how to do a percolation test

3 min read 31-12-2024

A percolation test, also known as a soakaway test or infiltration test, is crucial for determining the suitability of your land for drainage systems like septic tanks, soakaways, or French drains. This test measures the rate at which water is absorbed into the ground. Understanding how to perform a percolation test accurately is essential for any construction project involving subsurface drainage. This guide will walk you through the process.

What You'll Need for Your Percolation Test

Before you begin, gather these essential materials:

Auger or shovel: To dig the test hole.
Measuring tape: To accurately measure the hole's dimensions.
Level: To ensure the hole is level.
Water: A significant amount (at least 5 gallons). A garden hose is ideal.
Stopwatch or timer: To precisely measure the time it takes for the water to disappear.
Wooden stakes (optional): To mark the boundaries of the test hole and prevent it from collapsing.
Notebook and pen: To record your observations and calculations.

Step-by-Step Guide to Conducting a Percolation Test

Follow these steps for an accurate percolation test:

1. Digging the Test Hole

Location: Select a representative area of the land where you intend to install your drainage system. Avoid areas with obvious obstructions like rocks or tree roots.
Size: Dig a hole approximately 12 inches in diameter and 12 inches deep. Ensure the hole's bottom is level.
Cleaning: Remove any loose soil or debris from the bottom of the hole. This will provide an accurate result.

2. Adding Water to the Hole

Fill the Hole: Carefully fill the hole with water until the water reaches the level of the surrounding ground. You may need to add water gradually to avoid overflow.
Initial Measurement: Record the initial water level in your notebook. This is your starting point.

3. Measuring Absorption Rate

Begin Timing: Start your stopwatch as soon as the water reaches the surface of the hole.
Monitor the Water Level: Regularly check the water level in the hole. Note the time it takes for the water level to drop by a specific amount (usually 1 inch).
Repeat: Continue this process until the water has completely disappeared or reached a significantly lower level.

4. Calculating the Percolation Rate

Record Your Observations: Record all the times and water level measurements. The more data points you gather, the more accurate your calculation will be. Include any unusual observations, such as very slow absorption or very rapid absorption.
Percolation Rate: Calculate the percolation rate. This is typically expressed in inches per hour (in/hr). The formula is:

(Initial Water Level - Final Water Level) / Time (in hours)

For example: If the water level dropped 6 inches in 3 hours, the percolation rate is 2 inches per hour.

5. Interpreting the Results

Percolation Rates: Different authorities may use slightly different standards, but generally:
- Less than 0.5 inches/hour: Poor percolation, unsuitable for drainage systems relying on absorption.
- 0.5 to 1 inch/hour: Slow percolation, may require modifications to drainage design.
- 1 to 3 inches/hour: Good percolation, suitable for most drainage systems.
- Greater than 3 inches/hour: Excellent percolation.

Important Considerations

Multiple Tests: It’s best practice to conduct several percolation tests in different locations to account for variations in soil conditions.
Weather Conditions: Avoid conducting tests immediately after heavy rainfall, as this will skew your results.
Soil Type: The type of soil significantly influences percolation rates. Clay soils absorb water much more slowly than sandy soils.
Professional Advice: For complex drainage systems or significant construction projects, consult a qualified engineer or contractor. They can offer expert advice based on your specific site conditions. They may also have access to more sophisticated testing methods.

By following this step-by-step guide, you can accurately perform a percolation test and determine the suitability of your land for various drainage solutions. Remember, accurate testing is crucial for avoiding costly and potentially problematic drainage issues down the line.