Solving Ince's challenges
Ince Biomass Plant near Ellesmere, Cheshire challenged our HAC (Part of SOLID) team on several fronts. Today we’ll highlight the four main challenges that had to be overcome during the design and construction of this facility.
Ground conditions
The 7.5 hectare site is located in waste land with 9-12m of underlying alluvial clay with pockets/layers of peat. The site was not previously developed, and there was the possibility of unexploded ordnances. Hence, prior to piling, each of the 1342 pile locations were probed.
Based on the ground investigation report provided by the client, the piles were estimated at between 35-40m long. Ward and Burke Construction carried out further cone penetration tests, and appointed geotechnical specialist Geocentrix to interpret the results. They provided design parameters which enabled the piles to be designed at reduced lengths of 18-30m. Group pile analyses were carried out to verify settlements were within acceptable limits.
Long term settlement was estimated at between 200mm and 300mm due to surcharge. A 1.2m thick stone-fill platform reinforced with four layers of geogrid was constructed as a piling platform. The piling platform was installed over the topsoil with no strip off.
Differential and final displacements of the various structures were critical and simulating the piled foundations correctly with SCIA models were crucial. HAC (part of SOLID) liaised with Geocentrix to determine the spring constants to use for the structural design in the SCIA models.
Loadings information out of sync with the construction programme
Due to programme constraints, the final plant layout and column loads were not available until after the piles had been installed. The information available at that stage were unit loads for each of the main components of the equipment. There was no information on the framing structure, or how the loads were distributed.
The specialist suppliers could not commit to the column layout or loads until they had completed their design, which did not fit in with the construction programme.
HAC (Part of SOLID) persuaded the specialist supplier to provide information of similar plant that they had installed in the USA. Using this information, with an enhancement on the column loads, HAC (Part of SOLID) used their engineering judgement and experience to come up with a piling layout which would allow for some variation in column positions and loads. The assessed column loads were passed back to the specialist supplier to ensure that these were not exceeded.
The final column loads were checked and were all within out ‘not to exceed’ figures.
Designing for large temporary construction loadings
The construction programme was based on the substructure, piling and all structural ground slabs being completed before the gasifier and boiler would be installed, with the building envelope to follow after. The piles and the ground slabs were designed for the gasifier and boiler, the two heaviest items, to be installed using a 600T crane. However, due to the procurement process, the fabrication and delivery of the gasifier and the boiler were late and it was decided to proceed with the building envelope, leaving access hatches in the roof of the gasification building for the gasifier and boiler to be lowered into the building. All deliveries and lifting operations were carried out after completion of the permanent works
Due to the additional height required to lift the items over the roof of the building, a larger crane was required, which in turn meant the loads imposed on the substructure would be higher.
Permanent works had to be designed for the temporary construction loads, and the delivery and cranage loads. For instance, the hardstanding slab and piles were designed for an 800T crane. A 1750T crane was used to lift the gasifier and boiler into the building
Extensive design checks were carried out to ensure the integrity of the structure was not compromised. Heavy duty steel mats were used to distribute rigger loads
The assembly of the main crane for the boiler lift required additional smaller working cranes at the same time, in close proximity. The position of each crane had to be checked to verify that the structural slab and piles were not compromised.
Considerable negotiations were required with the crane supplier to get the necessary information for the design checks, and to convince them that much heavier duty steel mats were required than what was proposing.
In checking the substructure, the steel mats were modelled with the slab as the mats were not entirely rigid and would not load the slab/piles uniformly.
The loading sequence of the 140T boiler lifted into 40m tall Gasification Building captured on web cam.
Controlling vibrations on the Turbine table
The turbine table is elevated 2.85m off the ground floor and sits on 1m square RC columns which are supported off an isolated 1.5m thick reinforced slab on 20 piles. The connection between the turbine table and the RC columns is via two tuned springs at each column.
The structure was analysed with hinged supports at the column heads and the load/displacement at each location was advised to the dynamic analysis consultant, who specified the tuned springs required to attenuate resonant frequencies. The spring constants were then introduced to the structural model to verify that the load distribution to each column had not changed.
There were a considerable number of penetrations in the table top for pipework and holding down bolts and many embedded plates & fixtures for post fixing of equipment. There were also variations in the levels of the top surface of the 1.7m slab which included shallow recesses.
The turbine and the gearbox were bolted through the 1.7m thick slab but the generator was fixed to the 2.5m thick slab with M30 bolts embedded 400mm. Special ‘baskets’ were fabricated to form the voids for the holding down bolts. These were profiled to ensure the subsequently grouted bolts would be fully engaged with the surrounding concrete.
With a lot of delicate equipment on the turboset, this was to be installed after completion of the building. Consideration was given to an access hatch in the turbine hall roof to lower the individual elements and skidding them in to place. However, with the building complete, and the height of the roof requiring a much larger crane than had been allowed for, the crane had to be set back from the building to get sufficient clearance. The outer riggers would then sit on a structure which would be compromised.
Skidding the turboset in from the end of the turbine hall was then considered. This required liaising with the steelwork designer to allow for some structural members to be temporarily removed for the installation. With this method of installation, the whole operation of moving the turboset on to the turbine table was complete in one day, instead of the week that would have been required if installed through the roof.
