Soakage Testing Services
SOAKAGE TESTS
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You may be asked by the local authority, warranty provider, environment agency or consultants for the project to carry our soakage tests for storm or foul water soakaway design purposes.
This testing is required to prevent small sized generic traditional rubble filled soakaways being installed, which may not provide suitable infiltration and/or may not have a sufficient holding capacity. The consequence being that heavy rainfall events may result in the soakaway becoming inundated, possibly causing flooding to the site, or properties beyond.
New soakaways for storm or foul water now need to be adequately designed.
Our charges for soakage testing are dependant on many factors, including the expected geology, the number of tests required, test method and any access restrictions. Therefore, please contact us for a bespoke estimate.​
Storm Water Soakage Tests (BRE DG 365)
Storm water soakaways often use the guidance provided in the Building Research Establishment document BRE DG 365, typically including for 1 in 100 year rainfall events, also an allowance for future climate change.
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Normally this testing is carried out in trial pits formed by an excavator, typically the pits are 1.0-3.0m long, and 0.3-1.0m wide. The soakage test depths often vary between 1.0-3.0m, the depth often governed by the expected soakaway invert level, the area to be drained, the depth where permeable soils are expected, also the ground water depth (groundwater should not rise to the level of the base of the soakaway during annual variations in the water table).​
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Where possible, it would be prudent to drill a borehole(s) at the soakage test location(s) installed with a standpipe(s). This would help determine the most permeable soils to be targeted with the soakage tests, also enable monitoring of ground water level variations.
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It is recommended that the soakage tests should be carried out near the location of the proposed soakaways. If soakaway trenches may exceed 10m long, a second test is recommended, with further tests every 25m.
Once formed, the pits should be rapidly filled with water up to the proposed invert level, and allowed to drain three times, to near empty. After each fill, the water level and time from filling should be recorded at frequent intervals. Extrapolated results are not advocated. The results are then collated and soil infiltration rates calculated for soakaway design purposes.
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A standard tap supply is generally not suitable to fill soakage pits as the pits need to be rapidly filled, therefore water is normally supplied using a trailer towed 2,000 litre pumped water bowser, 1000 litre IBCs, or lorry mounted water bowser.
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In normal circumstances, where testing is required to adhere to BRE365, we recommend that one day 'open' soakage tests are only carried out where ‘good’ or 'very good' draining soils are present, otherwise ‘stone filled’ soakage tests should be performed.
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It should be noted that if one day ‘open’ soakage tests are requested, in all but ‘moderate’ and ‘good’ draining soils, it is possible even the first fill may fail to achieve the ¾ empty level in the 3 - 4 hour monitoring period typically available in a day (allowing for pit excavation and backfilling).
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It should be noted that BRE DG 365 recommends that soakage tests should be filled three times. The use of extrapolated figures are not advocated.
Although standard one day ‘open’ soakage tests are quick, simple and relatively cheap, the disadvantages are as follows:
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iGeo are not prepared to be responsible for open trial pits filled with water to be left unattended, or overnight (even if Heras fenced), therefore the normal monitoring period available is 3 - 4 hours before backfilling is required.
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Collapsing of the excavation sides may result in unsafe pits, inaccurate estimated monitoring values, also poor ability to backfill.
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Three fills may not be possible, indeed only one may be achieved with extrapolation attempted if the water does not drain within the monitoring period. Any infiltration rates and example soakaway sizes calculated may be rejected.
The alternative test method is ‘stone filled’ soakage tests, which attracts a higher cost, but has the following advantages:
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The pits are filled with stone on completion, resulting in:
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stable sides
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safe monitoring
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refills possible at any time
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less long-term settlement
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Voids typically ~40%, which results in less water being used, therefore a shortened test period
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Data logging usually carried out, which results in:
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Frequent day and night, accurate water level measurements
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Monitoring period can be over days or weeks
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However, additional costs would apply for supplying the single sized stone, removal of the trial pit arisings, datalogger costs, also return visits to site with water to refill the pits.
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Drainfield Design Soakage Tests (Foul water)
Drainfield soakage tests are carried out to BS6297:2007+A1:2008.
This testing aims to minimize environmental, amenity or public health problems which could arise from the inappropriate design/use of non-mains wastewater systems.
These soakage tests calculate the required length/floor area of drainfields based on the number of people serving the septic tank / package treatment works (e.g. Klargesters).
The tests carried out by forming 300mm x 300mm x 300mm hand dug soakage test pits at the proposed invert level of the drainage fields, ideally in permeable material.
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Typically, this requires an excavator to form a pit down to invert level, with the smaller test pit then hand excavated.
The British Standards recommend a minimum of two test pits in the proposed drainfield area, at a depth of the likely drainfield base. The standards also require three fills of the drainfield test.
As our engineer will need to enter the trial pits to form the soakage pits, trial pits below 0.5m deep would need to be battered back to a safe angle to enter without the risk of harm. Battering back will increase the footprint of the excavation and area of disturbed ground on completion. We do not have the training or equipment to shore up the trial pits. It is considered that battered back pits greater than 1.5m deep would not be practical to construct.
As BS6297 requires these tests to be repeated three times at each location, with a minimum of two locations tested, it may not be possible to complete this testing in the day due to poor outflow rates, or a limited site water supply. If compliance to the standards is required, it may be necessary to attend the site on multiple visits:
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If the test pit sides are likely to be unstable during the testing period, and/or, if infiltration is expected to be poor, where three fills would not be complete within the day the excavator is present, there is an option for us to install a 300mm cube mesh box in a woven filter bag into the pit, along with a monitoring standpipe. The pit could then be very carefully backfilled, leaving surplus spoil heaped over the pit, then filling and monitoring could be carried out at leisure (if necessary utilising dataloggers).
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Alternatively, assuming the client confirms in writing they will take full responsibility for the test positions whilst they remain open, also the backfilling on completion (it may be possible for shallow test pits 1.0m deep to be battered back to a very shallow angle, or surrounded by securely fixed Heras fencing).
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If requested, the engineer from iGeo could provide a number of 20 litre barrel(s) to fill each of the 300mm cube test pit(s). However, it would be ideal if there was a water supply on site to fill the barrels, otherwise we would be limited to one fill of each test pit, per visit to site.
Once testing was complete, Vp values would be calculated for using in the following calculations:
For domestic premises, the floor area of the drainage field required should be calculated as follows.
A = p × Vp × 0.25 (for septic tanks)
A = p × Vp × 0.20 (for effluents which have received secondary treatment)
A = required drainage field floor area in square metres (m2).
p = number of people served by the tank (for domestic applications this should be the maximum number of people that could live in the dwelling).
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Generally it is considered that Vp values should be between 15 and 100 (300mm of water fully draining between 1.25 and 8.3 hours), otherwise the regulator should be consulted for further options.
Full details can be found in BS6297:2007+A1:2008.
