Joseph Gallagher Ltd

Tangmere Wastewater Treatment Scheme

Delivering Essential Wastewater Infrastructure for Southern Water at Tangmere WwTW

By: Joseph Gallagher Ltd

Published: April 3, 2026

Aerial view of Tangmere Water Treatment Works during JGL`s upgrade works.

Project Overview

Joseph Gallagher Ltd supported Southern Water in delivering vital wastewater infrastructure upgrades at the Tangmere Water Treatment Works in Chichester, West Sussex. As installation contractor within the Galliford Try & Binnies (GTb) Joint Venture delivery team, JGL was responsible for the full construction of two 12.5‑metre‑internal‑diameter humus settlement tanks – key assets within the site’s clarified effluent treatment process.

The project forms part of Southern Water’s £7m investment programme aimed at increasing treatment capacity and improving the ecological quality of the Aldingbourne Rife, a sensitive and environmentally significant watercourse.

Joseph Gallagher Ltd delivering essential civil engineering works as part of the Tangmere improvements

Scope of Works

Joseph Gallagher Ltd delivered the full construction of both humus settlement tanks, from early enabling works through to final structural completion. The scope included forming reinforced shaft collars, sinking each caisson to 8 metres, installing tremie plug bases, pressure grouting, constructing base slabs, precision benching, installing ductile iron process pipework and completing reinforced capping beams for scraper bridge support.

Joseph Gallagher Ltd also installed FP McCann’s precast back‑bolted segmental rings using the jacked caisson method – an approach ideally suited to the site’s restricted footprint and ground conditions.

Engineering & Technical Approach

The tanks were constructed using FP McCann’s 12.5‑metre‑ID precast concrete rings, installed and sunk using the jacked caisson method. Excavation beneath each ring allowed controlled descent supported by hydraulic jacks, with back‑bolted EPDM‑sealed rings providing immediate watertightness. The works included installation of 4,000mm‑ID central caisson rings on tremie plug foundations, reinforced sloped benching, ductile pipework and, on one tank, a recessed choker ring for added shear resistance.

This methodology ensured accurate sinking, efficient progress and reduced the need for operatives to enter the shaft during construction.

Joseph Gallagher Ltd excavating within the caisson shaft during the Tangmere humus tank works

Project Challenges

The project faced several constraints, including a confined site, limited working hours and complex ground conditions that required switching to a wet‑caisson design with tremie plug bases. Despite these challenges, coordinated planning between Joseph Gallagher Ltd, GTb and Southern Water ensured successful delivery, including full shaft sinking, internal pipework installation, construction of benching and internal structures, completion of capping beams and final handover of both tanks.

Delivery Insight

All precast ring installations were completed on site, with each ring assembled and lowered using a combination of self‑weight and hydraulic assistance. The 12.5‑metre‑ID caisson design provided an efficient and safe construction solution suited to the limited working space.

The back‑bolted universal ring system minimised the need for operatives to enter the shafts during sinking, enhancing overall safety. From the installation of the first ring and jacking collar to full excavation and descent, each shaft was sunk within ten days – demonstrating highly efficient delivery and coordination.

“The two humus settlement tanks will play a vital role in improving the water quality of the local watercourse. From initial ring installation to full caisson sinking, the shafts were delivered efficiently and safely thanks to excellent collaboration and precise engineering.”