When evaluating the suitability of a soakaway for a particular site an evaluation of the whole life costs of the various options should be undertaken. This should include construction costs alongside the operating and maintenance costs of the soakaway during its design life and the cost of decommissioning or replacement at the end of the soakaways design life. Where the soakaway is to be adopted an evaluation of the operating and maintenance costs relevant to the adopting body will often be required.

In many cases a soakaway can be installed at a lower capital cost than an equivalent positive drainage solution, particularly where this would involve long drain runs, new outfalls or upgrades of existing drains. This is often a developer’s primary concern, particularly where the soakaway is to be adopted.

Construction Costs

These can usually be estimated from previous Contractor and supplier rates. Care should be taken when applying rates for shallow soakaways to deeper soakaways as the installation methods may be quite different. Soakaway construction advice and details are shown here.

Maintenance Costs

Soakaways require regular maintenance including sediment removal. Many soakaways do not receive adequate maintenance and this will lead to failure and replacement sooner than anticipated. The level of maintenance required is dependent on the soakaway design and the quality of the runoff.

Positive drainage systems and other alternatives all require regular inspections and some level of maintenance. These are often only practised when the drains or inlets become blocked.

Replacement Costs

When a soakaway has reached its design life it is likely to be replaced by removing the aggregate backfill and any surrounding soils which have become blocked and replacing the backfill. This can usually be done much cheaper than the original construction, unless there are access difficulties or paving over the top of the soakaway.

The frequency of replacement will depend on the design life of the soakaway. Soakaways are often designed with a 10 year design life, which means they will often need replacing several times before a positive drainage system would need to be replaced. The effective soakaway design life is dependent on the level of maintenance and can be extended through good design though this may increase capital costs.

Other Costs

There may be a number of other project specific costs which need to be included. These can include planning costs, connection charges and any potential environmental liabilities.

Future Cost Discount

In order to accurately compare costs which will not occur at the same time, it is common practice to convert all costs to a Present Value. Many methods are available for converting costs to a Present Value, the discount rate is often set in line with the development or developers expected rate of return.