Evaluation of temporarily flowable self-compacting backfill materials in large-scale sewer applications in Germany

Abstract

Ground subsidence due to inadequate compaction of backfill materials and damaged sewer pipelines poses significant risks to urban infrastructure. This study evaluates the performance of Temporarily Flowable Self-Compacting Backfill (TFSB) materials in large-scale sewer applications, addressing key properties such as flowability, volume stability, re-excavation capability, and recyclability. Five TFSB formulations were experimentally assessed using a large-scale test rig replicating real-world sewer construction conditions. The study employed a multi-criteria evaluation framework, integrating innovative testing methodologies such as the Mini-MAC system for pipe-soil stiffness measurement and a walkability test to determine early load-bearing capacity. Results demonstrated substantial variability in TFSB performance. While certain formulations exhibited superior flowability and bedding continuity, others faced challenges related to post-hardening and re-excavation difficulty. Compressive strength measurements revealed that materials exceeding 0.3 N/mm2 at 28 days hindered future removability, necessitating formulation adjustments for optimal structural integrity and maintainability. Additionally, environmental assessments identified gaps in existing standards, emphasizing the need for regulatory updates tailored to TFSB-specific properties. This research provides actionable insights for network operators and industry stakeholders, offering a framework for optimizing TFSB formulations to enhance urban infrastructure resilience. The findings contribute to the development of standardized guidelines for TFSB applications, promoting cost-effective, sustainable, and structurally reliable backfill solutions for sewer construction.