Project Overview

This £300m wastewater treatment project was designed to process 95m litres of wastewater per day, significantly improving environmental performance along the south coast.

A major element of the scheme was the installation of a one‑kilometre, 1800mm diameter sea outfall tunnel, discharging treated effluent into the sea via diffusers positioned on the seabed.

The works also included 4m diameter on‑shore flow transfer tunnels, delivered alongside complex marine and tunnelling operations.

Scope of Works

The project presented a uniquely challenging alignment. The thrust shaft at Peacehaven was located at cliff‑top level, approximately 40m above the beach. To accommodate this, a tunnel with a 1200m radius horizontal curve was required to connect into an intermediate shaft on the promenade.

This intermediate shaft was purpose‑built to allow inspection of the TBM roller cutters before relaunching the machine for the offshore drive.

Our scope included:

Shaft sinking at cliff‑top and promenade level
Installation of 1,000m of microtunnelling beneath the seabed
Operation and maintenance of an 1800mm slurry TBM
Compressed air interventions
Full recovery of the tunnelling machine from the seabed

Technical Challenges

Microtunnelling beneath the seabed

The tunnel was constructed with only 5m cover above the tunnel soffit beneath the seabed.
Ground conditions consisted of chalk with beds of flint, consistent with the exposed cliff face.
Despite the abrasive conditions, the TBM’s roller cutters performed effectively, with full face access available should tooling changes have been required.

Groundwater and face pressure control

A pipe brake system was deployed to counteract groundwater pressure that could otherwise push the pipes backward when jacking pressure was released between excavation cycles.
Compressed air interventions were required to safely manage face conditions and maintain progress.

Machine Recovery from the Seabed

While the microtunnelling drive itself was completed in 26 weeks, additional specialist resources were required to recover the Iseki tunnelling machine.

The reception pit had been backfilled with sand to support the machine during breakthrough.
Divers installed a lifting beam onto the machine cans.
The TBM was then jacked off the installed pipes using remote‑controlled jacks.
Recovery was completed using 12 buoyancy bags, allowing the machine to be floated safely to the surface.
A tug then towed the TBM to Newhaven Harbour, where demobilisation was completed.

This operation required precise coordination between tunnelling, marine, and diving specialists.

Added Value

The 1800mm slurry microtunnelling machine and pipe brake were designed and manufactured by Specialist Plant Associates, ensuring equipment was fully tailored to the project’s demands.
The compressed air lock was designed and manufactured by Specialist Plant.
90t crawler cranes were supplied by NRC, enabling safe and efficient lifting operations.
All shaft sinking and microtunnelling works were delivered by our in‑house tunnelling division, ensuring full control over programme, quality, and safety.