A soakaway size calculator spreadsheet makes iterative soakaway design much quicker and easier, allowing the designer to complete an optimal design in minutes, writes Stephen Webster.

Disposes of rainwater runoff

The purpose of a soakaway is very simple. As the name suggests a soakaway disposes of rainwater runoff through infiltration into the surrounding soils, that is, the water is allowed to soak away.

In almost all cases the rate at which the water is able to infiltrate into the soil is much slower than the rate at which rainwater runoff is collected. For this reason a soakaway must include a chamber large enough to store the excess rainwater allowing it to slowly disperse into the ground between storm events.

While the principles of soakaway design are very simple, the design of a soakaway is not straightforward. This is particularly the case when the size or shape of the soakaway must be optimised for economic reasons or due to limitations in the space available.

Soakaway storage volume.

The complexity in the calculations is largely due to the fact that the two main parameters are interlinked. The size of the soakaway affects both how quickly the water drains and how much water the soakaway can store.

Plus the soakaway size itself is dependent on three dimensions which do not all affect the outcome equally. Therefore it can be a time-consuming iterative process optimising the soakaway size. One way to speed up the process is to use design software such as this commercial soakaway design spreadsheet.

The first parameter to be calculated is the volume of rainwater runoff which the soakaway will be required to accommodate. This will depend on the size of the catchment draining into the soakaway, the location and rainfall characteristics of the particular site, and the desired frequency with which the deign runoff volume will be exceeded and the soakaway will flood.

This in turn will depend upon the consequences of the flooding and the cost in increasing the size of the soakaway and thereby lessening the flooding risk.

Calculating required rainwater storage volume

There are many different methods of calculating the required rainwater storage volume. The method chosen will depend on the size of the catchment and the accuracy required as well as any national or international standards which are applicable.

For detailed soakaway design it is beneficial to use a reasonably complex runoff calculation method as this will include more detailed information on the rainfall intensity for different storm durations.

This will allow the designer to optimise the soakaway for the worst-case design storm which can be either short intense rainfall events or longer less intense storms.

This leads to a much more economical and reliable solution than simply specifying a rainfall intensity and storm duration from a standard or from experience of the site.

The most important factor determining the required size of a soakaway is the infiltration rate of the surrounding soils. There are several theoretical ways of estimating the infiltration performance or hydraulic conductivity of a soil, but these can never capture the full picture of a specific site.

Many site specific factors can greatly affect the infiltration rate achieved through the soils such as the depth and variability of the ground water table, the compaction of the soils (which can take place during the construction of the soakaway), the lithology of the underlying bedrock which can contain fractures and fissures which greatly increase infiltration and the vertical and horizontal extents of the soils.

Many of these factors would not even be found by a typical site investigation programme and if they were it would be difficult to measure the extents of the features or to estimate the likely effects on the infiltration rate.

Infiltration performance of local soils

The simplest, cheapest and most effective way to estimate the infiltration performance of the local soils is to conduct an infiltration test. This involves digging a small hole preferably of a similar size and depth as the proposed soakaway, filling it with water and then measuring the rate at which the water disperses.

BRE Digest 365 describes the procedure in detail. If this test is conducted in accordance with this procedure the infiltration test will give the designer a good indication of the infiltration rate which can be expected onsite.

Where an infiltration test is not possible for some reason or where a preliminary design is required, infiltration rates can be estimated from general site investigation info. These typical infiltration rates, however, come with many caveats and are usually given in extremely broad bands.

The reason for this is as described above - site specific factors greatly influence the infiltration rates achieved on the particular site. Typical infiltration rates should only ever be used for preliminary design and even then only with caution. An example of typical infiltration rates of different soils is shown below.

When the volume of rainwater and the infiltration rate have been determined the soakaway can be sized. In most cases there will be a maximum desired depth, even if only for buildability reasons.

In many cases there will also be a limitation on the area of the soakaway in its preferred location. For this reason the size of the soakaway often needs to be optimised. Generally it is best to install the soakaway as deep as is practically possible.

Leads to more economical design

This is because the depth of the soakaway has a greater influence on the infiltration rate achieved; therefore a deeper soakaway will require less storage volume than a shallow soakaway. This minimises the footprint of the soakaway and generally leads to a more economical design.

The maximum practical depth of the soakaway can be affected by many factors including the ease at which the soils can be excavated with or without support, any contaminated ground layers and the depth of any impermeable strata such as clays and silts.

The most important factor is the groundwater level. For obvious reasons the soakaway should not extend below the groundwater level as this part of the soakaway will fill with water and will not achieve any infiltration or allow storage of runoff water.

Also it is important that there is about a 1.0m gap between the bottom of the soakaway and the groundwater level. This provides a layer of soils which filter out any contaminants which are washed into the soakaway with the runoff water before they can reach the groundwater.

Finally a design check is required to estimate the time that it will take for the soakaway to empty. This is because some slowly infiltrating soils can lead to soakaway designs which take many days or even longer to empty after they have been filled by a storm event.

While this is acceptable for a single storm event, there is a risk that a smaller storm event occurring a day or two after the first can cause the soakaway to fail as it is still mostly full from the first event.

To avoid this risk many design standards including BRE Digest 365 specify that the soakaway must be able to empty half its storage volume within 24 hours. This also prevents unsanitary or anaerobic conditions from forming within a soakaway which has a pool stagnant water at the bottom for long periods following rainfall.

Once the maximum practical depth is determined, the size can be calculated. Again the size of the soakaway in plan also affects the infiltration rate and the required volume, requiring the calculations to be undertaken again.

This can be a tedious process to complete by hand, with a first guess at the required size, infiltration rates calculated, required storage volume calculated and then checked against the proposed storage volume and finally a check completed on the emptying time.

This process will inevitably need to be repeated multiple times before an acceptable size can be found and must be repeated many more times if an optimised size is required. The process can be completed with greater accuracy and much easier and faster with a simple design spreadsheet.

A spreadsheet programme or similar software can complete all the calculations instantly, allowing a much faster and more accurate optimisation process.

Author: Stephen Webster, chartered civil engineer, runs a civil and structural engineering design software company, CivilWeb Spreadsheets, based in the UK. It designs and builds advanced excel spreadsheets for completing engineering design calculations.