How to build and maintain a retarding basin


How to build and maintain a retarding basin


Retarding basins can be built as either open or closed basins. Open basins are usually created as soakaways so that they form part of the landscape. These soakaways can be designed as empty soakaways i.e. they only fill up with water when it rains.

The retarding basin can also be designed as an open basin with standing water. BIO-BLOK® elements are very useful here if you want to prevent accidental drownings. In places with not much space, it is often a better solution to have the retarding basin as a closed basin.

If you look back in history, closed retarding basins are usually made of reinforced concrete. Development in production methods etc. has led to it often being cheaper to build these basins using plastic elements today.

Building a retarding basin is quick and easy to do.

After levelling the bottom, laying out the membrane/fibre cloth (if any) and installing the inlet pipe, you can install the BIO-BLOK® elements. » Read more

You must set out blocks 1 and 2 and join them using at least two stainless steel clips. Then repeat the process by clipping block 3 to block 2 and block 4 to block 3.

Press block 5 against block 1 and they will be joined. Press block 6 against block 5 and block 2, and clip block 6 to these. Then repeat the process with block 7 and block 8 and so on until the first layer has been completed. Remember to use at least two stainless steel clips each time you clip two blocks together.

Please note that the BIO-BLOK® elements are made of extruded net tubes which are then welded together into blocks measuring approx. 54 x 54 x 55cm. As a consequence of this process not all blocks can be equal in size, i.e. there may be a difference of several millimetres. There may also be blocks where the net tubes have a slight curve which can make it difficult to place the blocks right next to each other. This tolerance in production can be solved by pressing the blocks against each other as described above.

To assemble BIO-BLOK® elements in the layer structure, you must use two stacking pipes in the external elements, i.e. in blocks 1, 2, 3, 4, 5, 8, 9, 12, 13, 14, 15 and 16. In all other BIO-BLOK® elements, use just one stacking pipe, i.e. in blocks 6, 7, 10 and 11.

Clipping together using stainless steel clips     Assembly using stacking pipess

The assembly of the second layer of BIO-BLOK® elements is done in the same way as for the first layer. The only difference is that stacking pipes are also used here, as mentioned earlier.

When all the layers have been completed, the pipe connections can be made, depending on the type of connection.

Then wrap the BIO-BLOK® elements in fibre cloth/membrane and cover the structure with sand.

Sand must be filled in and the bottom built up so that the requirements of the construction going above are fulfilled, although the filling etc. must be compressed to a min. of 95% standard Proctor (Isotope method) » Read more

Please note also that it may be a good idea to protect the fibre cloth/membrane with a plastic net over the sand layer. You well then obtain a better pressure distribution so that only an evenly distributed load appears in the retarding basin.



The BIO-BLOK® element is made of the imperishable and eco-friendly material polyethylene, which is also highly resistant to chemicals.

The system therefore does not require regular inspection at fixed intervals, although the installation should be included in the municipality’s set inspection and maintenance programme.

The inspection programme should consist of the following:
♦ Observation of whether any changes have been made to and around the retarding basin.
♦ Inspection of the associated inspection chamber.
♦ Inspection of the catch pit during any emptying.
♦ Inspection of the unit for holding back water.

Inspection of any ventilation.

The reason for this is that holes may appear in the fibre cloth or membrane which normally covers the retarding basin. If this happens, there will be settling in the surrounding ground and the basin may also fill up with soil material.

Number of ventilations
The retarding basin must be equipped with at least one vent pipe, measuring Ø110mm, which must be led at least 0.8 m above the ground or to the closest ventilated inlet.

Catch pit
The retarding basin should be secured with a catch pit of the correct volume, so that sand, particles and leaves etc. do not enter the basin.

The catch pit is to be inspected and emptied as and when necessary.