How to save a 30 tonne tank from collapse

The tank in question is big. Its capacity is about 220,000 litres. To be precise, it is 11m long, 7m wide and 2.9m deep. It is constructed from cast iron panels which are bolted together. Its self-weight alone is 30 tonnes.

The tank is located at Tone Works, a former textile finishing works near Wellington in Somerset. It is just one small part of a large Grade II* listed site filled with fascinating buildings, machinery and underground waterways being conserved by Architectural THREAD for Somerset Council . At one time the tank held water, then fuel-oil for the machinery.?

The problem? This tank, until a couple of weeks ago, was supported 2m off the ground on beams, columns and props which were decaying after decades of exposure to harsh conditions. The risk of collapse was very real. One beam had already given way, causing the tank to drop in one corner and large cracks to appear in the tank’s cast iron walls. If the tank did fall to the ground, it would take the adjacent buildings with it. Integral was tasked with rescuing it.?

The Constraints

There is an obvious answer to this problem: just build some new supports under it! Not possible, as it was not safe to go under the tank. In any case, hidden voids in the ground would make founding new vertical structure very difficult.

What about demolishing it? Is that cheaper and safer than trying to save it? Does that align with conservation principles? Discussions with the design team returned a resounding “no” to those questions.

So, the tank was to be rescued from collapse and it had to be done without going underneath it or entering it. Any solution also had to account for the restricted site access and the tank’s condition. As a bolted cast-iron structure of unknown age, care would need to be taken when applying new forces to it. ?

We quickly concluded the tank had to be hung from above. It made sense to utilise the brick masonry walls either side of the tank. These could support new structure spanning over the tank.?

Tension hangers would need to be connected to the tank but half a metre of tar-like sludge and vegetation prevented access to the base. Therefore, a two-stage plan was formed: The tank could be hung from its walls initially and the base would hang in tension. With the tank’s full self-weight supported, but the original beams and columns still in place underneath, it could be accessed for cleaning. With the base exposed, more tension hangers could be installed. This would ultimately allow for the inspection, removal and/or replacement of the original structure underneath.?

The Design

This is where our close relationship with Corbel Conservation Ltd , the main contractor, showed its value. With Corbel already on site having just completed Phase 2 in an adjacent area, they were on hand to provide invaluable support to our design work. Our design also evolved as Corbel encountered challenges and constraints on site.?

Seven primary steel beams span over the tank and are supported off new padstones in the existing walls. Secondary beams span perpendicular to these and are aligned with the tank walls below. 62 threaded rods hang from the beams to suspend the tank.

A critical part is the connection between the hangers and tank. Following an exchange of ideas with Corbel, we designed a clamp which utilises a flange at the top of the tank walls. This could be installed safely and speedily from above. It also minimised damage to the historical material by avoiding drilling completely.

?All we knew about the tank material was that it was cast iron. We had no information about its date of construction. Therefore, we assumed a very conservative allowable stress of 18N/mm2 based on guidance in the Historical Structural Steelwork Handbook. The PFC component of the clamp acts as a spreader to distribute the load along the wall flange.??

Next came the methodology for taking up the load of the tank in the new hanging structure. Unusually for us, we were fully involved in developing the method alongside Corbel as the lifting process was inextricably linked to the hanger design. Crucially, we had to find a way of ensuring the load was taken up by the hangers and beams correctly. An overloaded hanger could lead to failure of the entire system.?

With costs a key consideration, we came up with a solution which used hand-operated hydraulic jacks combined with load cells. Although strain gauges on every hanger would have provided accurate, real-time data on the loads in the rods, they would have been prohibitively expensive. Our chosen method only required two jacks and two load cells. These kits were moved between hangers in sequence. We provided Corbel with a loading plan which they used to tension the hangers to 50%, 75%, 90% and 100% load. This incremental loading ensured the whole system was loaded evenly. At each tensioning stage, the nuts at the top of the hanger rods were simply tightened off.

Western Fabrications of Taunton were the final part of this complex operation. With Corbel’s guidance they templated and fabricated steelwork which was installed ahead of programme and without any issues; an impressive achievement given the irregularity of the tank.?

With the walls of the tank now fully supported, the cleaning process is underway. In the next couple of weeks, 24 additional hangers will be installed down to the base of the tank. They will be secured with a PFC stub ‘anchor’ on the underside to spread the load on the cast iron plate.?

The Result

This project has secured a historically significant tank, in turn saving the surrounding buildings from potential destruction. The new supports allow safe access under the tank, not only for the purposes of assessing and repairing the original structure, but for future access by site users and visitors.

Despite being very technically challenging and under budget and programme pressures, the collaborative approach by the team meant that these challenges never seemed insurmountable.

To learn a little about hydraulic jacking and designing details which combined the temporary and permanent cases was a great challenge. Even more rewarding was to see the designs go from sketch to site in a matter of weeks, with no formal construction drawings issued at all. This was only possible because of the close collaboration between Integral, Corbel and the rest of the design team.?