Percolation Test: Step-by-Step Guide

What You'll Need

• Spade or mini excavator — For digging test pits
• Measuring tape — Marked in millimetres
• Stopwatch or phone timer — For timing water drop
• Water supply — Hose connection or water bowser
• Recording sheet and pen — To note results
• Wooden stakes — To mark water levels

Estimated time: 3-4 hours (longer if pre-soaking is needed)

Step-by-Step Instructions

Step 1: Choose Test Pit Locations

Select 2-3 locations within your proposed drainage field area. Space them to capture any soil variation across the site.

• At least 5 metres from any building
• At least 2 metres from boundaries
• At least 50 metres from any water source or borehole
• Away from areas that may have been disturbed or compacted

Step 2: Dig the Test Pits

Excavate square pits approximately 300mm x 300mm, to the depth of your proposed drainage field trenches (usually 400-900mm). Keep the sides vertical and the base level.

Step 3: Pre-Soak the Pits

For BS 6297 tests, fill pits with water and allow to drain completely. Repeat until the soil is saturated. This may take several hours or overnight for clay soils. This step normalises dry conditions and gives more accurate results.

Step 4: Fill to Starting Level

Fill each pit with water to a depth of 300mm. Mark this level clearly on the pit wall or use a stake. This is your starting measurement point.

Step 5: Time the Water Drop

Start your timer. Measure how long it takes for the water level to drop from 300mm to 225mm (a 75mm drop). Record this time in seconds.

Step 6: Repeat for Accuracy

Refill to 300mm and repeat the test at least 3 times per pit. Record each result. If results vary significantly, continue until you get 3 consistent readings.

Step 7: Calculate the Vp Value

Average your consistent readings. Divide the time (in seconds) by 75 (the mm drop) to get your Vp value:

Interpreting Your Results

• Vp below 12 s/mm: Soil drains too fast. Insufficient treatment time—may need design adaptation or alternative solution.
• Vp 12-100 s/mm: Suitable for a drainage field. Use the Vp value to size your trenches according to BS 6297.
• Vp above 100 s/mm: Soil drains too slowly. Drainage field likely unsuitable—consider a treatment plant with watercourse discharge instead.

The lower the Vp, the faster water drains. Very fast drainage means wastewater doesn't get enough treatment before reaching groundwater.

UK Standards: BRE365 vs BS 6297

Two testing methods are commonly used in the UK:

• BS 6297 — For septic tank and treatment plant drainage fields. Uses 300mm to 225mm drop.
• BRE365 — For rainwater soakaways. Uses different pit dimensions and falling head method.

Check with your Building Control which method they require for your application.

What to Record for Building Control

• Location plan showing pit positions with measurements
• Date and weather conditions
• Pit depths and dimensions
• All timing results and calculations
• Average Vp value for each pit
• Soil description and any groundwater observations
• Design calculations for proposed drainage field size