New cement works are rarer than greenfield motorways these days. The last to be built in the UK was Ketton in 1986. Now Rugby Cement is investing 120M in the reconstruction of its Rugby works, a major feature of which will be a giant 120,000t clinker store. Eight other silos will be needed as well, and all will be designed and built by silo and cooling tower specialist Bierrum under subcontract to main contractor Amec.
In fact we built the silos at Ketton but not the clinker store, says Bierrum director and chief engineer John Turner. On this contract we began working with Amec a couple of years ago during the preliminary tender stage.
After 18 months the clinker storage had changed from two 50,000t silos to a single 100,000 tonner to a 120,000t version with an internal diameter of 55m and a wall height of 35m.
The 6M design and construct contract was awarded in May last year. Bierrum had already decided to slipform the eight smaller silos, but the big clinker store which represented half the value of the contract was a different matter. Sheer size and the anticipated complexity of the post-tensioning ducts made it a natural for the lift over lift technique developed by the contractor more than 30 years ago to cope with hyperbolic cooling towers (see box below).
But, although Bierrum had done all the preliminary design on the clinker silo and proposed to continue with the detail design of the smaller silos, it felt it needed help with the big one.
So we turned to Oscar Faber, just down the road, which we know has the expertise needed, says Turner. The Ketton clinker store was also designed by Oscar Faber.
For the first five months of 1997 the consultant worked on the final design, prior to Bierrums tender being accepted. Oscar Faber associate director Philip Benson says there have been some significant advances since the practice designed the Ketton silo.
Although we used finite element analysis then, methods now are much more sophisticated. Were specifying Grade 50 concrete rather than Grade 35. But the real difference is the height of the stored material inside.
At 30m this is significantly higher than the next highest one we know of, in Thailand, which is 25m. In fact there is nothing exceptional about the diameter or even the capacity of this silo, it is the height that makes it special.
This height is a direct function of what Benson describes as reasonable ground conditions a bearing pressure of 800kN/m2 is used in the design of the 5.5m wide by 1.5m deep ring foundation. Other design parameters are less favourable.
Benson explains: Before the new, more thermally efficient works comes into operation there will be 12 months when clinker will be entering the store at 160C. After this clinker temperatures will be half that, but the thermal gradient through the wall is still one of the worst design cases.
A temperature differential of the order of 80C produces a significant straightening force in the curved wall. which is generally 500mm thick except where it is beefed up around vehicle access door openings and the like. Another complication comes from the stress changes generated as the silo is filled and emptied.
We spent a lot of time discussing unloading scenarios with Rugby, Benson recalls. There are 13 outlets in the base of the silo leading to three conveyor tunnels, and most of the time it is the central conveyor that will be doing the unloading.
But if the outlets above that tunnel ever get blocked, the side conveyors will have to be used, producing an eccentric load on the walls.
In the worst case scenario these eccentric loads could produce a distortion of 60mm in the outer wall, which is why the silo roof will be a self-supporting steel structure. Oscar Faber also expects the slow loading cycle it takes three months in theory to fill the silo, but it only ever gets completely full over the Christmas holiday to tilt the silo floor on to a 1:400 slope at times. Total projected long term settlement is 80mm.
In all, 58 rings of tendon ducts will be installed by Bierrum, mostly concentrated in the lower half of the walls where they have to weave around the various openings. Individual tendons will run around one third of the silos circumference only.
Only concrete additive is a plasticiser. For the first 21m of the silos height the concrete was placed by pump, but after that increasing congestion around the silos base dictated a switch to crane and skip. Progress has been remorseless, with two 1.5m lifts completed every three days on average and final pours due this week.
By the end of March, PSC Freyssinet should have installed all the tendons, ready for the start of post-tensioning after Easter. When Amec hands over the new works Rugby reckons it will be the most efficient plant in the UK, if not in Europe.