Case Study: Permeation Grouting for Flood Defence Scheme in Otley
Project Overview
Location: Otley, West Yorkshire
Project: Flood Defence Scheme
Client: Bam Nuttall
Objective: To explore the incorporation of permeation grouting to effectively seal a 6m gap in the sheet piles, avoiding excavation and reducing risks to a historic bridge.
Background
The flood defence structure in Otley was constructed over gravel and sand with a high water table. The embankment included a sheet pile cut-off wall intended to prevent seepage beneath the embankment. However, due to the presence of a historic bridge, the sheet piles stop approximately 6 meters from the bridge, creating a potential seepage path. The design solution originally involved excavating a trench to 4.5m and backfilling it with clay, which posed risks related to high soil permeability and water pressure.
Problem Statement
The primary concern was the risk of excavation boiling (an upward water flow into the bottom of the cut) and the inability to effectively place the clay plug due to high permeability soils and differential water pressure. Geobear was contacted to design a permeation grouting solution using geopolymer. This would seal the gap and mitigate the associated risks without excavation. Careful monitoring was also required due to a surface sewer pipe.
Proposed Solution
Design: Geobear Permeation Grouting / Soil Permeability Reduction Solution with geopolymer
Objective: To reduce soil permeability in the 6m gap adjacent to the historic bridge, preventing water seepage without extensive excavation.
- Area of Treatment: A soil volume measuring 6.0m x 1.4m x 2.5m.
- Materials: Geopolymer
- Equipment: Specialist geopolymer injection unit.
Execution Plan
- Site Preparation: Bam Nuttall reduced the ground level to the top of the 900mm surface water sewer pipe. They exposed the top of the 450mm surface water sewer pipe for monitoring during injection.
- Injection Process: Angled injection to treat soils beneath the pipes, avoiding interference with existing services, followed by drilling to the required depth and inserting 12mm injection tubes. Geopolymer was injected into the surrounding soils via hoses from the injection unit.
- Duration: The main works were completed within four days.
Benefits
- Risk Mitigation: Reduced the risk of excavation boiling and water pressure issues.
- Preservation: Protected the integrity of the historic bridge by avoiding extensive excavation.
Conclusion
The solution successfully addressed the seepage path concern without the risks associated with traditional excavation methods. By implementing this approach, the project team ensured the structural integrity of the flood defence scheme and the historic bridge, while maintaining project timelines and budget constraints. The wider Otley Flood Alleviation scheme was successful and received an ICE Award.