As part of the wider decommissioning process of the two reactors buildings at Berkeley Nuclear Power station, OBR were awarded the contract of constructing a 2,000m2 waste processing facility building and apron slab. The brief was to excavate the ground which use to be on the footprint of the pond’s location, install a new raft slab and construct a steel portal frame and clad with single skin cladding. 

This was a design and build project for OBR, and we were engaged with our client and designers for 6 months prior to mobilisation in working around the site constraints. Because of the possible contaminated land below the location the slab was to be constructed, we were only allowed to excavate within 1m of the current site levels. This had an impact on our slab and drainage design, and we worked through many iterations of the design before landing on the clients prefer option.
We were to excavate down 800mm, import sub-base and form the external ring and internal thickening ribs of the raft slab. We laid concrete blinding to the whole area which would then allow an accurate install of the steel reinforcement structures. However, the underground services drawings that were provided were incomplete and many clashes with historic concrete structures within he ground was found during our excavation. To ensure that the excavator was unable to dig lower than the strictly set 1m depth, and avoid tracking over known underground live services, we installed a machine control system on the 13t excavator. This allowed us to set limits on the machine that would physically stop the plant operative from exceeding depths and staying within allowable limits. It also allowed our engineer to update the excavation model on the fly and upload to the machine while it was working, and review excavation quantities and working hours. 

The formwork was constructed from timber off-site by OBR and transported in to provide greater progress as we did not have much room on site, and our off-site warehouse would allow our joiners to work in adverse weather conditions. The external rebar cages were prefabricated off-site and laved together in-situ, again to increase quality and decrease programme. The hold down bolts boxes were constructed and installed to accuracy of 20mm, supported by our engineers.
Strict quality control was undertaken at each step of the slab formation and inspection and tests plans captures each step, laying out the requitements, standards used and results to prove compliance.  The 500m3 slab was poured over two days, working 14hr days to avoid unnecessary joints in the raft slab. The concrete was laid in warm weather, and we applied a number of curing control techniques to minimise the risk of any cold joints, or cracking of the completed slab. The slab tolerance was +-5mm from end to end, and our team of engineers were continually checking the concrete gangs work to ensure we were within this range.