Thrust Wall Formwork in Doka Frami for a Water Main Diversion, Banwell Bypass, North Somerset

Location: Banwell, North Somerset
Project time: September 2026

SEL designed and supplied Doka Frami wall formwork for two L-shaped reinforced concrete thrust walls on a DN600 ductile iron water main diversion for the Banwell Bypass. SEL set out the Frami panels around a timber infill at the pipe, so the flanged pipe could be cast through each wall without cutting hire panels. Both walls were cast as planned in September 2026, and the Temporary Works Design was issued the day after the request.

The Banwell Bypass required existing water mains to be diverted along a new alignment. Where a pressurised main changes direction, the hydraulic thrust at the bend has to be resisted by a mass of reinforced concrete, and the diversion included thrust walls built for that purpose. Each structure comprised two walls meeting at a 90-degree corner on a reinforced concrete thrust slab measuring 6.0m by 5.5m. Wall 1 was 6.0m long and 0.50m thick, and Wall 2 was 5.5m long and 0.60m thick, both cast on a 100mm kicker in C32/40 concrete.

The 4.5 tonne Frami package formed a lower thrust wall with a 1.40m pour height and a higher thrust wall with a 1.815m pour height. Each wall was cast in a single pumped pour, with the 600mm diameter ductile iron pipe and puddle flange cast through its 600mm-thick leg. The design was based on a full hydrostatic concrete pressure of 45kN/m² and checked as a Category 2 design under BS 5975.

The Challenge: Casting a Flanged DN600 Ductile Iron Pipe Through a Thrust Wall Formed in Doka Frami

The permanent works design required the wall reinforcement to be fixed in one operation with the puddle flanges already in place, and the formwork had to accommodate the pipework passing through the shutter. The DN600 ductile iron pipe was therefore fixed in position before the second face of formwork was closed. The request reached SEL on 2 September with the design required for the following day, and the drawing was issued on 3 September.

Doka Frami Design Criteria for the Banwell Thrust Walls

The package totalled 4.5 tonnes and included 72 Frami panels in 1.5m and 1.2m sizes, corner units for the 90-degree junction, 16 push-pull props, lifting hooks for crane handling, and access brackets with edge protection for the higher wall. On the higher thrust wall the 1.2m panels were stacked to a formwork height of 2.40m for the 1.815m pour. The Frami was designed for the full hydrostatic head of concrete at 45kN/m², within the 50kN/m² permissible pressure of the system, so the rate of rise did not limit the pour.

Why We Set Out the Frami Around a Timber Infill at the Pipe Penetration

Frami panels are modular hire components with fixed widths and tie positions, and a 600mm pipe with a puddle flange cannot pass through a panel face without cutting it. Because the reinforcement and flanges were fixed before the shutter closed, the formwork on both thrust walls also had to be assembled around a pipe that was already in its final position. SEL's layout arranged the Frami panels on either side of a 775mm zone at the pipe, where a timber infill section was cut to the pipe and flange profile. The Frami walers continued above and below the pipe to carry the infill, and the remainder of both walls, including the corner, was formed in standard Frami panels and corner units.

Frami Restraint, Push-Pull Props and Access

The two faces of each wall were tied through the concrete with tie rods in plastic sleeves, which left the walls free of cast-in ties once the rods were withdrawn at striking. One face was braced by SEL-modified push-pull props set at 50 degrees, No.0 props on the lower thrust wall and No.1 props on the higher, with the longer props matching the additional 415mm of pour height. On the higher wall, Frami access brackets at a maximum of 1.2m centres carried a working platform of scaffold boards with barriers and safety posts for the pour. After the lower thrust wall was struck, the clamps, walers and props were reused on the higher wall, so the package was sized to the larger of the two requirements rather than to both walls combined. The Temporary Works Design was prepared to BS 5975 and Formwork: A Guide to Good Practice, and was checked as a Category 2 design under BS 5975.

The Result:

The lower and higher thrust walls were both cast in single pumped pours in September 2026, as planned. The DN600 ductile iron pipe and puddle flange were cast through the 600mm-thick leg of each wall at the timber infill. After the lower thrust wall was struck, its Frami clamps, walers and props were reused on the higher wall within the same 4.5 tonne package.

Services and Equipment Provided

• Temporary Works Design for the Doka Frami formwork, checked to BS 5975 Category 2 and issued the day after request
• Doka Frami panels, inside and outside corners and walers
• SEL-modified No.0 and No.1 push-pull props
• Tie rods, sleeves and cones
• Frami access brackets, barriers, safety posts and scaffold boards
• Lifting hooks
• Supply from SEL's Bristol depot

Other Applications for Doka Frami Formwork

• Pipeline thrust blocks
• Valve and meter chamber walls
• Pump station and wet well walls
• Retaining walls
• Pad and strip foundations
• Pile caps and ground beams
• Culvert headwalls and wing walls
• Lift pit and basement walls
• Attenuation tank walls
• Plinths and equipment bases

This project was supplied from SEL's Bristol depot, which covers South West England. For Doka Frami formwork and Temporary Works Design, contact the Bristol depot or your nearest SEL depot to arrange a no-obligation site visit. SEL provides Temporary Works Design and engineering support whether a job calls for full design or kit alone.

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