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Utilities

Ground improvement beneath a sewer system

Challenge

A failed sewer in Gorleston suffered 200 mm settlement and voiding due to ground washout. Excavation to 7.5 m depth was unfeasible in the busy town-centre junction.

Solution

Geobear injected geopolymer resin to 7.5 m depth, tripling soil strength, removing sinkhole risk, and enabling safe sewer relining — all completed ahead of schedule.

3×

SPT values improved from 0–1 to 5–15

7.5 metres

Targeted injections to 7.5 m beneath sewer alignment

5 days

Completed five days ahead of schedule

Background

A road and sewer system managed by the Anglian Water One Alliance experienced severe settlement of 100–200 mm following flash flooding. The washout beneath a critical sewer pipe caused it to bow, with failed joints accelerating erosion and ground loss. This created the risk of a localised sinkhole and threatened service continuity.

Standard Penetration Testing (SPT) classified the ground beneath the sewer as very loose, with values between 0–1 at depths of 4.5–7.5 m. The client required the soils to be improved to at least medium density before relining works could proceed.

Traditional excavation to a depth of 6.5 m was ruled out. The site was in a constrained town-centre location — a busy road junction beside a supermarket. Full excavation would have required:

  • Sheet piling and heavy temporary works
  • Utility diversions
  • Extended road closures lasting several weeks
The client sought an alternative that would stabilise the sewer, mitigate settlement risk, and minimise community disruption.

Challenge

The client faced both engineering and logistical challenges.

From an engineering perspective, the soils were dangerously weak and required densification without excavation. Full pipe collapse was a risk if ground support was not reinstated. The treatment had to reach depths of 7.5 m in very loose, water-affected soils, while ensuring accuracy to avoid clashes with utilities.
From a logistical perspective, the works were located in a busy town centre. Excavation would have meant major disruption, extended road closures, and high community impact. Anglian Water needed a solution that was fast, precise, and minimally disruptive.

Client testimonial:

Geobear’s engineering team worked collaboratively with us to develop an innovative solution. The site team exceeded expectations and clearly met all of our objectives.

Anglian Water Alliance

Solution

Geobear worked closely with the Anglian Water Alliance to develop an innovative geopolymer injection programme.

  • Plaxis modelling was used to demonstrate achievable ground improvement, giving the client confidence in the proposed solution.
  • BIM integration: For the first time in the industry, Geobear’s injection design was incorporated into a Building Information Model (BIM). This enabled accurate mapping of injection points, avoiding buried services and ensuring safe, targeted delivery. (See image below)
  • Injection methodology: Resin tubes were installed to depths of 7.5 m on a grid pattern beneath the sewer. Expansive geopolymer resin was injected to densify the loose soils, compacting voids and reinstating support to the pipe.
  • Programme optimisation: The original design anticipated a working platform for rod-driving. On site, Geobear’s engineers developed a direct rod-driving method, eliminating the need for the platform and saving five days from the programme.
  • Monitoring & verification: Pre- and post-treatment SPT testing demonstrated that soil density increased from 0–1 to between 5–15, exceeding the client’s requirement for medium-density ground.
BIM model Baker Street sewer

Results

The project was successfully completed ahead of programme and enabled full sewer relining to proceed safely.

  • Ground strength tripled: Post-treatment SPT values confirmed soils were improved to medium density.
  • Sinkhole risk eliminated, restoring stability beneath the road and sewer.
  • No disruption to the community, as excavation and extended road closure were avoided.
  • Carbon savings realised: Compared with 6.5 m excavation and reconstruction, the geopolymer approach cut embodied carbon significantly.
  • Innovation milestone: First-ever use of BIM modelling to design and deliver a geopolymer injection scheme in the utilities sector.

Client Representative, Anglian Water Alliance:

“The unforeseen circumstances presented us with a particularly challenging situation. Geobear’s engineering team worked collaboratively with us to develop an innovative solution. The site team exceeded expectations and clearly met all of our objectives. The result was delivered ahead of programme, with ground conditions improved well beyond the requirement.

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