FP MCCANN BACK-BOLTED PRECAST CONCRETE SEGMENTAL SHAFT FORMS COMBINED WASTEWATER ATTENUATION TANK

Project: Combined Wastewater Holding Tank – Snitterfield, Stratford-upon-Avon, Warwickshire

Client: Severn Trent Water

Installation Contractor: CPC Civils

Installation Process: Jacked Caisson Method

Product Supplied:  9.0m id Back-Bolted Precast Concrete Segmental Shaft; 9.0m diameter Precast Concrete Cover Slab

FP McCann’s back-bolted segmental shaft rings have been used to build a 9.00m id combined sewage/stormwater holding tank as part of Severn Trent Water’s ‘Green Recovery’ programme of works. Severn Trent is implementing planned improvements to several of its wastewater treatment works and overflow networks, alongside pioneering monitoring and ozone technology to enhance wastewater cleaning, a move set to revolutionise the future of wastewater treatment.

The segmental shaft attenuation tank has been installed adjacent to the Snitterfield sewage treatment works near Stratford-upon-Avon, Warwickshire, by Solihull-based specialist civil engineering and water management contractor CPC Civils.

The combined wastewater temporary holding tank will capture excessive volumes of stormwater and then release volumes at a system-controlled level into the Snitterfield sewage treatment works for processing.

The shaft has been sunk 7 metres deep using the jacked caisson method of construction, with the individual back-bolted precast concrete rings forming the shaft wall as each was jacked down into position. The design of the structure features a universal transition recessed choker ring, manufactured by FP McCann, which is positioned on top of an FP McCann concrete cutting edge. The choker incorporates recesses cast into the intrados, providing shear interaction with the concrete base to resist flotation.

Each load of precast concrete segments was delivered on specialist vehicles to ensure easy access to the site. The CPC Civils team then formed the individual 9.0m id rings. To complete the shaft build, a 9.0-metre-diameter FP McCann precast concrete cover slab was installed in four separate sections. Both the segmental rings and cover slab were manufactured and transported from FP McCann’s Cadeby factory near Nuneaton on a just-in-time delivery schedule.

Commenting on behalf of CPC Civils, Project Manager Neil Perrins says, “The concrete segments supplied by FP McCann were simply connected on site and each ring descended with hydraulic jacking assistance under its weight as the ground was excavated beneath. The precast segmental caisson shaft provided the most efficient solution to deal with the limited working space available. The much smaller footprint compared to other options considered reduced the volume of material to be removed. From a safety aspect, the back-bolted rings meant that our operatives didn’t need to enter the shaft until the base plug was poured. From the installation of the first ring and jacking collar to the bulk excavation and caisson sinking, the shaft building has taken 8 weeks. Once commissioned, this 7-metre-deep chamber will receive excess combined wastewater from the local drainage network”.

FP McCann’s shaft segments are all factory-fitted with EPDM rubber gaskets, which provide an immediate watertight seal upon construction. Cross segment connections are made by passing a spear bolt through a pocket in one segment and screwing it into a threaded plastic socket in the adjacent segment. Circle joint connections are made using a ‘T’ bolt passing through a hole in one segment into a ‘T’ box in the adjacent segment. Bolts are designed to compress the gasket fully.

Key Features and benefits of FP McCann’s back and front bolted smoothbore shafts are:-
• Smooth internal faces
• Simple locking process
• Speedy installation
• Increased site productivity
• Safe build features
• Technical advice and support