Soakaway and infiltration tunnel: local stormwater management

01What is a soakaway and an infiltration tunnel?

A soakaway is an underground void that receives stormwater from roofs and impermeable surfaces, stores it temporarily and lets it infiltrate down into the ground. Traditionally soakaways were made of crushed stone, but modern plastic infiltration tunnels have a far higher void ratio: they take more water in less space, are easy to handle and can be assembled in modules as needed. Norrloop supplies infiltration tunnels in rotationally moulded polyethylene (PE).

02Why local stormwater management?

More rainfall and densification mean the sewer network does not always have the capacity for all the stormwater. That is why more and more local authorities require stormwater to be managed locally on the property itself. A three-step approach to stormwater is the guiding principle:

• Step 1: capture and infiltrate the small, frequent rainfall amounts locally (this is where the soakaway belongs)
• Step 2: attenuate and detain larger rainfall amounts before they are released onward
• Step 3: provide safe flood routes for the extreme downpours

A soakaway solves step 1 by letting the rainwater seep back into the ground where it falls, rather than sending everything to the network.

03How is a soakaway sized?

The right size depends on three factors:

The infiltration capacity of the ground is decisive: sand and gravel take water away quickly, while dense clay barely infiltrates at all. That is why an infiltration test of the ground should be done before the soakaway is sized. Norrloop sizes the reservoir volume to the property's area, rainfall and ground conditions.

04Do I need a sand trap in front of the soakaway?

Yes. A sand trap in front of the soakaway captures sand, grit and particles by sedimentation, so they do not follow the water in and clog the voids. It keeps the infiltration capacity high and extends the service life considerably. The sand trap should be accessible for emptying and inspection.

05Where should the soakaway be placed?

• At a good distance from the building and basement, so infiltrated water does not draw towards the foundation wall
• With sufficient distance up to the highest groundwater level
• In soils that actually infiltrate (not dense clay)
• With cover that gives a frost-free and trafficable solution where needed

The placement is determined by ground conditions and the local authority's requirements. Where infiltration is poor, attenuation is an alternative, see the next point.

06Soakaway or attenuation tank?

The two solutions answer to different ground conditions:

In practice they are often combined: the soakaway takes the small rainfall amounts, while an attenuation tank detains the larger ones. Both are part of steps 1 and 2 of the three-step approach.

07Why an infiltration tunnel in PE?

Infiltration tunnels in polyethylene are corrosion-free, easy to handle and modular, so the reservoir can be built exactly as large as the property requires. The high void ratio gives a lot of storage volume on little area compared with a traditional crushed-stone soakaway.

08Frequently asked questions