Calculate soil infiltration rate (f) for soakaway design
The length of the test pit in meters.
The width of the test pit in meters.
Depth of water at the start of the test (below pipe invert) in meters.
Time taken for water to fall from 75% to 25% effective depth in minutes.
Volume Outflow (Vp75-25):0 m³
Mean Surface Area (ap50):0 m²
Infiltration Rate (f):0 m/s
Understanding the BRE Digest 365 Soakaway Test
Designing an effective surface water drainage system requires accurate data regarding the soil's ability to absorb water. The BRE Digest 365 provides the authoritative methodology in the United Kingdom for determining the soil infiltration rate, a critical parameter for sizing soakaways.
What is the Soil Infiltration Rate?
The soil infiltration rate (denoted as f) represents the speed at which water enters the soil. It is usually expressed in meters per second (m/s). This value is derived from a percolation test performed in a trial pit on the site where the soakaway is proposed.
The Calculation Formula
The BRE Digest 365 calculation determines the infiltration rate based on the time it takes for the water level in a trial pit to fall from 75% full to 25% full. The formula is:
f = Vp75-25 / (ap50 × tp75-25)
Where:
Vp75-25: The effective storage volume of water in the trial pit between 75% and 25% effective depth (m³).
ap50: The internal surface area of the trial pit up to 50% effective depth, including the base area (m²).
tp75-25: The time taken for the water level to fall from 75% to 25% effective depth (seconds).
How to Perform the Calculation
To use this calculator effectively, you need data from a standard trial pit test. Here is the step-by-step logic used in the calculation:
1. Calculate Volume Outflow (Vp75-25)
This is the volume of water lost during the measurement period. Since the measurement is taken between 75% and 25% of the effective depth, this represents exactly 50% of the total effective volume.
V = Length × Width × (Effective Depth × 0.5)
2. Calculate Mean Surface Area (ap50)
The infiltration rate assumes the water is infiltrating through the base and the sides of the pit. The relevant surface area is calculated at the point where the pit is 50% full.
Area of Base = Length × Width
Area of Sides = 2 × (Length + Width) × (Effective Depth × 0.5)
ap50 = Area of Base + Area of Sides
3. Determine Time (t)
The time input is usually recorded in minutes during field tests but must be converted to seconds for the final calculation to result in meters per second (m/s).
Example Calculation
Consider a trial pit with the following dimensions:
Step 2: Surface Area (at 50% depth)
50% Depth = 0.75 m
Base Area = 2.0 m²
Side Area = 2 × (2.0 + 1.0) × 0.75 = 4.5 m²
Total Area (ap50) = 6.5 m²
Step 3: Time
100 minutes = 6000 seconds
Final Rate (f)
f = 1.5 / (6.5 × 6000) = 3.85 × 10-5 m/s
Why is this important?
If the infiltration rate is calculated incorrectly, the soakaway may be undersized (leading to flooding) or oversized (leading to unnecessary construction costs). The BRE Digest 365 method ensures a standardized approach to managing stormwater runoff compliant with UK building regulations.
function calculateInfiltration() {
// 1. Get input values
var pitLength = document.getElementById("pitLength").value;
var pitWidth = document.getElementById("pitWidth").value;
var effectiveDepth = document.getElementById("effectiveDepth").value;
var timeTakenMinutes = document.getElementById("timeTaken").value;
// 2. Element references for output
var resultBox = document.getElementById("resultBox");
var errorBox = document.getElementById("errorBox");
var resVolume = document.getElementById("resVolume");
var resArea = document.getElementById("resArea");
var resRate = document.getElementById("resRate");
// 3. Validation
// Reset display
resultBox.style.display = "none";
errorBox.style.display = "none";
if (!pitLength || !pitWidth || !effectiveDepth || !timeTakenMinutes) {
errorBox.innerText = "Please fill in all fields.";
errorBox.style.display = "block";
return;
}
var L = parseFloat(pitLength);
var W = parseFloat(pitWidth);
var D = parseFloat(effectiveDepth);
var T_min = parseFloat(timeTakenMinutes);
if (L <= 0 || W <= 0 || D <= 0 || T_min <= 0) {
errorBox.innerText = "All values must be greater than zero.";
errorBox.style.display = "block";
return;
}
// 4. Calculations (BRE Digest 365 Logic)
// The effective depth for calculation is the volume between 75% and 25%.
// This represents a slice of the pit that is 50% of the total effective depth.
// Height of the slice = D * 0.75 – D * 0.25 = D * 0.5
var sliceHeight = D * 0.5;
// Vp75-25: Volume of water lost
var volumeOutflow = L * W * sliceHeight;
// ap50: Internal surface area of the pit at 50% effective depth
// Depth at 50% = D * 0.5
var depthAt50 = D * 0.5;
// Area of base
var areaBase = L * W;
// Area of sides at 50% depth = Perimeter * DepthAt50
var perimeter = 2 * (L + W);
var areaSides = perimeter * depthAt50;
// Total ap50
var meanSurfaceArea = areaBase + areaSides;
// Convert time to seconds
var timeSeconds = T_min * 60;
// Calculate Infiltration Rate f = V / (Area * Time)
var infiltrationRate = volumeOutflow / (meanSurfaceArea * timeSeconds);
// 5. Formatting Results
// Format scientific notation for very small numbers
var rateFormatted = infiltrationRate.toExponential(3); // e.g., 2.345e-5
// Update DOM
resVolume.innerText = volumeOutflow.toFixed(3) + " m³";
resArea.innerText = meanSurfaceArea.toFixed(3) + " m²";
resRate.innerText = rateFormatted + " m/s";
// Show result box
resultBox.style.display = "block";
}