Soakage Tests

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Why Do I Need A Soakage Test?

A soakaway test, also known as a percolation test, is necessary to determine if installation of a soakaway system (sometimes referred to as a drainage field) can be performed at a site. Providing the soil is appropriate a soakage test will determine the size of your required soakaway system. If you are currently considering building a new soakaway system or upgrading an existing one, it is vital that a soakage test is conducted to prevent any potential soakaway issues.

Types of Soakage Test

This is a Design Guide by the Building Research Establishment originally published in 1991 and revised a number of times, the latest being in 2016. Sometimes this is referred to as DG365. The test procedure is referenced in building regulations Approved Document H3 for soakaways serving areas larger than 25m2. This test is routinely required.

The test requires a pit of minimum size 1m long and 0.3m wide, to a depth of around 1.5-2.5m depending on the ground, groundwater and likely size of the soakaways. It should have vertical sides trimmed square and, if necessary, for stability, should be filled with granular material and perforated vertical observation tube. A lot of water is used and the pit must be filled rapidly in a short space of time, so water tanks and pumps are needed.

Critically the specification states: “Fill the soakage trial pit and allow it to drain three times to near empty… The filling of the soakage trial pit should be on the same or consecutive days.”

The method does not specify a minimum rate for the water level to fall. However, consecutive days implies each test can be up to 24hrs, and the design guide implies that a soakaway must get to 50% empty in 24hrs.

But here lies the problem: the test could be finished in a matter of hours or potentially up to 4 days.

What is the answer? We have a 4-day test package and offer a daily discount for each day the tests do not run.

We do also offer a 1-day test version, as many competitors only offer that service, so you can compare like-for-like. However, in 24 years of commercial experience our Director can count on one hand the sites that have fully drained 3 times in one day! That said, a single moderately fast test will at least give a good indication that soakaways will work, they can be designed conservatively, and tested once built to confirm they perform satisfactorily (although that does come with some risks).

These tests are far less intrusive as the test hole is in fact drilled using a small drilling rig. Another benefit is that testing can be targeted to different levels, and can be taken deeper than a BRE365 test. The upper part is usually hand dug to check for buried services and so is around 300mm square, with the borehole typically starting in 100mm diameter. A perforated liner pipe is inserted into the borehole to prevent instability. The tests can be used to calculate soil infiltration rate following BS5930 but the procedure can be modified to follow the principles of BRE365. Again, three repeat tests are ideal. There is no maximum duration for the test, but typically they are run once, for 3hrs.

The test procedure is in accordance with BS5930: 2015; Falling Head Test in Boreholes.

These test holes are usually run for borehole soakaways. The trial borehole(s) are sunk with a drilling rig in a midsized diameter (typically 200mm) and to depth typically 10m or more depending on the ground. The ground conditions are vitally important as the soakaway is dependent on a relatively limited surface area, and they cannot penetrate the water table.

The test procedure is described under BS5930: 2015; Falling Head Test in Boreholes. We are also able to undertake the Constant Head version of the test if required.

These tests are used for calculating drainage fields for sewage treatment plants but can also be used for designing soakaways serving smaller impermeable areas up to 25m2. The test is described in Approved Document H1 (and cross referenced in H3) and the test requires a minimum of two trial pits. The test pit is a specific size (300mm square) and can be taken down to any depth up to 1.50m, but should be 300mm below the intended drainage level (of course, this may not be known, so a range of depths could be evaluated). Each hole must be filled once with water and left to sweep away overnight, and then the test proper has to be run 3 times. There is no maximum duration for the test but if longer than 250mins (4hrs 10mins) it may be necessary to have secondary treatment system. This does therefore imply that the full test can run longer than 250mins, and consequently the full test programme can potentially extend into 4 days.

This testing also corresponds to BS 6297: 2007; Percolation Test for Septic Tank Insulations.

This is a specific test procedure for small soakaways for residential dwellings which is similar to the Falling Head Soakage Test (FHST) but carried out in an un-lined hole of a specific diameter and at several depth intervals.

The borehole is to be 150mm in diameter to a depth of 2m, with the lower 300mm section filled with water (with an unspecified number of repeats); the hole is drilled to 2m and the procedure repeated. Further depth increments may be considered where the soil permeability increases with depth.

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