Project Overview

At Joseph Gallagher, we were proud to support Southern Water in the delivery of critical wastewater infrastructure upgrades at the Tangmere Water Treatment Works in Chichester, West Sussex. Working as the installation contractor within the wider delivery team led by Galliford Try & Binnies (GTB JV), we were responsible for the full construction of two 12.5‑metre-internal diameter humus settlement tanks, an essential component of the site’s treatment process.

These works contributed to Southern Water’s £7m programme to enhance treatment capacity and improve the quality of the Aldingbourne Rife, an environmentally important watercourse.

Our Scope of Works

Joseph Gallagher Ltd delivered the complete construction of both humus tanks from the initial enabling works through to final structural completion. Our responsibilities included:

Forming reinforced shaft collars
Sinking each shaft to its full 8‑metre-depth
Installing reinforced tremie plugs to provide watertight and stable working bases
Undertaking grouting operations to ensure structural integrity
Constructing the base slabs
Completing precision benching to ensure correct hydraulic performance
Installing ductile iron pipework
Final capping beam construction to provide structural support and a foundation for the scraper bridge mechanisms

In parallel, we installed precast back‑bolted segmental shaft rings provided by FP McCann, forming each tank using the jacked caisson method. This facilitated safe and efficient construction in a restricted working area.

Engineering and Technical Approach

To construct the tanks, we used FP McCann’s 12.5-metre-ID precast concrete segmental rings, jacked into position as excavation proceeded beneath each ring. Key technical features included:

Back‑bolted universal shaft rings with EPDM gaskets for immediate watertightness
Hydraulic jacking assistance allowing each ring to descend smoothly
Installation of 4,000mm-ID central caisson rings on reinforced tremie bases
Construction of sloped reinforced benching and ductile pipework to optimise flow performance
Use of a universal recessed choker ring on one tank to provide shear resistance and prevent flotation

Project Challenges

The project presented several challenges:

A restricted site footprint that limited access and equipment manoeuvrability
Limited working hours, requiring tight sequencing and programme discipline
Complex ground conditions leading to a required redesign to a wet‑caisson system incorporating tremie plugs

By adapting our methods, coordinating closely with GTB and Southern Water, and maintaining strong safety and quality standards, we successfully delivered:

Completion of both shafts
Installation of internal pipework
Benching and internal structures
Capping beams
Full handover

Delivery Insight

Our team delivered all precast ring installations on site, forming each 12.5-metre-ID ring with precision. Once assembled, each ring descended under its own weight with hydraulic assistance, enabling rapid progress.

The concrete segments supplied by FP McCann were connected on site and each ring descended under its own weight with hydraulic jacking assistance as the ground was excavated beneath. The 12.5m ID precast concrete segmental caisson shaft provided the most efficient solution to deal with the limited working space available. From a safety perspective, the universal rings which were back-bolted meant that our operatives didn’t need to enter the shaft until the central caisson rings, concrete benching and ductile pipework were installed. From the installation of the first ring and jacking collar to the bulk excavation and caisson sinking, the shafts themselves were sunk in 10 days. The two humus settlement tanks will help to significantly improve the water quality in the local watercourse.
Ionel Croitoriu
Site Agent, Joseph Gallagher Ltd