Retention of rainwater in structures of plastic blocks

Are you afraid of silting?

Within the design of safe drainage of rainwater in case of high or even extreme rainfall construction of underground accumulation structures with the use of plastic blocks is no longer a novelty. However, this construction method of underground retention tanks using the installation of plastic blocks often makes decision-makers worried about the possibility of uncontrolled silting.

Out of a relatively wide offer of various types of blocks you can select from two basic installation systems only:

1. Retention tank of blocks with direct inflow into the blocks (without a bottom distribution drain)
2. Retention tank of blocks with a bottom distribution drain

Out of these two systems the above mentioned problems of silting can be avoided in the bottom drain system. Most of the silt is further drained to the sewerage system via this drain and is not retained in the blocks.

This fact was verified by an independent study that was carried out at the University of Sheffield (UK), the Department of Construction Materials and Structures. The objective of the study was to analyze the behaviour of various types of soiling in rainwater retention tanks made of plastic blocks.
It was just the above mentioned configurations of installation and block types - without a drain and with a drain - that were tested.

Test tanks with the dimensions of 3.6 x 1.8 x 1.2 m equipped with an inlet and outlet pipeline with controlled inflow of 7 l/s were subject to the tests.

• The first configuration consisted of a tank of 160 blocks in 4 layers arranged on each other with 8 x 5 boxes each
• The other configuration consisted of AS-NIDAPLAST 2400x1200x520 mm blocks in 2 layers, seated on a gravel bed with the thickness of 40 cm with a Ø 225 mm drainage pipeline.

Zkušební instalace bez drenáže

Figure 1: Test installation without a drain

Zkušební instalace s drenáží pod bloky AS-NIDAPLAST

Figure 2: Test installation with a drain under

Three various factors were studied in a regime with stabilized flow of 7 l/s:
1. Tracking substance with the measurement of concentration of a fluorescence substance
2. Water with sand with granularity of up to 75 microns in the concentration of 75%
3. Flow condition with large waste (plastic bags)

Measurement results
- In the first case of water with a tracking substance water flow in the tank was mapped,
which showed disorderly flow of water in the tank blocks

Konfigurace bez drenáže

Figure 3: Configuration without a drain

Konfigurace systému AS-NIDAPLAST s drenáží

Figure 4: Configuration of the AS-NIDPALAST system with a drain

- The second inflow regime with a settling substance (fine sand) was to prove proneness of both the tank configurations to silting. It was proved that sediments that are commonly found in rainwater are almost 100% removed from the retention system in the case of the AS-NIDAPLAST system with a bottom drain, which prevents the risk of silting and blocking up. On the other hand, in a block system with direct inflow without a bottom drain 75% of sediments remain inside the system, which indicates a serious risk of silting and blocking up of the tank during operation.

- The last simulated regime was the flow of large waste items represented by plastic bags from supermarkets, which corresponds to the real situation again. The configuration with the AS-NIDAPLAST blocks with a bottom drain managed to keep such waste in the sewerage system and all the bags were washed out of the system. In the configuration with direct inflow into the blocks all the bags remained stuck inside the blocks (caught on the first block).


The study of the Sheffield University (UK) has clearly proved that the configuration of AS-NIDAPLAST blocks with a bottom drain is more reliable and safer than tank configurations with blocks with direct inflow.
At the same time it has shown that in tanks with a bottom drain there is only a minimum risk of silting with fine sediments and almost a zero risk of blocking the tank with large waste items of the plastic bag type.

Ing.Oldřich Pírek