Percolation Value (Vp)

What Is Percolation Value (Vp)?

The percolation value (Vp) is a measure of how long it takes for water to drain through the soil. It represents the average time (in seconds) for the water level in a test hole to drop 1 mm.

• - Low Vp → water drains quickly
• - High Vp → water drains slowly

Vp is not a direct measure of permeability, but it is a practical field indicator of how well the soil can accept treated effluent.

Percolation Test Requirements (BS 6297)

BS 6297 sets out a clear methodology for determining Vp. The test must be carried out after the groundwater level has stabilised and when the soil is at its natural moisture content—not during drought or saturation.

• Test Hole Dimensions

- Square hole: 300 mm × 300 mm

- Depth: 300 mm below the proposed invert level of the drainage trench (typically 700–900 mm deep)

• Preparation

- Scarify the sides and base to remove smearing

- Add 50 mm of clean gravel to the base

- Fill the hole with water to a depth of 300 mm

- Allow to soak for at least 30 minutes (longer in clay soils)

• Test Procedure

- Refill to 300 mm

- Measure the time for the water level to fall from 300 mm to 75%, 50%, and 25% depth

- Convert the drop time into seconds per millimetre

- Repeat the test three times per hole

- Use at least two test holes

- Calculate the average Vp across all valid readings

How to Calculate Vp

Vp = t / d, where t = time (seconds) and d = depth of fall (mm).

Example: If the water level drops 150 mm in 900 seconds, Vp = 900 / 150 = 6 s/mm.

Acceptable Vp Values for Drainage Field Design

• - 12–100 s/mm → Suitable for drainage fields
• - < 12 s/mm → Too fast (risk of groundwater contamination)
• - > 100 s/mm → Too slow (risk of ponding and system failure)
• - 100–140 s/mm → Possible with caution and increased trench area
• - > 140 s/mm → Not permitted

Using Vp to Size Drainage Fields

Formula: A = 0.25 × Vp × P, where A = trench length (m), Vp = percolation value (s/mm), P = daily flow (L/day).

Interpreting Vp in Real‑World UK Soils

Soil Type | Typical Vp | Suitability

Sand / Gravel | 5–20 | Often too fast

Sandy Loam | 15–40 | Ideal

Clay Loam | 40–90 | Acceptable

Clay | 90–200+ | Often too slow

Common Testing Mistakes

• - Testing during wet season → artificially high Vp
• - Testing during drought → artificially low Vp
• - Not pre‑soaking clay soils → misleadingly fast results
• - Smearing the hole sides → slows infiltration
• - Using only one test hole → non‑compliant

Why Vp Matters for System Approval

Authorities rely on Vp to confirm soil suitability, correct trench sizing, pollution prevention, and compliance with BS 6297 and Building Regulations Part H.

Key Takeaways

• - Vp is essential for drainage field design
• - Acceptable range: 12–100 s/mm
• - Vp < 12 → too fast
• - Vp > 100 → too slow
• - Testing must follow BS 6297
• - Vp determines trench length and layout

The CivilWeb Drainage Field Design Spreadsheet is able to complete all the required calculations fully in accordance with BS 6297:2007+A1:2008, and includes unique optimization tools to speed up the design.