The expansion of a distillery in West Lothian promises to boost production to 20M litres a year. But, as Jessica Rowson discovers, the site proved an exotic cocktail of surprises.
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A new distillery taking shape 30km west of Edinburgh is a welcome sight for the local economy and those keen on a dram or two. But for the team charged with delivering the project for distiller Glen Turner, it has proved challenging.
The 13.5ha site at Bathgate includes five process buildings - a fermentation hall including 11, 450t vats, a dryer enclosure enclosing an 80t drier, a waste mash store, an effluent treatment building and an energy centre. By locating the complex next to the existing bottling facility, the overall production process could be made more efficient, but the site was far from ideal.
An assessment of the geology of the site established that oil shale seams were likely to have been worked at shallow depth at some time in the past and a number of mine shafts were thought to be located on the site.
“When Arup carried out the feasibility study, the initial site investigation identified some mineral stability issues,” says Arup project manager Christopher Kitchen.
“What we have is a historical mining legacy, with shallow coal workings and beneath that oil shale working, which are similar to coal seams, but thicker,” explains Arup geotechnical engineer Colin McCreath.
“A coal seam may be about 1.5m thick whereas shale might be around 3m thick. They leave voids. If they’re never infilled and the mine is near the surface, the void progresses upwards and the ground collapses - which is a problem for any structure above,” he says.
Subsequent intrusive site investigation works established that the oil shale seams and specifically the Dunnet shales were worked beneath the site.
The Dunnet shales are present as three seams; Upper Dunnet, Dunnet and Under Dunnet, and the investigation suggested that all three seams had been worked in some areas. The individual seams are believed to range in thickness from 0.6m to 3m.
“Initially we tried to determine which areas were safe to build on without treating the workings,” says McCreath. “Because of the thickness of the seams, the cost of infilling with cementitious grout was very high. The value of the ground becomes worthless and it would have made sense to go elsewhere. “
“We did a lot of drilling into the mine working to get really accurate maps and to find where there was compact rock above the mine workings”
Colin McCreath, Arup
Following the site investigation, it became clear that the best route would be to adapt the design and avoid building on the coal seams, rather than treat the ground. “The existing building had avoided them,” says Kitchen. “We knew from initial phases that the seam was shallow, but further west, they went deeper.”
Arup were looking for rock cover of eight to 10 times the worked seam thickness as this minimises the risk of void propagation affecting the surface structures. This was calculated to be a rock cover of about 50m in this area.Where worked seams were present within this depth range, this area of the site was not considered suitable for development without some form of remediation being undertaken.
To validate the preferred building area, a probe drill exercise using rotary percussive grouting plant, which can determine approximate extents of identified areas of instability, was undertaken.
“We did a lot of drilling into the mine working to get really accurate maps and to find where there was compact rock above the mine workings,” says Mc- Creath. “We had to make some minor alterations to the layout, but no grouting was needed.”
South of the investigation footprint was considered to be at high risk from mine collapses so proposed warehouses in this area were relocated to the west, while avoiding other potentially unstable areas. “We couldn’t go further west as there were more shale workings and we were hemmed in,” says McCreath.
The design team also had to take account of the location of a river and flood plain to the south of the site as well as the mine workings to the east and west.
“Each warehouse will hold around 4M litres of whisky,” says McCreath. “We needed to avoid the risk of the mine collapsing underneath them or the whole lot floating down river.”
The site slopes steeply to a flood plain in the southern portion of the site, adjacent to an existing burn. Arup carried out a Level 2 Flood Risk Assessment and reconfigured the masterplan to avoid putting the development on the flood plain.
“When we looked at the southern area, it was marshy and next to a stream,” says Kitchen. “We did a flood risk assessment as a result of which we changed the layout and moved the buildings further north as to the south was a flood plain. In addition to changing the site layout, we changed the site levels. It was a two-pronged approach. We took 100,000m3 of fill from another site to build up the levels.”
Arup had to balance the risk of the site being flooded and incoming water with the risk of spillages and outgoing fluids causing environmental damage. Ongoing consultations with th Scottish Environmental Protection Agency and the Health and Safety Executive, identified the requirement to contain spillages on site to prevent the contamination of adjacent watercourses.
“We did a flood risk assessment and as a result we moved the buildings further north as to the south was a flood plain”
Christopher Kitchen, Arup
“A triple fail-safe solution was adopted to address this risk,” says Kitchen. “Each area is locally bunded with reinforced concrete retaining walls. A dedicated spillage drainage system is used to channel spillages into an underground storage tank.
“Larger spillages are contained on site through the use of a bunded area of the flood plain. In the event of spillage we can store 4M litres in a dedicated lagoon. It’s quite ingenious - floodplains to the south contain water in the event of flooding and spillages in the event of a spillage. It’s a two-way sluice.”
Contractor Colorado Group started on site in January and superstructure started coming out of the ground in March. The first phase is due to be commissioned in March 2010 and from then on the distillery will start producing 20M litres of whisky a year.
Why height equals safety
All that whisky can really weigh a building down - the foundations for the new distillery are considerable.
There are 11 fermentation vats which weigh 500t. For this area of the distillery, a 600mm deep raft foundation designed using a raftsoil interaction software package resulting in a 30% reduction in steel reinforcement content.
Even more onerous were the conditions for the 35m high cantilever distillation columns.
“For the client it’s all about the process,” says Arup project manager Christopher Kitchen. “The higher the distillation columns, the more efficient it is.”
The foundations for the 35m high cantilever distillation columns required a 350m3 concrete base - designed to meet strict overturning, dynamic and buoyancy criteria.
To overcome early thermal cracking effects, two solutions were adopted. The concrete mix was specified to include a high proportion of GGBS binder content to reduce early thermal gain during the concrete curing process.
As well, the formwork and surface were insulated to minimise heat loss from the foundation. The pour took two days and used 50 wagons of concrete.
Whisky galore: the West Lothian distillery