IKT – Institute for Underground Infrastructure | What Goes Back Into the Sewer Trench? – Flowable Backfill Materials Put to the Test

What Goes Back Into the Sewer Trench? – Flowable Backfill Materials Put to the Test

Posted 14. July 2025

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Test setup: One of a total of five chambers before the Temporarily Flowable Self-Compacting Backfill (TFSB) was filled. Set up in the IKT’s large-scale test rig with dimensions 6 x 6 x 15 m.

Settlement damage, uneven bedding, and poor re-excavation capability – these are common issues on sewer construction sites. Often, the cause lies in the backfill.

Temporarily Flowable Self-Compacting Backfill (TFSB) materials promise better performance – but which ones actually deliver?

We tested five different TFSB products in full-scale sewer trench simulations. The aim: to evaluate how these materials perform under realistic construction conditions.

The Approach: Large-Scale Testing, Real-Site Conditions

At the IKT’s large-scale test facility in Gelsenkirchen, we set up actual sewer construction scenarios, including pipes, manholes, trench shoring, and bedding layers.

Pipe enclosure without defects (DN 100)

We tested the materials for:

Flowability and installation quality
Walkability and early load-bearing capacity
Re-excavatability after hardening
Volume stability and risk of subsidence
Environmental performance and recyclability

We used innovative inspection tools like the 🔗 MAC system for pipe-soil stiffness and a walkability test after 3 hours to assess real-life performance.

Key results for sewer construction:

Not all flowable backfills are created equal: Some harden too much, while others remain unsuitable for overbuilding, e.g. for road construction, for up to 56 days.
Re-excavation is a challenge – for good excavation, TSFB must not exceed a compressive strength of 0.3 N/mm² after 28 days.
New test methods like the Mini-MAC and walkability test ensure practical and measurable evaluation.
Environmental compliance, shrinkage, and recyclability vary widely between products.
Two materials stood out: RSS® Flüssigboden and carbofill® showed the best balance between performance, usability, and long-term suitability.

Pipe enclosure with minor defects (DN 100)

Comparative Test Results: Two Materials Performed Best – Others Failed Key Criteria

We graded each material on a scale from 1.0 (“very good”) to 6.0 (“inadequate”).

Special emphasis was placed on re-excavation capability and early workability, both essential for practical use.

Top performers:
✅RSS® Flüssigboden – well-balanced, walkable, easy to re-excavate
✅ carbofill® – strong technical performance and consistent usability

We observed critical issues with:
❌ Terrapact© – hardened too much, impossible to excavate
❌ WBM-Flüssigboden® – excessive shrinkage, ammonia emissions, and safety concerns

Materials that failed key performance or environmental criteria were ruled out for sewer construction use.

Pipe enclosure with minor defects (DN 300)

Why This Matters for Sewer Network Owners and the Industry

Our study provides clear guidance for sewer network operators, planners, and contractors:
✔️ Backfill material can now be selected systematically and criteria-based
✔️ Supports long-term performance and maintainability
✔️ Promotes safer, more cost-effective, and more sustainable construction

Especially in urban environments, where tight schedules and complex infrastructures are the norm, choosing the right backfill is critical – not just technically, but also economically and environmentally.

Access the Full Study

The full paper is published in the journal:

Tunnelling and Underground Space Technology (Open Access)
Published: July 2025
Authors: Nicole Kimmling, Matteo Rubinato, Bert Bosseler et al.
🔗 Read the publication

Inspection of the installation track: The TFSB has broken into the cavity of the installation box and does not show the desired flush cavity filling.

Acknowledgements

We would like to thank the members of the municipal steering committee for this project, who have closely accompanied our project and provided us with tremendous support.

The steering committee was composed of representatives from the cities:
Burscheid (Chair), Düsseldorf, Gladbeck, Hamm, Lünen, Mülheim, Oberhausen, Recklinghausen, Solingen, Troisdorf, Wuppertal

They also contributed to the financing of the project, as did the Ministry for the Environment, Nature Conservation and Transport of the State of North Rhine-Westphalia, Germany. We would like to express our sincere gratitude for this as well.

We would like to thank also the Cologne District Government, as well as Leibniz University of Hannover and Koblenz University of Applied Sciences for their support.