Retention basin: Small but mighty!

Bentofix® X5F shows its full strength in beautiful Franconia, Germany

In Untersteinach near Kulmbach (Franconia), a small project reveals the great strengths of Bentofix® X5F. Among other things, a large capacity, high impermeability and sustainability from installation to stability were relevant for the rainwater retention basin.

The realisation of the construction measure was initiated by structural changes to the railway line, which runs parallel to the national highway 289. Here, the drainage planning had to be rethought, and a new rainwater retention basin had to be created between the federal road and the railway tracks. One planning challenge was the limited space: on the one hand, the basin was to have a maximum capacity, so a conventional solution with a 1 m thick mineral clay layer was ruled out from the start. On the other hand, geosynthetics require a sufficiently wide anchor trench at the head of the basin edges.

The intelligent solution came from BBG Bauberatung Geokunststoffe GmbH & Co. KG.

Experts for geotextile sealing work there and design corresponding projects almost daily. For the given space, they calculated basin edges that were as steep as possible to gain sufficient capacity and, at the same time, narrowed the anchor trench to the necessary minimum. They proposed the Bentofix® X5F BFG 5300 bentonite mat as a sealing system for the 25m x 10m basin. Only this special bentonite mat has a permanently applied structured PE layer, which is why it can also be installed on steeper slopes and a nonwoven fabric interspersed with bentonite over the entire surface self-sealing overlaps. In contrast to bentonite mats laminated with foils, the permanently applied PE layer does not create an additional sliding joint or a dissolving adhesive joint, which could cause sliding safety problems.

The project planners were immediately enthusiastic about this proposal because the all-rounder Bentofix® could score with further properties. The geosynthetic clay liner (GTD) Bentofix®, also called bentonite mat, consists of highly swellable sodium bentonite at its core, which stands for a high sealing effect. The clay layer is encapsulated at the top and bottom by geosynthetics that are permanently needle-punched together over the entire surface, transmitting shear force. This holds the sodium bentonite in place, resulting in high internal shear strength. The Naue bentonite mats are therefore particularly suitable for installation on steeper pool edges.

In addition, the Bentofix® X5F BFG 5300 type installed in Untersteinach is coated with polyethylene

This layer increases the overall impermeability and prevents the bentonite from drying out even during longer dry phases. Therefore, the bentonite mat in the stormwater retention basin manages with a relatively low cover of only 30cm, which benefits the capacity. At the same time, the rainwater retention basin is always watertight and fulfils its function immediately when it rains.

A unique feature of Naue’s Bentofix® X types is that the polyethylene coating bonds homogeneously with the fibres of the geotextile. This has two effects: The PE coating additionally stabilises the fibre reinforcement and increases the long-term shear force transmission. In the long term, a high level of stability is thus achieved because the different layers are inseparably bonded.

The sealing system was installed in March 2021 by Enig GmbH, Arnstadt, Germany. The company specialises in waterproofing and is very familiar with high-quality Naue products. Due to the cramped conditions, the installation on the relatively steep edge of the pool was no real challenge for the professionals.

The ecological and economic advantages of sealing with a geosynthetic clay liner compared to conventional mineral solutions should not be forgotten here. Quarrying the clay and transporting it to the construction site with many lorry loads would have caused many times more CO2 emissions. In addition, the bentonite mat is more durable and thus more sustainable in this respect as well.

The construction on the levelled, stone-free subgrade looked as follows:

  • Bentofix® X5F BFG 5300, bonded at the overlaps with X-Tape butyl rubber
  • Secutex® R401: The protective fleece protects the polyethylene coating of the bentonite mat from damage
  • Lining with a gravel-concrete layer
  • Coarse natural stone paving

Bentofix® X5F sealing system