Module 3: Soakaways, Drainage Fields & Percolation

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CORE Off-Mains Drainage · Module 3 of 4

Content

Slide 11

Why soakaways block

Trainer note:

Sludge builds up in the primary settlement tank

Slide 12

Why soakways block

Trainer note:

When the system isn’t emptied regularly Sludge builds up and enters the soakaway or if the T pipes or H pipes are damaged

Damage to the baffle can also occur if the tank is not emptied regularly as the sludge builds up and pushes on the baffle.

Slide 13

Why soakways block

Trainer note:

Sludge enters the soakaway eventually completely blocking it.

Other issues can be damaged pipework from tree roots or ground movement as usual damage

Slide 14

Septic Tank and herring bone soakaway

Slide 15

Perforated soakaway pipe

Trainer note:

Most Perforated pipe soakaways use a shingle bed, A shingle covering and a membrane. This allows the effluence to drain from the pipe and into the shingle and then into the ground strata,

Slide 24

Percolation test method

Step 1: Excavate a test hole 300mm square x 300mm deep below proposed
invert level of the drainage field trench bottom
Step 2: Fill the test hole with water and allow drain away over night
Step 3: Refill to a depth of 300mm and note time taken in seconds to drain
away from 75% full to 25% full (i.e. 150mm drop in level from 225mm to
75mm)
Step 4: Repeat the procedure in two more test holes and calculate the
average of the three results as follows: test 1 + test 2+ test 3 = average
3 time taken
Step 5: Calculate the Vp (average time in seconds for the water to drop 1mm)
as follows:
For example: If average time above took 2700 seconds
(i) Divide 2700 seconds by 150mm depth of water
(ii) 2700 = 18 Vp* (see note below*)
150
(iii) Area of trench = number of persons to use property X Vp X 0.25
Therefore: 5 persons X 18 X 0.25 = 13.5m2 of effluent drain required.
(iv) To calculate actual length of trench divide 13.5 by width of the trench
required therefore: 13.5m2 = 22m (Minimum permitted area is 30m
0.6m wide long x 0.6m wide)
* Vp should range between 12 and 100 to be successful; otherwise the system should be designed by a drainage specialist.

Trainer note:

A percolation test needs to be carried out to check if the soil makeup is acceptable for a soakaway system and to calculate the size of the soakaway required

Cross-references

Catalyst Percolation Test Methodology course — full calculator and worked examples.
Drainage Claims (WRc) Module 4 — Private Treatment Systems — for full Part 4 treatment.

Knowledge check

Pass mark: 80%. You'll get immediate feedback with the correct answer + rationale on each question. Wrong answers can be retaken without penalty.

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