.0) applied to the average flow to account for surges in water usage, such as morning showers or laundry times.
Effluent
Liquid waste or sewage discharged into the environment. In this context, it refers to the wastewater leaving your septic tank and entering the soak pit.
Percolation
The process of a liquid slowly passing through a filter. For septic systems, this is the movement of wastewater down through the soil profile where it is treated by bacteria.
How to Use This Calculator
Enter Usage Data
Input your total Sewage Discharge (liters/day). A typical person uses 135–150 liters per day. Adjust the Peak Factor if you expect heavy simultaneous usage (default is usually 2.5).
Select Soil Type
Choose your soil from the dropdown (e.g., Sandy Loam, Clay Loam). The calculator automatically fills in the standard Infiltration Rate. If you have a percolation test report, select “Custom” and enter the exact rate.
Choose Design Type
Toggle between Soakpit (vertical pit) and Drain Field (horizontal trenches). Enter your desired dimensions (e.g., depth of the pit or width of the trenches) to see if the design is sufficient.
Using the 7-Day Simulation
After designing your system, scroll down to the Simulation section. This unique feature stress-tests your design.
- Adjust the sliders to mimic your household’s routine (e.g., high usage in the morning/evening).
- Click Start Simulation to watch how the pit fills and drains over a week.
- If the “Height Filled” reaches 100% (Overflow), you need to increase your pit dimensions or switch to a drain field.
Worked Out Examples
1. Standard Family Home (Soak Pit)
Moderate Soil- Scenario: 5 family members, Sandy Loam soil.
- Usage: 5 people × 135 L = 675 Liters/day.
- Soil Rate: Sandy Loam (5 mm/hr ≈ 120 L/m²/day).
Area = 675 / 120 = 5.625 m²
Result: A soak pit with 1.5m diameter and 1.2m effective depth provides adequate surface area.
2. Poor Soil Condition (Drain Field)
Clay/Slow Soil- Scenario: 4 users, Clay Loam soil.
- Usage: 600 Liters/day.
- Soil Rate: Clay Loam (0.5 mm/hr ≈ 12 L/m²/day).
Result: A soak pit is impractical here. Use a Drain Field with 3 trenches, each 20m long and 0.8m wide.
3. High Volume/Commercial (Custom)
Fast Infiltration- Scenario: Small Office, 20 people.
- Usage: 20 × 45 L (office use) = 900 Liters/day.
- Soil Rate: Coarse Sand (20 mm/hr).
Required Area = 900 / 480 = 1.875 m².
Result: A small, compact soak pit (1m x 1m) is sufficient due to excellent drainage.
4. Using the Simulation
Stress Test- Scenario: Design mathematically works, but usage is concentrated.
- Input: 80% of water used between 6 AM – 9 AM.
Adjustment: Increase Soak Pit depth by 0.5m to add “Buffer Volume” for the morning surge.
Frequently Asked Questions
1. What is the difference between a Soak Pit and a Drain Field?
A soak pit is a deep vertical hole, best for small spaces and good soil. A drain field is a horizontal network of trenches, better for shallow water tables or poor soil.
2. How do I know my soil type?
You can perform a “Percolation Test.” Dig a hole, fill it with water, and time how long it takes to drop. Compare the rate (mm/hr) with the calculator’s presets.
3. Can I build a soak pit in clay soil?
It is difficult. Clay has very low infiltration (0.1–0.5 mm/hr). You will need a very large surface area, so a Drain Field is usually preferred over a Soak Pit.
4. How deep should a soak pit be?
Typically 1.5m to 4m deep. However, the bottom must remain at least 1.5m above the seasonal high groundwater table to prevent contamination.
5. How far should it be from my house?
A minimum safe distance is usually 3 meters (10 feet) from building foundations to prevent structural damage from moisture.
6. Can I drain rainwater into the soak pit?
No. Soak pits for septic tanks are designed for biological treatment. Rainwater should go to a separate “Stormwater Soak Pit” to avoid flooding the septic system.
7. What is the lifespan of a soak pit?
A well-designed pit lasts 15–25 years. Failure occurs when the soil pores get clogged with sludge (“Bio-mat”) from the septic tank.
8. Why does the calculator show “Overflow”?
This means the volume of water entering is faster than the soil can absorb it. You need to increase the dimensions (surface area) or add a second pit.
9. How often should I clean the soak pit?
Soak pits shouldn’t need cleaning. You must clean the Septic Tank (every 2-3 years) to prevent solids from entering and clogging the pit.
10. Can I plant trees near the soak pit?
Avoid large trees near the pit. Roots seek water and can penetrate the masonry or clogging the voids, destroying the system.
11. What is the “Peak Factor” in the calculator?
It accounts for surge usage. Even if you use 500L/day, you might use 200L in one hour. The system must be sized to handle this surge without backing up.
12. Why use a “Storage Volume” design method?
This method sizes the pit to hold a specific retention time (e.g., 24 hours of flow) rather than relying solely on immediate infiltration. It’s safer for fluctuating usage.
13. Does the shape of the pit matter?
Circular pits are structurally stronger and easier to build. Rectangular pits are fine but require stronger sidewalls.
14. What if I have a high water table?
You cannot use a standard deep soak pit. You must use a “Mound System” or a raised Drain Field to ensure treated water doesn’t mix directly with groundwater.
15. Should I use gravel or plastic crates?
Both work. Gravel is traditional and cheap. Plastic crates (Soakaway Crates) offer 95% void space (vs 30% for gravel), allowing for smaller pits but are more expensive.