Percolation Test - local suggestions near Edinburgh?

[Meabh]

Hi - Any recommendations for a percolation tester for surface water near East Lothian? Or is it really a DIY job?

 

We have built a house on very sandy soil and as the scaffold is coming down we are ready to sort the surface water drainage.

 

We knocked down a house built in the 70's and discovered the rainwater was discharged under the house (no issues) but had thought it was fed into a local burn.

Original plan for new house was to discharge into the local burn but during groundworks last year we discovered the burn doesn't actually run through the end of our garden (it's a bit further over into a neighbours garden). After discussion we decided it would be a soakaway. BC and Scottish water happy with that and said we just needed percolation test. 

 

I'd love to get it done quickly so we can get the soakaway design sorted and get the drainage lines dug up in the garden while I have a mini digger in for services.

 

Thanks

 

 

[ProDave]

If you have the digger there, dig a 1 cubic metre hole in the ground, and then in the bottom of that dig a 300mm cube hole.  Fill that small hole with water and time how long it takes for the water to drain away.  

[Jilly]

It's quite easy to do, there are instructions on line. Why not ask your BC officer if he will accept your/builder's word for it and your proposed location for the soakaway? Sandy soil should give a good result. 

[Duncan62]

https://forum.buildhub.org.uk/topic/35099-streetfield-suds-system/?do=findComment&comment=513546

 

(And if you can spot the mistake in the very final calculation, you have understood it. The answer is correct but a number in the equation are not)

 

The Building Regulations 2010 Drainage and Waste Disposal Approved Document H 2015 Edition: Section 3.27 states “Soakaways should be designed to a return period of once in ten years”.

 

However, our LA wanted to see 1 in 100 calculations

 

 

 

The soil infiltration rate (f) is calculated with the equation:

 

 

f = Vol75-25 / ap50 x tp75-25

 

 

 

f: soil infiltration rate (m/s)

 

Vol75-25: volume of water in the trial pit between 75% - 25% effective depth (m³)

 

ap50: internal surface area of trial pit up to 50% effective depth, including the base area (m²)

 

tp75-25: time for water to fall from 75% - 25% effective depth (secs)

 

 

 

The storage volume should be calculated so that, over the duration of the storm, it is sufficient to contain the difference between the inflow volume and the outflow volume.

 

 

 

The inflow volume (I) is calculated from the rainfall depth and the area drained. The outflow volume (O) is calculated from the area of the side of the storage volume when filled to 50% of its effective depth and D, the duration of the storm in minutes.

 

 

 

 

 

Storage Volume Method

The soakaway storage volume (S):

 

S = I – O

 

S: Soakaway storage volume (m³)

 

I: Inflow from impermeable area drained to soakaway (m³)

 

O: Outflow infiltrating into the soil (m³)

 

 

 

Inflow (I):

 

I = A x R

 

A: Impermeable area to be drained to the soakaway (m²)

 

R: Total rainfall in design storm (m)

 

 

Outflow (O):

 

O = as50 x f x D

 

as50: Internal surface area of proposed soakaway up to 50% effective depth, excluding the base area (m²)

 

f: Soil infiltration rate (m/s)

 

D: Storm duration (sec)

 

 

 

 

 

The following 1 in 100 year return period rainfall values will be used for the infiltration proposals. This is calculated using the Wallingford Procedure for the Cambridge area with catchment details:

 

5-year return period rainfall of 60 minutes duration = 20

 

r Ratio Rainfall Value = 0.45

 

Standard Average Annual Rainfall = 550

 

WRAP Soil Index = 2

 

Runoff Coefficient = 100%

 

Climate Change Allowance = 1.4

 

Return Period = 100 years

 

 

 

Storm Duration (Minutes)

 

5

 

10

 

15

 

30

 

60

 

120

 

240

 

360

 

600

 

100 Year Return Period Rainfall (mm)

 

20.50

 

28.96

 

35.18

 

45.07

 

56.00

 

67.42

 

78.11

 

85.05

 

93.98

 

Table 4: 100 Year Rainfall Values

Infiltration Rate Calculation

The proposal presents a rainwater run-off of 115m² from the proposed dwelling roof area and 62m² from the proposed garage roof area.

 

The soil infiltration rate should be calculated as follows:

 

f = Vol75-25 / ap50 x tp75-25

 

 

 

f: Soil infiltration rate (m/s)

 

Vol75-25: Volume of water in the trial pit between 75% - 25% effective depth (m³)

 

ap50: Internal surface area of trial pit up to 50% effective depth, including the base area (m²)

 

tp75-25: Time for water to fall from 75% - 25% effective depth (sec)

 

Vol75-25 = 0.3 x 0.3 x (0.3 x 0.5) = 0.0135m³

 

 

 

ap50 = (0.15 x 0.3 x 4) + (0.3 x 0.3)

 

= 0.18 + 0.09

 

= 0.27m²

 

 

 

Average tp75-25 = 2870 sec

 

 

Average infiltration rate to be used for design,

 

 

 

f = 0.0135 / (0.27 x 2870) = 1.74 x 10-5 m/s

 

 

Storage Volume Calculations

The proposed soakaway is to be made with dimensions of 1.40m effective depth, 3m long and 3m wide.

 

 

 

Inflow I = A x R

 

A = 115m² + 62m² = 177m²

 

R = M100-5 = 20.50mm = 0.02050m

 

 

 

I = 177 x 0.02050 = 3.63m³

 

 

 

Outflow O = as50 x f x D

 

 

 

as50 = (L x h x 2) x0.5 + (W x h x 2) x0.5

 

= (3 x 1.4) + (3 x 1.4)

 

= 8.40 m²

 

f = 1.74 x 10-5 m/s

 

D = 5min = 300sec

 

 

O = 8.40 x (1.74 x 10-5) x 300

 

= 0.044m³

 

 

Storage Volume

S = I – O:

 

= 3.63 – 0.044

 

= 3.58 m³

 

 

 

Calculations were then repeated for a range of critical storm durations.

 

Storm Duration (min)

 

Required Soakaway Volume (m³)

 

5

 

3.58

 

10

 

5.04

 

15

 

6.09

 

30

 

7.71

 

60

 

9.38

 

120

 

10.87

 

240

 

11.71

 

360

 

11.88

 

600

 

11.34

 

1440

 

7.14

 

Table 5: Soakaway Volume for Range of Storm Durations

 

 

The calculations have indicated the critical storm duration to be 360 minutes in this situation, giving 11.88m³ soakaway volume requirement.

 

Using geocell soakaway crates with 95% void space gives an effective volume of the soakaway:

 

 

 

EVol = 3 x 3 x 1.4 x 0.95 = 11.97m³

 

 

Effective Volume 11.97m³ > Required Volume 11.88m³

 

 

 

 

It should also be ensured that any proposed soakaway discharge from full to half-volume within the required maximum 24 hour duration, to allow for subsequent storm inflow.

 

The half drain time (ts50) should be calculated as follows:

 

 

 

ts50 = S x 0.5 / as50 x f

 

= (11.88 x 0.5) / (11.88 x 1.74 x 10-5)

 

= 40716 sec

 

= 11.31 hrs

 

 

 

 

Half Drain Time 11.31hr < Required Drain Time 24hr

Edited February 26 by Duncan62

[MortarThePoint]

I dug it the hard way and it wasn't too bad 🙂 I'd say it's a good DIY job for anyone who enjoyed school science experiments

[Duncan62]

Content duplication, delete

Edited February 26 by Duncan62

[saveasteading]

3 hours ago, ProDave said:

Fill that small hole with water and time how long it takes for the water to drain away.  

Lots of good advice here. I think that one thing missed is that you should fill the hole once to soak the ground, and then do it again while timing it.

Don't cheat as it will,be your garden that floods.

There are alternative ways to build soakaways, then to fill a hole. So come back on when you have figures.