BoRSiS soil-pipe system – implemented for the first time

City of Leichlingen: Project realised in line with the sponge city strategy

The city of Leichlingen (Rhineland) has taken the first steps towards becoming a sponge city with the Friedensstraße rainwater disconnection project – a pilot project for the sponge city of Leichlingen. In order to be able to respond better to heavy rainfall, dry periods and heat waves, the town is focusing on innovative rainwater projects. In autumn 2025, under the leadership of the municipal wastewater utility and with the support of partners from the interdisciplinary R&D project ‘Soil-pipe system as an innovative element of climate-adapted urban drainage (BoRSiS)’, it launched and implemented a pilot project in line with the sponge city strategy in Friedensstraße (see also schwammstadt.de).

Innovative soil-pipe system (BoRSiS) in Leichlingen

Use rock wool and ductile cast iron pipes to cushion heavy rainfall and create sufficient root space in the pipe trench

The BoRSiS system combines the storage of rainwater from disconnected areas and the promotion of tree growth in a single linear construction measure: mixed water continues to be safely drained via root-resistant ductile cast iron pipes with cement mortar coating, while the pipe zone in the pipe trench is designed as a ventilated root space. The Rockflow rock wool in the main filling of the pipe trench stores both small and large amounts of rainwater and releases it over a longer period of time by capillary action via a compensating layer of sand into the root space. The system thus significantly reduces the risk of flooding during heavy rainfall events and at the same time provides a generous, expanded root space for roadside trees – without requiring any additional space!

Technical data

Pipe system: Ductile cast iron pipes with cement mortar coating. These pipes are root-resistant and ensure wastewater drainage despite the pipe zone being used as root space.
Root space: Ventilated coarse-grained substrate in the pipe zone serves as root space and also stores water.
Water storage: Rockflow rock wool is installed in the main backfill of the pipe trench. Rockflow stores large amounts of rainwater and releases water capillarily to the newly planted trees.
Infiltration: A layer of sand between the rock wool and the root space allows water to infiltrate into the coarse-grained substrate, while the root-resistant pipes continue to drain mixed water.
Sustainability: Rockflow rock wool and ductile cast iron pipe systems are not only recyclable, but also 100% reusable. The systems are durable and designed for easy access and cleaning.