Home » Projects » Void Filling Colt Cragg Reservoir

Project Overview

Injection hose to transfer material on reservoir embankment

Location: Colt Crag Reservoir, Hexham, Northumberland
Client: Esh Group – Stantec
Main Contractor: Northumberland Water
Project Duration: 1 week


Geobear was contracted to provide a void-filling solution for an abandoned spillway tunnel at Colt Crag Reservoir. This project required the use of a specialist geopolymer design, delivery team and equipment to ensure a precise and efficient execution at a difficult-to-access site.

Scope of Work

The primary objective was to fill the 232m³ void within the tunnel using Geobear’s expansive geopolymer resin, ensuring structural stability and integrity. The project required careful planning and execution due to restricted site access and specific logistical constraints.

Project Challenges

  • Restricted Access: The site had limited access, necessitating the use of smaller injection units and extended hoses.
  • Logistical Constraints: Geopolymer materials had to be stored off-site and transported in small loads using 4×4 vehicles and flatbed trailers.
  • Structural Preparations: The lower tunnel arches required formwork installation to retain the geopolymer in its liquid state. Blockwork formwork was constructed by the main contractor at both ends of the tunnel.
  • Carbon reduction is a key driver for many projects, Geobear solutions emit upto 75% less emissions than alternative methods due to the minimal plant and equipment use.  

View of restricted access into void


  • Site Preparation:
    • Injection unit and trailers were parked north of the spillway bridge.
    • Formwork installation at tunnel ends.
    • Removal of any standing water from the void area.
    • Provision of task lighting and ventilation within the tunnel.
  • Geopolymer Injection:
    • Technicians entered the tunnel from the bottom, using extended hoses to pump geopolymer from the injection unit.
    • A geopolymer resin with a compressive strength of 100kPa, was used.
    • Continuous visual monitoring ensured the precise filling of the void.

Execution and Timeline

The project was executed with one Geobear injection and the entire void-filling process was completed within the estimated one-week timeframe, subject to uninterrupted access and continuous workflow.

Materials Used

Geobear’s specialist geopolymer resin was employed, conforming to BS EN 826 standards for compressive strength. The resin was selected for its expansive properties, essential for effective void filling in confined spaces.

Visual of expanded geopolymer at mouth of site

Site-Specific Considerations

  • Safe access steps and designated waste disposal arrangements were managed by the main contractor.
  • A detailed Method Statement and Health, Safety, and Environmental assessments were prepared and adhered to throughout the project.

Benefits of the Geobear Solution

  1. Reduced Carbon Emissions: Geobear’s solutions emit up to 75% less carbon than traditional void-filling alternatives, contributing to a more sustainable and environmentally friendly approach.
  2. Efficient and Quick Installation: The use of specialist injection units and extended hoses allowed for a swift and efficient void-filling process, completed within the estimated one-week timeframe.
  3. Minimal Disruption: The project’s execution plan ensured minimal disruption to the site and surrounding areas, thanks to the precise and contained injection method.
  4. Structural Integrity: The expansive properties of the geopolymer resin provided robust structural support, enhancing the overall stability of the abandoned spillway tunnel.
  5. Versatility and Adaptability: The solution was tailored to address the site-specific constraints, demonstrating Geobear’s ability to adapt to challenging environments.


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